# Guide to GNU gcj

## Table of Contents

- GNU General Public License
- GNU Free Documentation License
  - ADDENDUM: How to use this License for your documents
- 1 Invoking gcj
  - 1.1 Input and output files
  - 1.2 Input Options
  - 1.3 Encodings
  - 1.4 Warnings
  - 1.5 Linking
  - 1.6 Code Generation
  - 1.7 Configure-time Options
- 2 Compatibility with the Java Platform
  - 2.1 Standard features not yet supported
  - 2.2 Extra features unique to gcj
- 3 Invoking jcf-dump
- 4 Invoking gij
- 5 Invoking gcj-dbtool.
- 6 Invoking jv-convert
- 7 Invoking grmic
- 8 Invoking gc-analyze
- 9 Invoking aot-compile
- 10 Invoking rebuild-gcj-db
- 11 About CNI
  - 11.1 Basic concepts
    - 11.1.1 Limitations
  - 11.2 Packages
    - 11.2.1 Leaving out package names
  - 11.3 Primitive types
    - 11.3.1 Reference types associated with primitive types
  - 11.4 Reference types
  - 11.5 Interfaces
  - 11.6 Objects and Classes
    - 11.6.1 Classes
    - 11.6.2 Object fields
    - 11.6.3 Access specifiers
  - 11.7 Class Initialization
  - 11.8 Object allocation
  - 11.9 Memory allocation
  - 11.10 Arrays
    - 11.10.1 Creating arrays
  - 11.11 Methods
    - 11.11.1 Overloading
    - 11.11.2 Static methods
    - 11.11.3 Object Constructors
    - 11.11.4 Instance methods
    - 11.11.5 Interface methods
  - 11.12 Strings
  - 11.13 Interoperating with C/C++
    - 11.13.1 RawData
    - 11.13.2 RawDataManaged
    - 11.13.3 Native memory allocation
    - 11.13.4 Posix signals
  - 11.14 Exception Handling
  - 11.15 Synchronization
  - 11.16 Invocation
    - 11.16.1 Handling uncaught exceptions
    - 11.16.2 Example
  - 11.17 Reflection
- 12 System properties
  - 12.1 Standard Properties
  - 12.2 GNU Classpath Properties
  - 12.3 libgcj Runtime Properties
- 13 Resources
- Index

# Introduction

This manual describes how to use `gcj`, the GNU compiler for the Java programming language. `gcj` can generate both `.class` files and object files, and it can read both Java source code and `.class` files.

## GNU General Public License

Version 3, 29 June 2007

    Copyright © 2007 Free Software Foundation, Inc. http://fsf.org/

    Everyone is permitted to copy and distribute verbatim copies of this
    license document, but changing it is not allowed.

### Preamble

The GNU General Public License is a free, copyleft license for software and other kinds of works.

The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program–to make sure it remains free software for all its users. We, the Free Software Foundation, use the GNU General Public License for most of our software; it applies also to any other work released this way by its authors. You can apply it to your programs, too.

When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for them if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs, and that you know you can do these things.

To protect your rights, we need to prevent others from denying you these rights or asking you to surrender the rights. Therefore, you have certain responsibilities if you distribute copies of the software, or if you modify it: responsibilities to respect the freedom of others.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.

Developers that use the GNU GPL protect your rights with two steps: (1) assert copyright on the software, and (2) offer you this License giving you legal permission to copy, distribute and/or modify it.

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Finally, every program is threatened constantly by software patents. States should not allow patents to restrict development and use of software on general-purpose computers, but in those that do, we wish to avoid the special danger that patents applied to a free program could make it effectively proprietary. To prevent this, the GPL assures that patents cannot be used to render the program non-free.

The precise terms and conditions for copying, distribution and modification follow.

### TERMS AND CONDITIONS

1. Definitions.

   “This License” refers to version 3 of the GNU General Public License.

   “Copyright” also means copyright-like laws that apply to other kinds of works, such as semiconductor masks.

   “The Program” refers to any copyrightable work licensed under this License. Each licensee is addressed as “you”. “Licensees” and “recipients” may be individuals or organizations.

   To “modify” a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a “modified version” of the earlier work or a work “based on” the earlier work.

   A “covered work” means either the unmodified Program or a work based on the Program.

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   To “convey” a work means any kind of propagation that enables other parties to make or receive copies. Mere interaction with a user through a computer network, with no transfer of a copy, is not conveying.

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   You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License, in one of these ways:

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8. Additional Terms.

   “Additional permissions” are terms that supplement the terms of this License by making exceptions from one or more of its conditions. Additional permissions that are applicable to the entire Program shall be treated as though they were included in this License, to the extent that they are valid under applicable law. If additional permissions apply only to part of the Program, that part may be used separately under those permissions, but the entire Program remains governed by this License without regard to the additional permissions.

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   You may not propagate or modify a covered work except as expressly provided under this License. Any attempt otherwise to propagate or modify it is void, and will automatically terminate your rights under this License (including any patent licenses granted under the third paragraph of section 11).

   However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

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    Nothing in this License shall be construed as excluding or limiting any implied license or other defenses to infringement that may otherwise be available to you under applicable patent law.

13. No Surrender of Others’ Freedom.

    If conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot convey a covered work so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not convey it at all. For example, if you agree to terms that obligate you to collect a royalty for further conveying from those to whom you convey the Program, the only way you could satisfy both those terms and this License would be to refrain entirely from conveying the Program.

14. Use with the GNU Affero General Public License.

    Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU Affero General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such.

15. Revised Versions of this License.

    The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

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    If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Program.

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16. Disclaimer of Warranty.

    THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

17. Limitation of Liability.

    IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

18. Interpretation of Sections 15 and 16.

    If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.

### END OF TERMS AND CONDITIONS

### How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.

    one line to give the program's name and a brief idea of what it does.
    Copyright (C) year name of author

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or (at
    your option) any later version.

    This program is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see http://www.gnu.org/licenses/.

Also add information on how to contact you by electronic and paper mail.

If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:

    program Copyright (C) year name of author
    This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
    This is free software, and you are welcome to redistribute it
    under certain conditions; type ‘show c’ for details.

The hypothetical commands ‘`show w`’ and ‘`show c`’ should show the appropriate parts of the General Public License. Of course, your program’s commands might be different; for a GUI interface, you would use an “about box”.

You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/.

The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html.

## GNU Free Documentation License

Version 1.3, 3 November 2008

    Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
    http://fsf.org/

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

1. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

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   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

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   If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

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11. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

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12. RELICENSING

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    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

### ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

      Copyright (C)  year  your name.
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.3
      or any later version published by the Free Software Foundation;
      with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
      Texts.  A copy of the license is included in the section entitled ``GNU
      Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

        with the Invariant Sections being list their titles, with
        the Front-Cover Texts being list, and with the Back-Cover Texts
        being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

## 1 Invoking gcj

As `gcj` is just another front end to `gcc`, it supports many of the same options as gcc. See Option Summary in Using the GNU Compiler Collection (GCC). This manual only documents the options specific to `gcj`.

### 1.1 Input and output files

A `gcj` command is like a `gcc` command, in that it consists of a number of options and file names. The following kinds of input file names are supported:

`file.java`

  Java source files.

`file.class`

  Java bytecode files.

`file.zip`
`file.jar`

  An archive containing one or more `.class` files, all of which are compiled. The archive may be compressed. Files in an archive which don’t end with ‘`.class`’ are treated as resource files; they are compiled into the resulting object file as ‘`core:`’ URLs.

`@file`

  A file containing a whitespace-separated list of input file names. (Currently, these must all be `.java` source files, but that may change.) Each named file is compiled, just as if it had been on the command line.

`library.a`
`library.so`
`-llibname`

  Libraries to use when linking. See the `gcc` manual.

You can specify more than one input file on the `gcj` command line, in which case they will all be compiled. If you specify a `-o FILENAME` option, all the input files will be compiled together, producing a single output file, named FILENAME. This is allowed even when using `-S` or `-c`, but not when using `-C` or `--resource`. (This is an extension beyond the what plain `gcc` allows.) (If more than one input file is specified, all must currently be `.java` files, though we hope to fix this.)

### 1.2 Input Options

`gcj` has options to control where it looks to find files it needs. For instance, `gcj` might need to load a class that is referenced by the file it has been asked to compile. Like other compilers for the Java language, `gcj` has a notion of a class path. There are several options and environment variables which can be used to manipulate the class path. When `gcj` looks for a given class, it searches the class path looking for matching `.class` or `.java` file. `gcj` comes with a built-in class path which points at the installed `libgcj.jar`, a file which contains all the standard classes.

In the text below, a directory or path component can refer either to an actual directory on the filesystem, or to a `.zip` or `.jar` file, which `gcj` will search as if it is a directory.

