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Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free
Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, the Front-Cover Texts being (a) (see below), and
with the Back-Cover Texts being (b) (see below). A copy of the license
is included in the section entitled "GNU Free Documentation License".
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development. man end
INFO-DIR-SECTION Software development
* Gcj: (gcj). Ahead-of-time compiler for the Java language
INFO-DIR-SECTION Individual utilities
* jcf-dump: (gcj)Invoking jcf-dump.
Print information about Java class files
* gij: (gcj)Invoking gij. GNU interpreter for Java bytecode
* gcj-dbtool: (gcj)Invoking gcj-dbtool.
Tool for manipulating class file databases.
* jv-convert: (gcj)Invoking jv-convert.
Convert file from one encoding to another
* grmic: (gcj)Invoking grmic.
Generate stubs for Remote Method Invocation.
* gc-analyze: (gcj)Invoking gc-analyze.
Analyze Garbage Collector (GC) memory dumps.
* aot-compile: (gcj)Invoking aot-compile.
Compile bytecode to native and generate databases.
* rebuild-gcj-db: (gcj)Invoking rebuild-gcj-db.
Merge the per-solib databases made by aot-compile
into one system-wide database.
Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free
Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, the Front-Cover Texts being (a) (see below), and
with the Back-Cover Texts being (b) (see below). A copy of the license
is included in the section entitled "GNU Free Documentation License".
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development. man end

File:, Node: Top, Next: Copying, Up: (dir)
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.
* Menu:
* Copying:: The GNU General Public License
* GNU Free Documentation License::
How you can share and copy this manual
* Invoking gcj:: Compiler options supported by `gcj'
* Compatibility:: Compatibility between gcj and other tools for Java
* Invoking jcf-dump:: Print information about class files
* Invoking gij:: Interpreting Java bytecodes
* Invoking gcj-dbtool:: Tool for manipulating class file databases.
* Invoking jv-convert:: Converting from one encoding to another
* Invoking grmic:: Generate stubs for Remote Method Invocation.
* Invoking gc-analyze:: Analyze Garbage Collector (GC) memory dumps.
* Invoking aot-compile:: Compile bytecode to native and generate databases.
* Invoking rebuild-gcj-db:: Merge the per-solib databases made by aot-compile
into one system-wide database.
* About CNI:: Description of the Compiled Native Interface
* System properties:: Modifying runtime behavior of the libgcj library
* Resources:: Where to look for more information
* Index:: Index.

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To do so, attach the following notices to the program. It is safest
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This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
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Also add information on how to contact you by electronic and paper
If the program does terminal interaction, make it output a short
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This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
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File:, Node: GNU Free Documentation License, Next: Invoking gcj, Prev: Copying, Up: Top
GNU Free Documentation License
Version 1.2, November 2002
Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
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Copyright (C) YEAR YOUR NAME.
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or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
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If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
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File:, Node: Invoking gcj, Next: Compatibility, Prev: GNU Free Documentation License, Up: Top
1 Invoking gcj
As `gcj' is just another front end to `gcc', it supports many of the
same options as gcc. *Note Option Summary: (gcc)Option Summary. This
manual only documents the options specific to `gcj'.
* Menu:
* Input and output files::
* Input Options:: How gcj finds files
* Encodings:: Options controlling source file encoding
* Warnings:: Options controlling warnings specific to gcj
* Linking:: Options for making an executable
* Code Generation:: Options controlling the output of gcj
* Configure-time Options:: Options you won't use

File:, Node: Input and output files, Next: Input Options, Up: Invoking 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:
Java source files.
Java bytecode files.
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.
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.
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.)

File:, Node: Input Options, Next: Encodings, Prev: Input and output files, Up: Invoking gcj
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.
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.
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.
Deprecated synonym for `--classpath'.
Where to find the standard builtin classes, such as
For each directory in the PATH, place the contents of that
directory at the end of the class path.
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
* 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
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.)
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.
This option is used to choose the source version accepted by
`gcj'. The default is `1.5'.