`-Idir`

  All directories specified by `-I` are kept in order and prepended to the class path constructed from all the other options. Unless compatibility with tools like `javac` is important, we recommend always using `-I` instead of the other options for manipulating the class path.

`--classpath=path`

  This sets the class path to path, a colon-separated list of paths (on Windows-based systems, a semicolon-separate list of paths). This does not override the builtin (“boot”) search path.

`--CLASSPATH=path`

  Deprecated synonym for `--classpath`.

`--bootclasspath=path`

  Where to find the standard builtin classes, such as `java.lang.String`.

`--extdirs=path`

  For each directory in the path, place the contents of that directory at the end of the class path.

`CLASSPATH`

  This is an environment variable which holds a list of paths.

The final class path is constructed like so:

- First come all directories specified via `-I`.
- If `--classpath` is specified, its value is appended. Otherwise, if the `CLASSPATH` environment variable is specified, then its value is appended. Otherwise, the current directory (`"."`) is appended.
- If `--bootclasspath` was specified, append its value. Otherwise, append the built-in system directory, `libgcj.jar`.
- Finally, if `--extdirs` was specified, append the contents of the specified directories at the end of the class path. Otherwise, append the contents of the built-in extdirs at `$(prefix)/share/java/ext`.

The classfile built by `gcj` for the class `java.lang.Object` (and placed in `libgcj.jar`) contains a special zero length attribute `gnu.gcj.gcj-compiled`. The compiler looks for this attribute when loading `java.lang.Object` and will report an error if it isn’t found, unless it compiles to bytecode (the option `-fforce-classes-archive-check` can be used to override this behavior in this particular case.)

`-fforce-classes-archive-check`

  This forces the compiler to always check for the special zero length attribute `gnu.gcj.gcj-compiled` in `java.lang.Object` and issue an error if it isn’t found.

`-fsource=VERSION`

  This option is used to choose the source version accepted by `gcj`. The default is ‘`1.5`’.

### 1.3 Encodings

The Java programming language uses Unicode throughout. In an effort to integrate well with other locales, `gcj` allows `.java` files to be written using almost any encoding. `gcj` knows how to convert these encodings into its internal encoding at compile time.

You can use the `--encoding=NAME` option to specify an encoding (of a particular character set) to use for source files. If this is not specified, the default encoding comes from your current locale. If your host system has insufficient locale support, then `gcj` assumes the default encoding to be the ‘`UTF-8`’ encoding of Unicode.

To implement `--encoding`, `gcj` simply uses the host platform’s `iconv` conversion routine. This means that in practice `gcj` is limited by the capabilities of the host platform.

The names allowed for the argument `--encoding` vary from platform to platform (since they are not standardized anywhere). However, `gcj` implements the encoding named ‘`UTF-8`’ internally, so if you choose to use this for your source files you can be assured that it will work on every host.

### 1.4 Warnings

`gcj` implements several warnings. As with other generic `gcc` warnings, if an option of the form `-Wfoo` enables a warning, then `-Wno-foo` will disable it. Here we’ve chosen to document the form of the warning which will have an effect – the default being the opposite of what is listed.

`-Wredundant-modifiers`

  With this flag, `gcj` will warn about redundant modifiers. For instance, it will warn if an interface method is declared `public`.

`-Wextraneous-semicolon`

  This causes `gcj` to warn about empty statements. Empty statements have been deprecated.

`-Wno-out-of-date`

  This option will cause `gcj` not to warn when a source file is newer than its matching class file. By default `gcj` will warn about this.

`-Wno-deprecated`

  Warn if a deprecated class, method, or field is referred to.

`-Wunused`

  This is the same as `gcc`’s `-Wunused`.

`-Wall`

  This is the same as `-Wredundant-modifiers -Wextraneous-semicolon -Wunused`.

### 1.5 Linking

To turn a Java application into an executable program, you need to link it with the needed libraries, just as for C or C++. The linker by default looks for a global function named `main`. Since Java does not have global functions, and a collection of Java classes may have more than one class with a `main` method, you need to let the linker know which of those `main` methods it should invoke when starting the application. You can do that in any of these ways:

- Specify the class containing the desired `main` method when you link the application, using the `--main` flag, described below.
- Link the Java package(s) into a shared library (dll) rather than an executable. Then invoke the application using the `gij` program, making sure that `gij` can find the libraries it needs.
- Link the Java packages(s) with the flag `-lgij`, which links in the `main` routine from the `gij` command. This allows you to select the class whose `main` method you want to run when you run the application. You can also use other `gij` flags, such as `-D` flags to set properties. Using the `-lgij` library (rather than the `gij` program of the previous mechanism) has some advantages: it is compatible with static linking, and does not require configuring or installing libraries.

These `gij` options relate to linking an executable:

`--main=CLASSNAME`

  This option is used when linking to specify the name of the class whose `main` method should be invoked when the resulting executable is run.

`-Dname[=value]`

  This option can only be used with `--main`. It defines a system property named name with value value. If value is not specified then it defaults to the empty string. These system properties are initialized at the program’s startup and can be retrieved at runtime using the `java.lang.System.getProperty` method.

`-lgij`

  Create an application whose command-line processing is that of the `gij` command.

  This option is an alternative to using `--main`; you cannot use both.

`-static-libgcj`

  This option causes linking to be done against a static version of the libgcj runtime library. This option is only available if corresponding linker support exists.

  Caution: Static linking of libgcj may cause essential parts of libgcj to be omitted. Some parts of libgcj use reflection to load classes at runtime. Since the linker does not see these references at link time, it can omit the referred to classes. The result is usually (but not always) a `ClassNotFoundException` being thrown at runtime. Caution must be used when using this option. For more details see: http://gcc.gnu.org/wiki/Statically%20linking%20libgcj

### 1.6 Code Generation

In addition to the many `gcc` options controlling code generation, `gcj` has several options specific to itself.

`-C`

  This option is used to tell `gcj` to generate bytecode (`.class` files) rather than object code.

`--resource resource-name`

  This option is used to tell `gcj` to compile the contents of a given file to object code so it may be accessed at runtime with the core protocol handler as ‘`core:/resource-name`’. Note that resource-name is the name of the resource as found at runtime; for instance, it could be used in a call to `ResourceBundle.getBundle`. The actual file name to be compiled this way must be specified separately.

`-ftarget=VERSION`

  This can be used with `-C` to choose the version of bytecode emitted by `gcj`. The default is ‘`1.5`’. When not generating bytecode, this option has no effect.

`-d directory`

  When used with `-C`, this causes all generated `.class` files to be put in the appropriate subdirectory of directory. By default they will be put in subdirectories of the current working directory.

`-fno-bounds-check`

  By default, `gcj` generates code which checks the bounds of all array indexing operations. With this option, these checks are omitted, which can improve performance for code that uses arrays extensively. Note that this can result in unpredictable behavior if the code in question actually does violate array bounds constraints. It is safe to use this option if you are sure that your code will never throw an `ArrayIndexOutOfBoundsException`.

`-fno-store-check`

  Don’t generate array store checks. When storing objects into arrays, a runtime check is normally generated in order to ensure that the object is assignment compatible with the component type of the array (which may not be known at compile-time). With this option, these checks are omitted. This can improve performance for code which stores objects into arrays frequently. It is safe to use this option if you are sure your code will never throw an `ArrayStoreException`.

`-fjni`

  With `gcj` there are two options for writing native methods: CNI and JNI. By default `gcj` assumes you are using CNI. If you are compiling a class with native methods, and these methods are implemented using JNI, then you must use `-fjni`. This option causes `gcj` to generate stubs which will invoke the underlying JNI methods.

`-fno-assert`

  Don’t recognize the `assert` keyword. This is for compatibility with older versions of the language specification.

`-fno-optimize-static-class-initialization`

  When the optimization level is greater or equal to `-O2`, `gcj` will try to optimize the way calls into the runtime are made to initialize static classes upon their first use (this optimization isn’t carried out if `-C` was specified.) When compiling to native code, `-fno-optimize-static-class-initialization` will turn this optimization off, regardless of the optimization level in use.

`--disable-assertions[=class-or-package]`

  Don’t include code for checking assertions in the compiled code. If `=class-or-package` is missing disables assertion code generation for all classes, unless overridden by a more specific `--enable-assertions` flag. If class-or-package is a class name, only disables generating assertion checks within the named class or its inner classes. If class-or-package is a package name, disables generating assertion checks within the named package or a subpackage.

  By default, assertions are enabled when generating class files or when not optimizing, and disabled when generating optimized binaries.

`--enable-assertions[=class-or-package]`

  Generates code to check assertions. The option is perhaps misnamed, as you still need to turn on assertion checking at run-time, and we don’t support any easy way to do that. So this flag isn’t very useful yet, except to partially override `--disable-assertions`.

`-findirect-dispatch`

  `gcj` has a special binary compatibility ABI, which is enabled by the `-findirect-dispatch` option. In this mode, the code generated by `gcj` honors the binary compatibility guarantees in the Java Language Specification, and the resulting object files do not need to be directly linked against their dependencies. Instead, all dependencies are looked up at runtime. This allows free mixing of interpreted and compiled code.

  Note that, at present, `-findirect-dispatch` can only be used when compiling `.class` files. It will not work when compiling from source. CNI also does not yet work with the binary compatibility ABI. These restrictions will be lifted in some future release.

  However, if you compile CNI code with the standard ABI, you can call it from code built with the binary compatibility ABI.

`-fbootstrap-classes`

  This option can be use to tell `libgcj` that the compiled classes should be loaded by the bootstrap loader, not the system class loader. By default, if you compile a class and link it into an executable, it will be treated as if it was loaded using the system class loader. This is convenient, as it means that things like `Class.forName()` will search ‘`CLASSPATH`’ to find the desired class.

`-freduced-reflection`

  This option causes the code generated by `gcj` to contain a reduced amount of the class meta-data used to support runtime reflection. The cost of this savings is the loss of the ability to use certain reflection capabilities of the standard Java runtime environment. When set all meta-data except for that which is needed to obtain correct runtime semantics is eliminated.

  For code that does not use reflection (i.e. serialization, RMI, CORBA or call methods in the `java.lang.reflect` package), `-freduced-reflection` will result in proper operation with a savings in executable code size.

  JNI (`-fjni`) and the binary compatibility ABI (`-findirect-dispatch`) do not work properly without full reflection meta-data. Because of this, it is an error to use these options with `-freduced-reflection`.

  Caution: If there is no reflection meta-data, code that uses a `SecurityManager` may not work properly. Also calling `Class.forName()` may fail if the calling method has no reflection meta-data.

### 1.7 Configure-time Options

Some `gcj` code generations options affect the resulting ABI, and so can only be meaningfully given when `libgcj`, the runtime package, is configured. `libgcj` puts the appropriate options from this group into a ‘`spec`’ file which is read by `gcj`. These options are listed here for completeness; if you are using `libgcj` then you won’t want to touch these options.

`-fuse-boehm-gc`

  This enables the use of the Boehm GC bitmap marking code. In particular this causes `gcj` to put an object marking descriptor into each vtable.

`-fhash-synchronization`

  By default, synchronization data (the data used for `synchronize`, `wait`, and `notify`) is pointed to by a word in each object. With this option `gcj` assumes that this information is stored in a hash table and not in the object itself.

`-fuse-divide-subroutine`

  On some systems, a library routine is called to perform integer division. This is required to get exception handling correct when dividing by zero.

`-fcheck-references`

  On some systems it’s necessary to insert inline checks whenever accessing an object via a reference. On other systems you won’t need this because null pointer accesses are caught automatically by the processor.

`-fuse-atomic-builtins`

  On some systems, GCC can generate code for built-in atomic operations. Use this option to force gcj to use these builtins when compiling Java code. Where this capability is present it should be automatically detected, so you won’t usually need to use this option.

## 2 Compatibility with the Java Platform

As we believe it is important that the Java platform not be fragmented, `gcj` and `libgcj` try to conform to the relevant Java specifications. However, limited manpower and incomplete and unclear documentation work against us. So, there are caveats to using `gcj`.

### 2.1 Standard features not yet supported

This list of compatibility issues is by no means complete.

- `gcj` implements the JDK 1.2 language. It supports inner classes and the new 1.4 `assert` keyword. It does not yet support the Java 2 `strictfp` keyword (it recognizes the keyword but ignores it).
- `libgcj` is largely compatible with the JDK 1.2 libraries. However, `libgcj` is missing many packages, most notably `java.awt`. There are also individual missing classes and methods. We currently do not have a list showing differences between `libgcj` and the Java 2 platform.
- Sometimes the `libgcj` implementation of a method or class differs from the JDK implementation. This is not always a bug. Still, if it affects you, it probably makes sense to report it so that we can discuss the appropriate response.
- `gcj` does not currently allow for piecemeal replacement of components within `libgcj`. Unfortunately, programmers often want to use newer versions of certain packages, such as those provided by the Apache Software Foundation’s Jakarta project. This has forced us to place the `org.w3c.dom` and `org.xml.sax` packages into their own libraries, separate from `libgcj`. If you intend to use these classes, you must link them explicitly with `-l-org-w3c-dom` and `-l-org-xml-sax`. Future versions of `gcj` may not have this restriction.

### 2.2 Extra features unique to gcj

The main feature of `gcj` is that it can compile programs written in the Java programming language to native code. Most extensions that have been added are to facilitate this functionality.

- `gcj` makes it easy and efficient to mix code written in Java and C++. See About CNI, for more info on how to use this in your programs.
- When you compile your classes into a shared library using `-findirect-dispatch` then add them to the system-wide classmap.db file using `gcj-dbtool`, they will be automatically loaded by the `libgcj` system classloader. This is the new, preferred classname-to-library resolution mechanism. See Invoking gcj-dbtool, for more information on using the classmap database.
- The old classname-to-library lookup mechanism is still supported through the `gnu.gcj.runtime.VMClassLoader.library_control` property, but it is deprecated and will likely be removed in some future release. When trying to load a class `gnu.pkg.SomeClass` the system classloader will first try to load the shared library `lib-gnu-pkg-SomeClass.so`, if that fails to load the class then it will try to load `lib-gnu-pkg.so` and finally when the class is still not loaded it will try to load `lib-gnu.so`. Note that all ‘`.`’s will be transformed into ‘`-`’s and that searching for inner classes starts with their outermost outer class. If the class cannot be found this way the system classloader tries to use the `libgcj` bytecode interpreter to load the class from the standard classpath. This process can be controlled to some degree via the `gnu.gcj.runtime.VMClassLoader.library_control` property; See libgcj Runtime Properties.
- `libgcj` includes a special ‘`gcjlib`’ URL type. A URL of this form is like a `jar` URL, and looks like ‘`gcjlib:/path/to/shared/library.so!/path/to/resource`’. An access to one of these URLs causes the shared library to be `dlopen()`d, and then the resource is looked for in that library. These URLs are most useful when used in conjunction with `java.net.URLClassLoader`. Note that, due to implementation limitations, currently any such URL can be accessed by only one class loader, and libraries are never unloaded. This means some care must be exercised to make sure that a `gcjlib` URL is not accessed by more than one class loader at once. In a future release this limitation will be lifted, and such libraries will be mapped privately.
- A program compiled by `gcj` will examine the `GCJ_PROPERTIES` environment variable and change its behavior in some ways. In particular `GCJ_PROPERTIES` holds a list of assignments to global properties, such as would be set with the `-D` option to `java`. For instance, ‘`java.compiler=gcj`’ is a valid (but currently meaningless) setting.

## 3 Invoking jcf-dump

This is a class file examiner, similar to `javap`. It will print information about a number of classes, which are specified by class name or file name.

`-c`

  Disassemble method bodies. By default method bodies are not printed.

`--print-constants`

  Print the constant pool. When printing a reference to a constant also print its index in the constant pool.

`--javap`

  Generate output in `javap` format. The implementation of this feature is very incomplete.

`--classpath=path`
`--CLASSPATH=path`
`-Idirectory`
`-o file`

  These options as the same as the corresponding `gcj` options.

`--help`

  Print help, then exit.

`--version`

  Print version number, then exit.

`-v, --verbose`

  Print extra information while running. Implies `--print-constants`.

## 4 Invoking gij

`gij` is a Java bytecode interpreter included with `libgcj`. `gij` is not available on every platform; porting it requires a small amount of assembly programming which has not been done for all the targets supported by `gcj`.

The primary argument to `gij` is the name of a class or, with `-jar`, a jar file. Options before this argument are interpreted by `gij`; remaining options are passed to the interpreted program.

If a class name is specified and this class does not have a `main` method with the appropriate signature (a `static void` method with a `String[]` as its sole argument), then `gij` will print an error and exit.

If a jar file is specified then `gij` will use information in it to determine which class’ `main` method will be invoked.

`gij` will invoke the `main` method with all the remaining command-line options.

Note that `gij` is not limited to interpreting code. Because `libgcj` includes a class loader which can dynamically load shared objects, it is possible to give `gij` the name of a class which has been compiled and put into a shared library on the class path.

`-cp path`
`-classpath path`

  Set the initial class path. The class path is used for finding class and resource files. If specified, this option overrides the `CLASSPATH` environment variable. Note that this option is ignored if `-jar` is used.

`-Dname[=value]`

  This defines a system property named name with value value. If value is not specified then it defaults to the empty string. These system properties are initialized at the program’s startup and can be retrieved at runtime using the `java.lang.System.getProperty` method.

`-ms=number`

  Equivalent to `-Xms`.