File:, Node: Encodings, Next: Warnings, Prev: Input Options, Up: Invoking gcj
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.

File:, Node: Warnings, Next: Linking, Prev: Encodings, Up: Invoking gcj
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.
With this flag, `gcj' will warn about redundant modifiers. For
instance, it will warn if an interface method is declared `public'.
This causes `gcj' to warn about empty statements. Empty statements
have been deprecated.
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.
Warn if a deprecated class, method, or field is referred to.
This is the same as `gcc''s `-Wunused'.
This is the same as `-Wredundant-modifiers -Wextraneous-semicolon

File:, Node: Linking, Next: Code Generation, Prev: Warnings, Up: Invoking gcj
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:
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.
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.
Create an application whose command-line processing is that of the
`gij' command.
This option is an alternative to using `--main'; you cannot use
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:

File:, Node: Code Generation, Next: Configure-time Options, Prev: Linking, Up: Invoking gcj
1.6 Code Generation
In addition to the many `gcc' options controlling code generation,
`gcj' has several options specific to itself.
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
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.
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
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'.
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
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
Don't recognize the `assert' keyword. This is for compatibility
with older versions of the language specification.
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.
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
By default, assertions are enabled when generating class files or
when not optimizing, and disabled when generating optimized
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
`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.
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
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

File:, Node: Configure-time Options, Prev: Code Generation, Up: Invoking gcj
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.
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.
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.
On some systems, a library routine is called to perform integer
division. This is required to get exception handling correct when
dividing by zero.
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.
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.

File:, Node: Compatibility, Next: Invoking jcf-dump, Prev: Invoking gcj, Up: Top
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'.
* Menu:
* Limitations::
* Extensions::

File:, Node: Limitations, Next: Extensions, Up: Compatibility
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
* `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.

File:, Node: Extensions, Prev: Limitations, Up: Compatibility
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++. *Note About CNI::, for more info on how to use this in your
* 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. *Note 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
`', if that fails to load the class then
it will try to load `' and finally when the class is
still not loaded it will try to load `'. 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; *Note
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/!/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
`'. 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.

File:, Node: Invoking jcf-dump, Next: Invoking gij, Prev: Compatibility, Up: Top
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.
Disassemble method bodies. By default method bodies are not
Print the constant pool. When printing a reference to a constant
also print its index in the constant pool.
Generate output in `javap' format. The implementation of this
feature is very incomplete.
`-o FILE'
These options as the same as the corresponding `gcj' options.
Print help, then exit.
Print version number, then exit.
`-v, --verbose'
Print extra information while running. Implies

File:, Node: Invoking gij, Next: Invoking gcj-dbtool, Prev: Invoking jcf-dump, Up: Top
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
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.
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.
Equivalent to `-Xms'.
Equivalent to `-Xmx'.
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.
Supplying `-X' by itself will cause `gij' to list all the
supported `-X' options. Currently these options are supported:
Set the initial heap size.
Set the maximum heap size.
Set the thread stack size.
Unrecognized `-X' options are ignored, for compatibility with
other runtimes.
This indicates that the name passed to `gij' should be interpreted
as the name of a jar file, not a class.
Print help, then exit.
Print version number and continue.
Print detailed version information, then exit.
Print version number, then exit.
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

File:, Node: Invoking gcj-dbtool, Next: Invoking jv-convert, Prev: Invoking gij, Up: Top
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.
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.
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.
Test a database.
List the contents of a database.
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.
Print a help message, then exit.
Print version information, then exit.

File:, Node: Invoking jv-convert, Next: Invoking grmic, Prev: Invoking gcj-dbtool, Up: Top
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'
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.
Swap the input and output encodings.
Print a help message, then exit.
Print version information, then exit.