`-mx=number`

  Equivalent to `-Xmx`.

`-noverify`

  Do not verify compliance of bytecode with the VM specification. In addition, this option disables type verification which is otherwise performed on BC-ABI compiled code.

`-X`
`-Xargument`

  Supplying `-X` by itself will cause `gij` to list all the supported `-X` options. Currently these options are supported:

  `-Xmssize`

    Set the initial heap size.

  `-Xmxsize`

    Set the maximum heap size.

  `-Xsssize`

    Set the thread stack size.

  Unrecognized `-X` options are ignored, for compatibility with other runtimes.

`-jar`

  This indicates that the name passed to `gij` should be interpreted as the name of a jar file, not a class.

`--help`
`-?`

  Print help, then exit.

`--showversion`

  Print version number and continue.

`--fullversion`

  Print detailed version information, then exit.

`--version`

  Print version number, then exit.

`-verbose`
`-verbose:class`

  Each time a class is initialized, print a short message on standard error.

`gij` also recognizes and ignores the following options, for compatibility with existing application launch scripts: `-client`, `-server`, `-hotspot`, `-jrockit`, `-agentlib`, `-agentpath`, `-debug`, `-d32`, `-d64`, `-javaagent`, `-noclassgc`, `-verify`, and `-verifyremote`.

## 5 Invoking gcj-dbtool.

`gcj-dbtool` is a tool for creating and manipulating class file mapping databases. `libgcj` can use these databases to find a shared library corresponding to the bytecode representation of a class. This functionality is useful for ahead-of-time compilation of a program that has no knowledge of `gcj`.

`gcj-dbtool` works best if all the jar files added to it are compiled using `-findirect-dispatch`.

Note that `gcj-dbtool` is currently available as “preview technology”. We believe it is a reasonable way to allow application-transparent ahead-of-time compilation, but this is an unexplored area. We welcome your comments.

`-n DBFILE [SIZE]`

  This creates a new database. Currently, databases cannot be resized; you can choose a larger initial size if desired. The default size is 32,749.

`-a DBFILE JARFILE LIB`
`-f DBFILE JARFILE LIB`

  This adds a jar file to the database. For each class file in the jar, a cryptographic signature of the bytecode representation of the class is recorded in the database. At runtime, a class is looked up by its signature and the compiled form of the class is looked for in the corresponding shared library. The `-a` option will verify that LIB exists before adding it to the database; `-f` skips this check.

`[`-`][`-0`] -m DBFILE DBFILE,[DBFILE]`

  Merge a number of databases. The output database overwrites any existing database. To add databases into an existing database, include the destination in the list of sources.

  If `-` or `-0` are used, the list of files to read is taken from standard input instead of the command line. For `-0`, Input filenames are terminated by a null character instead of by whitespace. Useful when arguments might contain white space. The GNU find -print0 option produces input suitable for this mode.

`-t DBFILE`

  Test a database.

`-l DBFILE`

  List the contents of a database.

`-p`

  Print the name of the default database. If there is no default database, this prints a blank line. If LIBDIR is specified, use it instead of the default library directory component of the database name.

`--help`

  Print a help message, then exit.

`--version`
`-v`

  Print version information, then exit.

## 6 Invoking jv-convert

`jv-convert` [`OPTION`] … [INPUTFILE [OUTPUTFILE]]

`jv-convert` is a utility included with `libgcj` which converts a file from one encoding to another. It is similar to the Unix `iconv` utility.

The encodings supported by `jv-convert` are platform-dependent. Currently there is no way to get a list of all supported encodings.

`--encoding name`
`--from name`

  Use name as the input encoding. The default is the current locale’s encoding.

`--to name`

  Use name as the output encoding. The default is the `JavaSrc` encoding; this is ASCII with ‘`\u`’ escapes for non-ASCII characters.

`-i file`

  Read from file. The default is to read from standard input.

`-o file`

  Write to file. The default is to write to standard output.

`--reverse`

  Swap the input and output encodings.

`--help`

  Print a help message, then exit.

`--version`

  Print version information, then exit.

## 7 Invoking grmic

`grmic` [`OPTION`] … class …

`grmic` is a utility included with `libgcj` which generates stubs for remote objects.

Note that this program isn’t yet fully compatible with the JDK `grmic`. Some options, such as `-classpath`, are recognized but currently ignored. We have left these options undocumented for now.

Long options can also be given with a GNU-style leading ‘`--`’. For instance, `--help` is accepted.

`-keep`
`-keepgenerated`

  By default, `grmic` deletes intermediate files. Either of these options causes it not to delete such files.

`-v1.1`

  Cause `grmic` to create stubs and skeletons for the 1.1 protocol version.

`-vcompat`

  Cause `grmic` to create stubs and skeletons compatible with both the 1.1 and 1.2 protocol versions. This is the default.

`-v1.2`

  Cause `grmic` to create stubs and skeletons for the 1.2 protocol version.

`-nocompile`

  Don’t compile the generated files.

`-verbose`

  Print information about what `grmic` is doing.

`-d directory`

  Put output files in directory. By default the files are put in the current working directory.

`-help`

  Print a help message, then exit.

`-version`

  Print version information, then exit.

## 8 Invoking gc-analyze

`gc-analyze` [`OPTION`] … [file]

`gc-analyze` prints an analysis of a GC memory dump to standard out.

The memory dumps may be created by calling `gnu.gcj.util.GCInfo.enumerate(String namePrefix)` from java code. A memory dump will be created on an out of memory condition if `gnu.gcj.util.GCInfo.setOOMDump(String namePrefix)` is called before the out of memory occurs.

Running this program will create two files: `TestDump001` and `TestDump001.bytes`.

    import gnu.gcj.util.*;
    import java.util.*;

    public class GCDumpTest
    {
        static public void main(String args[])
        {
            ArrayList<String> l = new ArrayList<String>(1000);

            for (int i = 1; i < 1500; i++) {
                l.add("This is string #" + i);
            }
            GCInfo.enumerate("TestDump");
        }
    }

The memory dump may then be displayed by running:

    gc-analyze -v TestDump001

`--verbose`
`-v`

  Verbose output.

`-p tool-prefix`

  Prefix added to the names of the `nm` and `readelf` commands.

`-d directory`

  Directory that contains the executable and shared libraries used when the dump was generated.

`--help`

  Print a help message, then exit.

`--version`

  Print version information, then exit.

## 9 Invoking aot-compile

`aot-compile` is a script that searches a directory for Java bytecode (as class files, or in jars) and uses `gcj` to compile it to native code and generate the databases from it.

`-M, --make=PATH`

  Specify the path to the `make` executable to use.

`-C, --gcj=PATH`

  Specify the path to the `gcj` executable to use.

`-D, --dbtool=PATH`

  Specify the path to the `gcj-dbtool` executable to use.

`-m, --makeflags=FLAGS`

  Specify flags to pass to `make` during the build.

`-c, --gcjflags=FLAGS`

  Specify flags to pass to `gcj` during compilation, in addition to ’-fPIC -findirect-dispatch -fjni’.

`-l, --ldflags=FLAGS`

  Specify flags to pass to `gcj` during linking, in addition to ’-Wl,-Bsymbolic’.

`-e, --exclude=PATH`

  Do not compile PATH.

## 10 Invoking rebuild-gcj-db

`rebuild-gcj-db` is a script that merges the per-solib databases made by `aot-compile` into one system-wide database so `gij` can find the solibs.

## 11 About CNI

This documents CNI, the Compiled Native Interface, which is is a convenient way to write Java native methods using C++. This is a more efficient, more convenient, but less portable alternative to the standard JNI (Java Native Interface).

### 11.1 Basic concepts

In terms of languages features, Java is mostly a subset of C++. Java has a few important extensions, plus a powerful standard class library, but on the whole that does not change the basic similarity. Java is a hybrid object-oriented language, with a few native types, in addition to class types. It is class-based, where a class may have static as well as per-object fields, and static as well as instance methods. Non-static methods may be virtual, and may be overloaded. Overloading is resolved at compile time by matching the actual argument types against the parameter types. Virtual methods are implemented using indirect calls through a dispatch table (virtual function table). Objects are allocated on the heap, and initialized using a constructor method. Classes are organized in a package hierarchy.

All of the listed attributes are also true of C++, though C++ has extra features (for example in C++ objects may be allocated not just on the heap, but also statically or in a local stack frame). Because `gcj` uses the same compiler technology as G++ (the GNU C++ compiler), it is possible to make the intersection of the two languages use the same ABI (object representation and calling conventions). The key idea in CNI is that Java objects are C++ objects, and all Java classes are C++ classes (but not the other way around). So the most important task in integrating Java and C++ is to remove gratuitous incompatibilities.

You write CNI code as a regular C++ source file. (You do have to use a Java/CNI-aware C++ compiler, specifically a recent version of G++.)

A CNI C++ source file must have:

    #include <gcj/cni.h>

and then must include one header file for each Java class it uses, e.g.:

    #include <java/lang/Character.h>
    #include <java/util/Date.h>
    #include <java/lang/IndexOutOfBoundsException.h>

These header files are automatically generated by `gcjh`.

CNI provides some functions and macros to make using Java objects and primitive types from C++ easier. In general, these CNI functions and macros start with the `Jv` prefix, for example the function `JvNewObjectArray`. This convention is used to avoid conflicts with other libraries. Internal functions in CNI start with the prefix `_Jv_`. You should not call these; if you find a need to, let us know and we will try to come up with an alternate solution.

#### 11.1.1 Limitations

Whilst a Java class is just a C++ class that doesn’t mean that you are freed from the shackles of Java, a CNI C++ class must adhere to the rules of the Java programming language.

For example: it is not possible to declare a method in a CNI class that will take a C string (`char*`) as an argument, or to declare a member variable of some non-Java datatype.

### 11.2 Packages

The only global names in Java are class names, and packages. A package can contain zero or more classes, and also zero or more sub-packages. Every class belongs to either an unnamed package or a package that has a hierarchical and globally unique name.

A Java package is mapped to a C++ namespace. The Java class `java.lang.String` is in the package `java.lang`, which is a sub-package of `java`. The C++ equivalent is the class `java::lang::String`, which is in the namespace `java::lang` which is in the namespace `java`.

Here is how you could express this:

    (// Declare the class(es), possibly in a header file:
    namespace java {
      namespace lang {
        class Object;
        class String;
        ...
      }
    }

    class java::lang::String : public java::lang::Object
    {
      ...
    };

The `gcjh` tool automatically generates the necessary namespace declarations.

#### 11.2.1 Leaving out package names

Always using the fully-qualified name of a java class can be tiresomely verbose. Using the full qualified name also ties the code to a single package making code changes necessary should the class move from one package to another. The Java `package` declaration specifies that the following class declarations are in the named package, without having to explicitly name the full package qualifiers. The `package` declaration can be followed by zero or more `import` declarations, which allows either a single class or all the classes in a package to be named by a simple identifier. C++ provides something similar with the `using` declaration and directive.

In Java:

    import package-name.class-name;

allows the program text to refer to class-name as a shorthand for the fully qualified name: `package-name.class-name`.

To achieve the same effect C++, you have to do this:

    using package-name::class-name;

Java can also cause imports on demand, like this:

    import package-name.*;

Doing this allows any class from the package package-name to be referred to only by its class-name within the program text.

The same effect can be achieved in C++ like this:

    using namespace package-name;

### 11.3 Primitive types

Java provides 8 primitives types which represent integers, floats, characters and booleans (and also the void type). C++ has its own very similar concrete types. Such types in C++ however are not always implemented in the same way (an int might be 16, 32 or 64 bits for example) so CNI provides a special C++ type for each primitive Java type:

Java type | C/C++ typename | Description
`char` | `jchar` | 16 bit Unicode character
`boolean` | `jboolean` | logical (true or false) values
`byte` | `jbyte` | 8-bit signed integer
`short` | `jshort` | 16 bit signed integer
`int` | `jint` | 32 bit signed integer
`long` | `jlong` | 64 bit signed integer
`float` | `jfloat` | 32 bit IEEE floating point number
`double` | `jdouble` | 64 bit IEEE floating point number
`void` | `void` | no value

When referring to a Java type You should always use these C++ typenames (e.g.: `jint`) to avoid disappointment.

#### 11.3.1 Reference types associated with primitive types

In Java each primitive type has an associated reference type, e.g.: `boolean` has an associated `java.lang.Boolean.TYPE` class. In order to make working with such classes easier GCJ provides the macro `JvPrimClass`:

macro: JvPrimClass type

  Return a pointer to the `Class` object corresponding to the type supplied.

      JvPrimClass(void) ⇒ java.lang.Void.TYPE

### 11.4 Reference types

A Java reference type is treated as a class in C++. Classes and interfaces are handled this way. A Java reference is translated to a C++ pointer, so for instance a Java `java.lang.String` becomes, in C++, `java::lang::String *`.

CNI provides a few built-in typedefs for the most common classes:

Java type | C++ typename | Description
`java.lang.Object` | `jobject` | Object type
`java.lang.String` | `jstring` | String type
`java.lang.Class` | `jclass` | Class type

Every Java class or interface has a corresponding `Class` instance. These can be accessed in CNI via the static `class$` field of a class. The `class$` field is of type `Class` (and not `Class *`), so you will typically take the address of it.

Here is how you can refer to the class of `String`, which in Java would be written `String.class`:

    using namespace java::lang;
    doSomething (&String::class$);

### 11.5 Interfaces

A Java class can implement zero or more interfaces, in addition to inheriting from a single base class.

CNI allows CNI code to implement methods of interfaces. You can also call methods through interface references, with some limitations.

CNI doesn’t understand interface inheritance at all yet. So, you can only call an interface method when the declared type of the field being called matches the interface which declares that method. The workaround is to cast the interface reference to the right superinterface.

For example if you have:

    interface A
    {
      void a();
    }

    interface B extends A
    {
      void b();
    }

and declare a variable of type `B` in C++, you can’t call `a()` unless you cast it to an `A` first.

### 11.6 Objects and Classes

#### 11.6.1 Classes

All Java classes are derived from `java.lang.Object`. C++ does not have a unique root class, but we use the C++ class `java::lang::Object` as the C++ version of the `java.lang.Object` Java class. All other Java classes are mapped into corresponding C++ classes derived from `java::lang::Object`.

Interface inheritance (the `implements` keyword) is currently not reflected in the C++ mapping.

#### 11.6.2 Object fields

Each object contains an object header, followed by the instance fields of the class, in order. The object header consists of a single pointer to a dispatch or virtual function table. (There may be extra fields in front of the object, for example for memory management, but this is invisible to the application, and the reference to the object points to the dispatch table pointer.)

The fields are laid out in the same order, alignment, and size as in C++. Specifically, 8-bit and 16-bit native types (`byte`, `short`, `char`, and `boolean`) are not widened to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types to 32 bits when on the VM stack or temporary registers.

If you include the `gcjh`-generated header for a class, you can access fields of Java classes in the natural way. For example, given the following Java class:

    public class Int
    {
      public int i;
      public Int (int i) { this.i = i; }
      public static Int zero = new Int(0);
    }

you can write:

    #include <gcj/cni.h>;
    #include <Int>;

    Int*
    mult (Int *p, jint k)
    {
      if (k == 0)
        return Int::zero;  // Static member access.
      return new Int(p->i * k);
    }

#### 11.6.3 Access specifiers

CNI does not strictly enforce the Java access specifiers, because Java permissions cannot be directly mapped into C++ permission. Private Java fields and methods are mapped to private C++ fields and methods, but other fields and methods are mapped to public fields and methods.

### 11.7 Class Initialization

Java requires that each class be automatically initialized at the time of the first active use. Initializing a class involves initializing the static fields, running code in class initializer methods, and initializing base classes. There may also be some implementation specific actions, such as allocating `String` objects corresponding to string literals in the code.

The GCJ compiler inserts calls to `JvInitClass` at appropriate places to ensure that a class is initialized when required. The C++ compiler does not insert these calls automatically—it is the programmer’s responsibility to make sure classes are initialized. However, this is fairly painless because of the conventions assumed by the Java system.

First, `libgcj` will make sure a class is initialized before an instance of that object is created. This is one of the responsibilities of the `new` operation. This is taken care of both in Java code, and in C++ code. When G++ sees a `new` of a Java class, it will call a routine in `libgcj` to allocate the object, and that routine will take care of initializing the class. Note however that this does not happen for Java arrays; you must allocate those using the appropriate CNI function. It follows that you can access an instance field, or call an instance (non-static) method and be safe in the knowledge that the class and all of its base classes have been initialized.

Invoking a static method is also safe. This is because the Java compiler adds code to the start of a static method to make sure the class is initialized. However, the C++ compiler does not add this extra code. Hence, if you write a native static method using CNI, you are responsible for calling `JvInitClass` before doing anything else in the method (unless you are sure it is safe to leave it out).

Accessing a static field also requires the class of the field to be initialized. The Java compiler will generate code to call `JvInitClass` before getting or setting the field. However, the C++ compiler will not generate this extra code, so it is your responsibility to make sure the class is initialized before you access a static field from C++.

### 11.8 Object allocation

New Java objects are allocated using a class instance creation expression, e.g.:

    new Type ( ... )

The same syntax is used in C++. The main difference is that C++ objects have to be explicitly deleted; in Java they are automatically deleted by the garbage collector. Using CNI, you can allocate a new Java object using standard C++ syntax and the C++ compiler will allocate memory from the garbage collector. If you have overloaded constructors, the compiler will choose the correct one using standard C++ overload resolution rules.