File:, Node: Invoking grmic, Next: Invoking gc-analyze, Prev: Invoking jv-convert, Up: Top
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.
By default, `grmic' deletes intermediate files. Either of these
options causes it not to delete such files.
Cause `grmic' to create stubs and skeletons for the 1.1 protocol
Cause `grmic' to create stubs and skeletons compatible with both
the 1.1 and 1.2 protocol versions. This is the default.
Cause `grmic' to create stubs and skeletons for the 1.2 protocol
Don't compile the generated files.
Print information about what `grmic' is doing.
Put output files in DIRECTORY. By default the files are put in
the current working directory.
Print a help message, then exit.
Print version information, then exit.

File:, Node: Invoking gc-analyze, Next: Invoking aot-compile, Prev: Invoking grmic, Up: Top
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
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);
The memory dump may then be displayed by running:
gc-analyze -v TestDump001
Verbose output.
Prefix added to the names of the `nm' and `readelf' commands.
Directory that contains the executable and shared libraries used
when the dump was generated.
Print a help message, then exit.
Print version information, then exit.

File:, Node: Invoking aot-compile, Next: Invoking rebuild-gcj-db, Prev: Invoking gc-analyze, Up: Top
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
`-e, --exclude=PATH'
Do not compile PATH.

File:, Node: Invoking rebuild-gcj-db, Next: About CNI, Prev: Invoking aot-compile, Up: Top
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

File:, Node: About CNI, Next: System properties, Prev: Invoking rebuild-gcj-db, Up: Top
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).
* Menu:
* Basic concepts:: Introduction to using CNI.
* Packages:: How packages are mapped to C++.
* Primitive types:: Handling primitive Java types in C++.
* Reference types:: Handling Java reference types in C++.
* Interfaces:: How Java interfaces map to C++.
* Objects and Classes:: C++ and Java classes.
* Class Initialization:: How objects are initialized.
* Object allocation:: How to create Java objects in C++.
* Memory allocation:: How to allocate and free memory.
* Arrays:: Dealing with Java arrays in C++.
* Methods:: Java methods in C++.
* Strings:: Information about Java Strings.
* Mixing with C++:: How CNI can interoperate with C++.
* Exception Handling:: How exceptions are handled.
* Synchronization:: Synchronizing between Java and C++.
* Invocation:: Starting the Java runtime from C++.
* Reflection:: Using reflection from C++.

File:, Node: Basic concepts, Next: Packages, Up: About CNI
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.

File:, Node: Packages, Next: Primitive types, Prev: Basic concepts, Up: About CNI
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
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:
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:
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;

File:, Node: Primitive types, Next: Reference types, Prev: Packages, Up: About CNI
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
*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
-- macro: JvPrimClass type
Return a pointer to the `Class' object corresponding to the type
JvPrimClass(void) => java.lang.Void.TYPE

File:, Node: Reference types, Next: Interfaces, Prev: Primitive types, Up: About CNI
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$);

File:, Node: Interfaces, Next: Objects and Classes, Prev: Reference types, Up: About CNI
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
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.

File:, Node: Objects and Classes, Next: Class Initialization, Prev: Interfaces, Up: About CNI
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
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>;
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.

File:, Node: Class Initialization, Next: Object allocation, Prev: Objects and Classes, Up: About CNI
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
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++.

File:, Node: Object allocation, Next: Memory allocation, Prev: Class Initialization, Up: About CNI
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);

File:, Node: Memory allocation, Next: Arrays, Prev: Object allocation, Up: About CNI
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
-- Function: void* JvRealloc (void* PTR, jsize SIZE)
Calls realloc. Throws `java.lang.OutOfMemoryError' if
reallocation fails.
-- Function: void JvFree (void* PTR)
Calls free.

File:, Node: Arrays, Next: Methods, Prev: Memory allocation, Up: About CNI
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
int length;
template<class T>
class JArray : public __JArray
T data[0];
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
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:
For example:
can be used to create an array of Java primitive boolean types.
The following function definition is the template for all such
-- Function: jbooleanArray JvNewBooleanArray (jint LENGTH)
Creates an array LENGTH indices long.
-- Function: jsize JvGetArrayLength (jarray ARRAY)
Returns the length of the ARRAY.