For example:

    java::util::Hashtable *ht = new java::util::Hashtable(120);

### 11.9 Memory allocation

When allocating memory in CNI methods it is best to handle out-of-memory conditions by throwing a Java exception. These functions are provided for that purpose:

Function: void* JvMalloc (jsize size)

  Calls malloc. Throws `java.lang.OutOfMemoryError` if allocation fails.

Function: void* JvRealloc (void* ptr, jsize size)

  Calls realloc. Throws `java.lang.OutOfMemoryError` if reallocation fails.

Function: void JvFree (void* ptr)

  Calls free.

### 11.10 Arrays

While in many ways Java is similar to C and C++, it is quite different in its treatment of arrays. C arrays are based on the idea of pointer arithmetic, which would be incompatible with Java’s security requirements. Java arrays are true objects (array types inherit from `java.lang.Object`). An array-valued variable is one that contains a reference (pointer) to an array object.

Referencing a Java array in C++ code is done using the `JArray` template, which as defined as follows:

    class __JArray : public java::lang::Object
    {
    public:
      int length;
    };

    template<class T>
    class JArray : public __JArray
    {
      T data[0];
    public:
      T& operator[](jint i) { return data[i]; }
    };

There are a number of `typedef`s which correspond to `typedef`s from the JNI. Each is the type of an array holding objects of the relevant type:

    typedef __JArray *jarray;
    typedef JArray<jobject> *jobjectArray;
    typedef JArray<jboolean> *jbooleanArray;
    typedef JArray<jbyte> *jbyteArray;
    typedef JArray<jchar> *jcharArray;
    typedef JArray<jshort> *jshortArray;
    typedef JArray<jint> *jintArray;
    typedef JArray<jlong> *jlongArray;
    typedef JArray<jfloat> *jfloatArray;
    typedef JArray<jdouble> *jdoubleArray;

Method on template<class T>: T* elements (JArray<T> array)

  This template function can be used to get a pointer to the elements of the `array`. For instance, you can fetch a pointer to the integers that make up an `int[]` like so:

      extern jintArray foo;
      jint *intp = elements (foo);

  The name of this function may change in the future.

Function: jobjectArray JvNewObjectArray (jsize length, jclass klass, jobject init)

  This creates a new array whose elements have reference type. `klass` is the type of elements of the array and `init` is the initial value put into every slot in the array.

    using namespace java::lang;
    JArray<String *> *array
      = (JArray<String *> *) JvNewObjectArray(length, &String::class$, NULL);

#### 11.10.1 Creating arrays

For each primitive type there is a function which can be used to create a new array of that type. The name of the function is of the form:

    JvNewTypeArray

For example:

    JvNewBooleanArray

can be used to create an array of Java primitive boolean types.

The following function definition is the template for all such functions:

Function: jbooleanArray JvNewBooleanArray (jint length)

  Creates an array length indices long.

Function: jsize JvGetArrayLength (jarray array)

  Returns the length of the array.

### 11.11 Methods

Java methods are mapped directly into C++ methods. The header files generated by `gcjh` include the appropriate method definitions. Basically, the generated methods have the same names and corresponding types as the Java methods, and are called in the natural manner.

#### 11.11.1 Overloading

Both Java and C++ provide method overloading, where multiple methods in a class have the same name, and the correct one is chosen (at compile time) depending on the argument types. The rules for choosing the correct method are (as expected) more complicated in C++ than in Java, but given a set of overloaded methods generated by `gcjh` the C++ compiler will choose the expected one.

Common assemblers and linkers are not aware of C++ overloading, so the standard implementation strategy is to encode the parameter types of a method into its assembly-level name. This encoding is called mangling, and the encoded name is the mangled name. The same mechanism is used to implement Java overloading. For C++/Java interoperability, it is important that both the Java and C++ compilers use the same encoding scheme.

#### 11.11.2 Static methods

Static Java methods are invoked in CNI using the standard C++ syntax, using the `::` operator rather than the `.` operator.

For example:

    jint i = java::lang::Math::round((jfloat) 2.3);

C++ method definition syntax is used to define a static native method. For example:

    #include <java/lang/Integer>
    java::lang::Integer*
    java::lang::Integer::getInteger(jstring str)
    {
      ...
    }

#### 11.11.3 Object Constructors

Constructors are called implicitly as part of object allocation using the `new` operator.

For example:

    java::lang::Integer *x = new java::lang::Integer(234);

Java does not allow a constructor to be a native method. This limitation can be coded round however because a constructor can call a native method.

#### 11.11.4 Instance methods

Calling a Java instance method from a C++ CNI method is done using the standard C++ syntax, e.g.:

    // First create the Java object.
    java::lang::Integer *x = new java::lang::Integer(234);
    // Now call a method.
    jint prim_value = x->intValue();
    if (x->longValue == 0)
      ...

Defining a Java native instance method is also done the natural way:

    #include <java/lang/Integer.h>

    jdouble
    java::lang:Integer::doubleValue()
    {
      return (jdouble) value;
    }

#### 11.11.5 Interface methods

In Java you can call a method using an interface reference. This is supported, but not completely. See Interfaces.

### 11.12 Strings

CNI provides a number of utility functions for working with Java Java `String` objects. The names and interfaces are analogous to those of JNI.

Function: jstring JvNewString (const jchar* chars, jsize len)

  Returns a Java `String` object with characters from the array of Unicode characters chars up to the index len in that array.

Function: jstring JvNewStringLatin1 (const char* bytes, jsize len)

  Returns a Java `String` made up of len bytes from bytes.

Function: jstring JvNewStringLatin1 (const char* bytes)

  As above but the length of the `String` is `strlen(bytes)`.

Function: jstring JvNewStringUTF (const char* bytes)

  Returns a `String` which is made up of the UTF encoded characters present in the C string bytes.

Function: jchar* JvGetStringChars (jstring str)

  Returns a pointer to an array of characters making up the `String` str.

Function: int JvGetStringUTFLength (jstring str)

  Returns the number of bytes required to encode the contents of the `String` str in UTF-8.

Function: jsize JvGetStringUTFRegion (jstring str, jsize start, jsize len, char* buf)

  Puts the UTF-8 encoding of a region of the `String` str into the buffer `buf`. The region to fetch is marked by start and len.

  Note that buf is a buffer, not a C string. It is not null terminated.

### 11.13 Interoperating with C/C++

Because CNI is designed to represent Java classes and methods it cannot be mixed readily with C/C++ types.

One important restriction is that Java classes cannot have non-Java type instance or static variables and cannot have methods which take non-Java types as arguments or return non-Java types.

None of the following is possible with CNI:

    class ::MyClass : public java::lang::Object
    {
       char* variable;  // char* is not a valid Java type.
    }

    uint
    ::SomeClass::someMethod (char *arg)
    {
      .
      .
      .
    }   // uint is not a valid Java type, neither is char*

Of course, it is ok to use C/C++ types within the scope of a method:

    jint
    ::SomeClass::otherMethod (jstring str)
    {
       char *arg = ...
       .
       .
       .
    }

#### 11.13.1 RawData

The above restriction can be problematic, so CNI includes the `gnu.gcj.RawData` class. The `RawData` class is a non-scanned reference type. In other words variables declared of type `RawData` can contain any data and are not checked by the compiler or memory manager in any way.

This means that you can put C/C++ data structures (including classes) in your CNI classes, as long as you use the appropriate cast.

Here are some examples:

    class ::MyClass : public java::lang::Object
    {
       gnu.gcj.RawData string;

       MyClass ();
       gnu.gcj.RawData getText ();
       void printText ();
    }

    ::MyClass::MyClass ()
    {
       char* text = ...
       string = text;
    }

    gnu.gcj.RawData
    ::MyClass::getText ()
    {
       return string;
    }

    void
    ::MyClass::printText ()
    {
      printf("%s\n", (char*) string);
    }

#### 11.13.2 RawDataManaged

`gnu.gcj.RawDataManaged` is another type used to indicate special data used by native code. Unlike the `RawData` type, fields declared as `RawDataManaged` will be "marked" by the memory manager and considered for garbage collection.

Native data which is allocated using CNI’s `JvAllocBytes()` function and stored in a `RawDataManaged` will be automatically freed when the Java object it is associated with becomes unreachable.

#### 11.13.3 Native memory allocation

Function: void* JvAllocBytes (jsize size)

  Allocates size bytes from the heap. The memory returned is zeroed. This memory is not scanned for pointers by the garbage collector, but will be freed if no references to it are discovered.

  This function can be useful if you need to associate some native data with a Java object. Using a CNI’s special `RawDataManaged` type, native data allocated with `JvAllocBytes` will be automatically freed when the Java object itself becomes unreachable.