File:, Node: Methods, Next: Strings, Prev: Arrays, Up: About CNI
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::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>
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. *Note Interfaces::.

File:, Node: Strings, Next: Mixing with C++, Prev: Methods, Up: About CNI
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
-- 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'
-- 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

File:, Node: Mixing with C++, Next: Exception Handling, Prev: Strings, Up: About CNI
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.
::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:
::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;
::MyClass::getText ()
return string;
::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
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()'.

File:, Node: Exception Handling, Next: Synchronization, Prev: Mixing with C++, Up: About CNI
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;
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.

File:, Node: Synchronization, Next: Invocation, Prev: Exception Handling, Up: About CNI
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
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)
might become this C++ code:
JvSynchronize dummy (OBJ);
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.

File:, Node: Invocation, Next: Reflection, Prev: Synchronization, Up: About CNI
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; gcj test.o'.
#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;
JvAttachCurrentThread(NULL, NULL);
String *message = JvNewStringLatin1("Hello from C++");
catch (Throwable *t)
System::err->println(JvNewStringLatin1("Unhandled Java exception:"));

File:, Node: Reflection, Prev: Invocation, Up: About CNI
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.

File:, Node: System properties, Next: Resources, Prev: About CNI, Up: Top
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.
* Menu:
* Standard Properties:: Standard properties supported by `libgcj'
* GNU Classpath Properties:: Properties found in Classpath based libraries
* libgcj Runtime Properties:: Properties specific to `libgcj'

File:, Node: Standard Properties, Next: GNU Classpath Properties, Up: System properties
12.1 Standard Properties
The following properties are normally found in all implementations of
the core libraries for the Java language.
The `libgcj' version number.
Set to `The Free Software Foundation, Inc.'
Set to `'.
The directory where `gcj' was installed. Taken from the `--prefix'
option given to `configure'.
The class format version number supported by the libgcj byte code
interpreter. (Currently `46.0')
The Virtual Machine Specification version implemented by `libgcj'.
(Currently `1.0')
The name of the Virtual Machine specification designer.
The name of the Virtual Machine specification (Set to `Java
Virtual Machine Specification').
The `gcj' version number.
Set to `The Free Software Foundation, Inc.'
Set to `GNU libgcj'.
The Runtime Environment specification version implemented by
`libgcj'. (Currently set to `1.3')
The Runtime Environment specification designer.
The name of the Runtime Environment specification (Set to `Java
Platform API Specification').
The paths (jar files, zip files and directories) used for finding
class files.
Directory path used for finding native libraries.
The directory used to put temporary files in.
Name of the Just In Time compiler to use by the byte code
interpreter. Currently not used in `libgcj'.
Directories containing jar files with extra libraries. Will be
used when resolving classes.
A `|' separated list of package names that is used to find classes
that implement handlers for `'.
A list of URLs that is used by the `java.rmi.server.RMIClassLoader'
to load classes from.
A list of class names that will be loaded by the
`java.sql.DriverManager' when it starts up.
The separator used in when directories are included in a filename
(normally `/' or `\' ).
The default character encoding used when converting platform
native files to Unicode (usually set to `8859_1').
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.)
The default line separator used on the platform (normally `\n',
`\r' or a combination of those two characters).
The class name used for the default policy provider returned by
The name of the user running the program. Can be the full name,
the login name or empty if unknown.
The default directory to put user specific files in.
The current working directory from which the program was started.
The default language as used by the `java.util.Locale' class.
The default region as used by the `java.util.Local' class.
The default variant of the language and region local used.
The default timezone as used by the `java.util.TimeZone' class.
The operating system/kernel name that the program runs on.
The hardware that we are running on.
The version number of the operating system/kernel.
The string to display when an untrusted applet is displayed.
Returned by `java.awt.Window.getWarningString()' when the window is
The class name used for initializing the default
`java.awt.Toolkit'. Defaults to `gnu.awt.gtk.GtkToolkit'.
Name of proxy host for http connections.
Port number to use when a proxy host is in use.