#### 11.13.4 Posix signals

On Posix based systems the `libgcj` library uses several signals internally. CNI code should not attempt to use the same signals as doing so may cause `libgcj` and/or the CNI code to fail.

SIGSEGV is used on many systems to generate `NullPointerExceptions`. SIGCHLD is used internally by `Runtime.exec()`. Several other signals (that vary from platform to platform) can be used by the memory manager and by `Thread.interrupt()`.

### 11.14 Exception Handling

While C++ and Java share a common exception handling framework, things are not yet perfectly integrated. The main issue is that the run-time type information facilities of the two languages are not integrated.

Still, things work fairly well. You can throw a Java exception from C++ using the ordinary `throw` construct, and this exception can be caught by Java code. Similarly, you can catch an exception thrown from Java using the C++ `catch` construct.

Here is an example:

    if (i >= count)
       throw new java::lang::IndexOutOfBoundsException();

Normally, G++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code:

    struct S { ~S(); };

    extern void bar();    // Is implemented in Java and may throw exceptions.

    void foo()
    {
      S s;
      bar();
    }

The usual effect of an incorrect guess is a link failure, complaining of a missing routine called `__gxx_personality_v0`.

You can inform the compiler that Java exceptions are to be used in a translation unit, irrespective of what it might think, by writing `#pragma GCC java_exceptions` at the head of the file. This `#pragma` must appear before any functions that throw or catch exceptions, or run destructors when exceptions are thrown through them.

### 11.15 Synchronization

Each Java object has an implicit monitor. The Java VM uses the instruction `monitorenter` to acquire and lock a monitor, and `monitorexit` to release it.

The corresponding CNI macros are `JvMonitorEnter` and `JvMonitorExit` (JNI has similar methods `MonitorEnter` and `MonitorExit`).

The Java source language does not provide direct access to these primitives. Instead, there is a `synchronized` statement that does an implicit `monitorenter` before entry to the block, and does a `monitorexit` on exit from the block. Note that the lock has to be released even when the block is abnormally terminated by an exception, which means there is an implicit `try finally` surrounding synchronization locks.

From C++, it makes sense to use a destructor to release a lock. CNI defines the following utility class:

    class JvSynchronize() {
      jobject obj;
      JvSynchronize(jobject o) { obj = o; JvMonitorEnter(o); }
      ~JvSynchronize() { JvMonitorExit(obj); }
    };

So this Java code:

    synchronized (OBJ)
    {
       CODE
    }

might become this C++ code:

    {
       JvSynchronize dummy (OBJ);
       CODE;
    }

Java also has methods with the `synchronized` attribute. This is equivalent to wrapping the entire method body in a `synchronized` statement. (Alternatively, an implementation could require the caller to do the synchronization. This is not practical for a compiler, because each virtual method call would have to test at run-time if synchronization is needed.) Since in `gcj` the `synchronized` attribute is handled by the method implementation, it is up to the programmer of a synchronized native method to handle the synchronization (in the C++ implementation of the method). In other words, you need to manually add `JvSynchronize` in a `native synchronized` method.

### 11.16 Invocation

CNI permits C++ applications to make calls into Java classes, in addition to allowing Java code to call into C++. Several functions, known as the invocation API, are provided to support this.

Function: jint JvCreateJavaVM (JvVMInitArgs* vm_args)

  Initializes the Java runtime. This function performs essential initialization of the threads interface, garbage collector, exception handling and other key aspects of the runtime. It must be called once by an application with a non-Java `main()` function, before any other Java or CNI calls are made. It is safe, but not recommended, to call `JvCreateJavaVM()` more than once provided it is only called from a single thread. The vmargs parameter can be used to specify initialization parameters for the Java runtime. It may be `NULL`.

  JvVMInitArgs represents a list of virtual machine initialization arguments. `JvCreateJavaVM()` ignores the version field.

      typedef struct JvVMOption
      {
        // a VM initialization option
        char* optionString;
        // extra information associated with this option
        void* extraInfo;
      } JvVMOption;

      typedef struct JvVMInitArgs
      {
        // for compatibility with JavaVMInitArgs
        jint version;

        // number of VM initialization options
        jint nOptions;

        // an array of VM initialization options
        JvVMOption* options;

        // true if the option parser should ignore unrecognized options
        jboolean ignoreUnrecognized;
      } JvVMInitArgs;

  `JvCreateJavaVM()` returns `0` upon success, or `-1` if the runtime is already initialized.

  Note: In GCJ 3.1, the `vm_args` parameter is ignored. It is recognized and used as of release 4.0.

Function: java::lang::Thread* JvAttachCurrentThread (jstring name, java::lang::ThreadGroup* group)

  Registers an existing thread with the Java runtime. This must be called once from each thread, before that thread makes any other Java or CNI calls. It must be called after `JvCreateJavaVM`. name specifies a name for the thread. It may be `NULL`, in which case a name will be generated. group is the ThreadGroup in which this thread will be a member. If it is `NULL`, the thread will be a member of the main thread group. The return value is the Java `Thread` object that represents the thread. It is safe to call `JvAttachCurrentThread()` more than once from the same thread. If the thread is already attached, the call is ignored and the current thread object is returned.

Function: jint JvDetachCurrentThread ()

  Unregisters a thread from the Java runtime. This should be called by threads that were attached using `JvAttachCurrentThread()`, after they have finished making calls to Java code. This ensures that any resources associated with the thread become eligible for garbage collection. This function returns `0` upon success, or `-1` if the current thread is not attached.

#### 11.16.1 Handling uncaught exceptions

If an exception is thrown from Java code called using the invocation API, and no handler for the exception can be found, the runtime will abort the application. In order to make the application more robust, it is recommended that code which uses the invocation API be wrapped by a top-level try/catch block that catches all Java exceptions.

#### 11.16.2 Example

The following code demonstrates the use of the invocation API. In this example, the C++ application initializes the Java runtime and attaches itself. The `java.lang.System` class is initialized in order to access its `out` field, and a Java string is printed. Finally, the thread is detached from the runtime once it has finished making Java calls. Everything is wrapped with a try/catch block to provide a default handler for any uncaught exceptions.

The example can be compiled with `c++ -c test.cc; gcj test.o`.

    // test.cc
    #include <gcj/cni.h>
    #include <java/lang/System.h>
    #include <java/io/PrintStream.h>
    #include <java/lang/Throwable.h>

    int main(int argc, char *argv[])
    {
      using namespace java::lang;

      try
      {
        JvCreateJavaVM(NULL);
        JvAttachCurrentThread(NULL, NULL);

        String *message = JvNewStringLatin1("Hello from C++");
        JvInitClass(&System::class$);
        System::out->println(message);

        JvDetachCurrentThread();
      }
      catch (Throwable *t)
      {
        System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
        t->printStackTrace();
      }
    }

### 11.17 Reflection

Reflection is possible with CNI code, it functions similarly to how it functions with JNI.

The types `jfieldID` and `jmethodID` are as in JNI.

The functions:

- `JvFromReflectedField`,
- `JvFromReflectedMethod`,
- `JvToReflectedField`
- `JvToFromReflectedMethod`

will be added shortly, as will other functions corresponding to JNI.

## 12 System properties

The runtime behavior of the `libgcj` library can be modified by setting certain system properties. These properties can be compiled into the program using the `-Dname[=value]` option to `gcj` or by setting them explicitly in the program by calling the `java.lang.System.setProperty()` method. Some system properties are only used for informational purposes (like giving a version number or a user name). A program can inspect the current value of a property by calling the `java.lang.System.getProperty()` method.

### 12.1 Standard Properties

The following properties are normally found in all implementations of the core libraries for the Java language.

`java.version`

  The `libgcj` version number.

`java.vendor`

  Set to ‘`The Free Software Foundation, Inc.`’

`java.vendor.url`

  Set to http://gcc.gnu.org/java/.

`java.home`

  The directory where `gcj` was installed. Taken from the `--prefix` option given to `configure`.

`java.class.version`

  The class format version number supported by the libgcj byte code interpreter. (Currently ‘`46.0`’)

`java.vm.specification.version`

  The Virtual Machine Specification version implemented by `libgcj`. (Currently ‘`1.0`’)

`java.vm.specification.vendor`

  The name of the Virtual Machine specification designer.

`java.vm.specification.name`

  The name of the Virtual Machine specification (Set to ‘`Java Virtual Machine Specification`’).

`java.vm.version`

  The `gcj` version number.

`java.vm.vendor`

  Set to ‘`The Free Software Foundation, Inc.`’

`java.vm.name`

  Set to ‘`GNU libgcj`’.

`java.specification.version`

  The Runtime Environment specification version implemented by `libgcj`. (Currently set to ‘`1.3`’)

`java.specification.vendor`

  The Runtime Environment specification designer.