File:, Node: GNU Classpath Properties, Next: libgcj Runtime Properties, Prev: Standard Properties, Up: System properties
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.
Enables printing serialization debugging by the
`' and `' classes when set
to something else then the empty string. Only used when running a
debug build of the library.
This is a succinct name of the virtual machine. For `libgcj',
this will always be `libgcj'.
A base URL used for finding system property files (e.g.,
`'). By default this is a `file:' URL pointing
to the `lib' directory under `java.home'.

File:, Node: libgcj Runtime Properties, Prev: GNU Classpath Properties, Up: System properties
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.
The combination of `' and `java.vm.version'.
Same as `java.fullversion'.
Used by the `' class when set to something
else then the empty string. When set all newly created
`DatagramSocket's will try to load a class
`[impl.prefix]DatagramSocketImpl' instead of the normal
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.
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.
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.
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.
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.
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, *Note Extensions::.
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.
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'.
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
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.
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'.

File:, Node: Resources, Next: Index, Prev: System properties, Up: Top
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 the online documentation at
The current `gcj' home page is `'.
For more information on gcc, see `'.
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 `'
for more information.

File:, Node: Index, Prev: Resources, Up: Top
* Menu:
* class path: Input Options. (line 6)
* class$: Reference types. (line 20)
* elements on template<class T>: Arrays. (line 46)
* FDL, GNU Free Documentation License: GNU Free Documentation License.
(line 6)
* GCJ_PROPERTIES: Extensions. (line 56)
* jclass: Reference types. (line 16)
* jobject: Reference types. (line 16)
* jstring: Reference types. (line 16)
* JvAllocBytes: Mixing with C++. (line 99)
* JvAttachCurrentThread: Invocation. (line 55)
* JvCreateJavaVM: Invocation. (line 11)
* JvDetachCurrentThread: Invocation. (line 68)
* JvFree: Memory allocation. (line 19)
* JvGetArrayLength: Arrays. (line 86)
* JvGetStringChars: Strings. (line 25)
* JvGetStringUTFLength: Strings. (line 29)
* JvGetStringUTFRegion: Strings. (line 34)
* JvMalloc: Memory allocation. (line 11)
* JvNewBooleanArray: Arrays. (line 83)
* JvNewObjectArray: Arrays. (line 57)
* JvNewString: Strings. (line 11)
* JvNewStringLatin1: Strings. (line 15)
* JvNewStringUTF: Strings. (line 21)
* JvPrimClass: Primitive types. (line 36)
* JvRealloc: Memory allocation. (line 15)

Tag Table:
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Node: Copying4221
Node: GNU Free Documentation License41771
Node: Invoking gcj64183
Node: Input and output files64946
Node: Input Options66472
Node: Encodings69746
Node: Warnings70952
Node: Linking72065
Node: Code Generation75004
Node: Configure-time Options81784
Node: Compatibility83524
Node: Limitations84008
Node: Extensions85590
Node: Invoking jcf-dump88684
Node: Invoking gij89629
Node: Invoking gcj-dbtool92880
Node: Invoking jv-convert95346
Node: Invoking grmic96425
Node: Invoking gc-analyze97811
Node: Invoking aot-compile99252
Node: Invoking rebuild-gcj-db100201
Node: About CNI100511
Node: Basic concepts101970
Node: Packages104866
Node: Primitive types107194
Node: Reference types108872
Node: Interfaces109961
Node: Objects and Classes110872
Node: Class Initialization113067
Node: Object allocation115409
Node: Memory allocation116199
Node: Arrays116831
Node: Methods119641
Node: Strings122462
Node: Mixing with C++123966
Node: Exception Handling127437
Node: Synchronization129071
Node: Invocation131061
Node: Reflection135997
Node: System properties136458
Node: Standard Properties137335
Node: GNU Classpath Properties141767
Node: libgcj Runtime Properties142814
Node: Resources147316
Node: Index148154

End Tag Table