`java.specification.name`

  The name of the Runtime Environment specification (Set to ‘`Java Platform API Specification`’).

`java.class.path`

  The paths (jar files, zip files and directories) used for finding class files.

`java.library.path`

  Directory path used for finding native libraries.

`java.io.tmpdir`

  The directory used to put temporary files in.

`java.compiler`

  Name of the Just In Time compiler to use by the byte code interpreter. Currently not used in `libgcj`.

`java.ext.dirs`

  Directories containing jar files with extra libraries. Will be used when resolving classes.

`java.protocol.handler.pkgs`

  A ‘`|`’ separated list of package names that is used to find classes that implement handlers for `java.net.URL`.

`java.rmi.server.codebase`

  A list of URLs that is used by the `java.rmi.server.RMIClassLoader` to load classes from.

`jdbc.drivers`

  A list of class names that will be loaded by the `java.sql.DriverManager` when it starts up.

`file.separator`

  The separator used in when directories are included in a filename (normally ‘`/`’ or ‘`\`’ ).

`file.encoding`

  The default character encoding used when converting platform native files to Unicode (usually set to ‘`8859_1`’).

`path.separator`

  The standard separator used when a string contains multiple paths (normally ‘`:`’ or ‘`;`’), the string is usually not a valid character to use in normal directory names.)

`line.separator`

  The default line separator used on the platform (normally ‘`\n`’, ‘`\r`’ or a combination of those two characters).

`policy.provider`

  The class name used for the default policy provider returned by `java.security.Policy.getPolicy`.

`user.name`

  The name of the user running the program. Can be the full name, the login name or empty if unknown.

`user.home`

  The default directory to put user specific files in.

`user.dir`

  The current working directory from which the program was started.

`user.language`

  The default language as used by the `java.util.Locale` class.

`user.region`

  The default region as used by the `java.util.Local` class.

`user.variant`

  The default variant of the language and region local used.

`user.timezone`

  The default timezone as used by the `java.util.TimeZone` class.

`os.name`

  The operating system/kernel name that the program runs on.

`os.arch`

  The hardware that we are running on.

`os.version`

  The version number of the operating system/kernel.

`awt.appletWarning`

  The string to display when an untrusted applet is displayed. Returned by `java.awt.Window.getWarningString()` when the window is “insecure”.

`awt.toolkit`

  The class name used for initializing the default `java.awt.Toolkit`. Defaults to `gnu.awt.gtk.GtkToolkit`.

`http.proxyHost`

  Name of proxy host for http connections.

`http.proxyPort`

  Port number to use when a proxy host is in use.

### 12.2 GNU Classpath Properties

`libgcj` is based on the GNU Classpath (Essential Libraries for Java) a GNU project to create free core class libraries for use with virtual machines and compilers for the Java language. The following properties are common to libraries based on GNU Classpath.

`gcj.dumpobject`

  Enables printing serialization debugging by the `java.io.ObjectInput` and `java.io.ObjectOutput` classes when set to something else then the empty string. Only used when running a debug build of the library.

`gnu.classpath.vm.shortname`

  This is a succinct name of the virtual machine. For `libgcj`, this will always be ‘`libgcj`’.

`gnu.classpath.home.url`

  A base URL used for finding system property files (e.g., `classpath.security`). By default this is a ‘`file:`’ URL pointing to the `lib` directory under ‘`java.home`’.

### 12.3 libgcj Runtime Properties

The following properties are specific to the `libgcj` runtime and will normally not be found in other core libraries for the java language.

`java.fullversion`

  The combination of `java.vm.name` and `java.vm.version`.

`java.vm.info`

  Same as `java.fullversion`.

`impl.prefix`

  Used by the `java.net.DatagramSocket` class when set to something else then the empty string. When set all newly created `DatagramSocket`s will try to load a class `java.net.[impl.prefix]DatagramSocketImpl` instead of the normal `java.net.PlainDatagramSocketImpl`.

`gnu.gcj.progname`

  The class or binary name that was used to invoke the program. This will be the name of the "main" class in the case where the `gij` front end is used, or the program binary name in the case where an application is compiled to a native binary.

`gnu.gcj.user.realname`

  The real name of the user, as taken from the password file. This may not always hold only the user’s name (as some sites put extra information in this field). Also, this property is not available on all platforms.

`gnu.gcj.runtime.NameFinder.use_addr2line`

  Whether an external process, `addr2line`, should be used to determine line number information when tracing the stack. Setting this to `false` may suppress line numbers when printing stack traces and when using the java.util.logging infrastructure. However, performance may improve significantly for applications that print stack traces or make logging calls frequently.

`gnu.gcj.runtime.NameFinder.show_raw`

  Whether the address of a stack frame should be printed when the line number is unavailable. Setting this to `true` will cause the name of the object and the offset within that object to be printed when no line number is available. This allows for off-line decoding of stack traces if necessary debug information is available. The default is `false`, no raw addresses are printed.

`gnu.gcj.runtime.NameFinder.remove_unknown`

  Whether stack frames for non-java code should be included in a stack trace. The default value is `true`, stack frames for non-java code are suppressed. Setting this to `false` will cause any non-java stack frames to be printed in addition to frames for the java code.

`gnu.gcj.runtime.VMClassLoader.library_control`

  This controls how shared libraries are automatically loaded by the built-in class loader. If this property is set to ‘`full`’, a full search is done for each requested class. If this property is set to ‘`cache`’, then any failed lookups are cached and not tried again. If this property is set to ‘`never`’ (the default), then lookups are never done. For more information, See Extensions.

`gnu.gcj.runtime.endorsed.dirs`

  This is like the standard `java.endorsed.dirs`, property, but specifies some extra directories which are searched after the standard endorsed directories. This is primarily useful for telling `libgcj` about additional libraries which are ordinarily incorporated into the JDK, and which should be loaded by the bootstrap class loader, but which are not yet part of `libgcj` itself for some reason.

`gnu.gcj.jit.compiler`

  This is the full path to `gcj` executable which should be used to compile classes just-in-time when `ClassLoader.defineClass` is called. If not set, `gcj` will not be invoked by the runtime; this can also be controlled via `Compiler.disable`.

`gnu.gcj.jit.options`

  This is a space-separated string of options which should be passed to `gcj` when in JIT mode. If not set, a sensible default is chosen.

`gnu.gcj.jit.cachedir`

  This is the directory where cached shared library files are stored. If not set, JIT compilation is disabled. This should never be set to a directory that is writable by any other user.

`gnu.gcj.precompiled.db.path`

  This is a sequence of file names, each referring to a file created by `gcj-dbtool`. These files will be used by `libgcj` to find shared libraries corresponding to classes that are loaded from bytecode. `libgcj` often has a built-in default database; it can be queried using `gcj-dbtool -p`.

## 13 Resources

While writing `gcj` and `libgcj` we have, of course, relied heavily on documentation from Sun Microsystems. In particular we have used The Java Language Specification (both first and second editions), the Java Class Libraries (volumes one and two), and the Java Virtual Machine Specification. In addition we’ve used Sun’s online documentation.

The current `gcj` home page is http://gcc.gnu.org/java/.

For more information on GCC, see http://gcc.gnu.org/.

Some `libgcj` testing is done using the Mauve test suite. This is a free software Java class library test suite which is being written because the JCK is not free. See http://www.sourceware.org/mauve/ for more information.

## Index

Jump to: | C E F G J

| Index Entry |  | Section

C |  |
| class path: |  | Input Options
| class$: |  | Reference types

E |  |
| `elements on template<class T>`: |  | Arrays

F |  |
| FDL, GNU Free Documentation License: |  | GNU Free Documentation License

G |  |
| `GCJ_PROPERTIES`: |  | Extensions
| GCJ_PROPERTIES: |  | Extensions

J |  |
| jclass: |  | Reference types
| jobject: |  | Reference types
| jstring: |  | Reference types
| `JvAllocBytes`: |  | Mixing with C++
| `JvAttachCurrentThread`: |  | Invocation
| `JvCreateJavaVM`: |  | Invocation
| `JvDetachCurrentThread`: |  | Invocation
| `JvFree`: |  | Memory allocation
| `JvGetArrayLength`: |  | Arrays
| `JvGetStringChars`: |  | Strings
| `JvGetStringUTFLength`: |  | Strings
| `JvGetStringUTFRegion`: |  | Strings
| `JvMalloc`: |  | Memory allocation
| `JvNewBooleanArray`: |  | Arrays
| `JvNewObjectArray`: |  | Arrays
| `JvNewString`: |  | Strings
| `JvNewStringLatin1`: |  | Strings
| `JvNewStringLatin1`: |  | Strings
| `JvNewStringUTF`: |  | Strings
| `JvPrimClass`: |  | Primitive types
| `JvRealloc`: |  | Memory allocation

Jump to: | C E F G J