     [file gcc-common.texi not included]

Copyright © 2001, 2002 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 the Invariant Sections being ``GNU General Public License'', 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.

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.

     [file gpl.texi not included]

     [file fdl.texi not included]

Invoking gcj
************

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

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 copmiled 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.)

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 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.

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.

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.

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 execitable:

--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.

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.

-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.

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.

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.

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.

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 they can be automatically loaded by the libgcj system classloader.  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.

Invoking gcjh
*************

The gcjh program is used to generate header files from class files.  It can generate both CNI and JNI header files, as well as stub implementation files which can be used as a basis for implementing the required native methods.

-stubs
     This causes gcjh to generate stub files instead of header files. By default the stub file will be named after the class, with a suffix of .cc.  In JNI mode, the default output file will have the suffix .c.

-jni
     This tells gcjh to generate a JNI header or stub.  By default, CNI headers are generated.

-add TEXT
     Inserts TEXT into the class body.  This is ignored in JNI mode.

-append TEXT
     Inserts TEXT into the header file after the class declaration. This is ignored in JNI mode.

-friend TEXT
     Inserts TEXT into the class as a friend declaration. This is ignored in JNI mode.

-prepend TEXT
     Inserts TEXT into the header file before the class declaration. This is ignored in JNI mode.

--classpath=PATH
--CLASSPATH=PATH
-IDIRECTORY
-d DIRECTORY
-o FILE
     These options are all identical to the corresponding gcj options.

-o FILE
     Sets the output file name.  This cannot be used if there is more than one class on the command line.

-td DIRECTORY
     Sets the name of the directory to use for temporary files.

-M
     Print all dependencies to stdout; suppress ordinary output.

-MM
     Print non-system dependencies to stdout; suppress ordinary output.

-MD
     Print all dependencies to stdout.

-MMD
     Print non-system dependencies to stdout.

--help
     Print help about gcjh and exit.  No further processing is done.

--version
     Print version information for gcjh and exit.  No further processing is done.

-v, --verbose
     Print extra information while running.

All remaining options are considered to be names of classes.

Invoking jv-scan
****************

The jv-scan program can be used to print information about a Java source file (.java file).

--no-assert
     Don't recognize the assert keyword, for backwards compatibility with older versions of the language specification.

--complexity
     This prints a complexity measure, related to cyclomatic complexity, for each input file.

--encoding=NAME
     This works like the corresponding gcj option.

--print-main
     This prints the name of the class in this file containing a main method.

--list-class
     This lists the names of all classes defined in the input files.

--list-filename
     If --list-class is given, this option causes jv-scan to also print the name of the file in which each class was found.

-o FILE
     Print output to the named file.

--help
     Print help, then exit.

--version
     Print version number, then exit.

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.

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
     This sets the initial heap size.

-mx=NUMBER
     This sets the maximum heap size.

-X
-XARGUMENT
     Supplying -X by itself will cause gij to list all the supported -X options.  Currently there are none.  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.

--version
     Print version number, then exit.

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

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.

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.

Invoking grmiregistry
*********************

grmic [OPTION] ... [PORT]

grmiregistry starts a remote object registry on the current host.  If no port number is specified, then port 1099 is used.

--help
     Print a help message, then exit.

--version
     Print version information, then exit.

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).

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.

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.

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.

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;

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.

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 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

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.

Objects and Classes
===================

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.

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-bite 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 Integer (int i) { this.i = i; }
       public static zero = new Integer(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);
     }

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.

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 Jv_InitClass 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++.

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);

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 typedefs which correspond to typedefs 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)
     Here klass is the type of elements of the array and init is the initial value put into every slot in the array.

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)
     Create's an array LENGTH indices long.

 -- Function: jsize JvGetArrayLength (jarray ARRAY)
     Returns the length of the ARRAY.

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.

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.

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)
     {
       ...
     }

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.

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;
     }

Interface methods
-----------------

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

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 char* CHARS, jsize LEN)
     Returns a Java String object with characters from the C string 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.

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 = ...
        .
        .
        .
     }

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);
     }

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.

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.

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.

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.

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 (void* 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. This function returns 0 upon success, or -1 if the runtime is already initialized.

     Note: In GCJ 3.1, the vm_args parameter is ignored. It may be used in a future release.

 -- 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.

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.

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++ test.cc -lgcj.

     // 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();
       }
     }

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.

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.

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.  Currently not used in libgcj.

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.

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 succint 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.

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 DatagramSockets will try to load a class java.net.[impl.prefix]DatagramSocketImpl instead of the normal java.net.PlainDatagramSocketImpl.

gnu.gcj.progname
     The name that was used to invoked the program.

gnu.gcj.runtime.NameFinder.demangle
     Whether names in a stack trace should be demangled.  Defaults to true.

gnu.gcj.runtime.NameFinder.sanitize
     Whether calls to initialize exceptions and starting the runtime system should be removed from the stack trace.  Only done when names are demangled.  Defaults to true.

gnu.gcj.runtime.NameFinder.remove_unknown
     Whether calls to unknown functions (class and method names are unknown) should be removed from the stack trace.  Only done when the stack is sanitized.  Ignored if this means no stack trace information would be available anymore.  Defaults to true.

gnu.gcj.runtime.NameFinder.remove_interpreter
     Whether runtime interpreter calls (methods in the _Jv_InterpMethod class and functions starting with ffi_) should be removed from the stack trace.  Only done when the stack is sanitized.  Defaults to true.

gnu.gcj.runtime.NameFinder.use_addr2line
     Whether an external process (addr2line or addr2name.awk) should be used as fallback to convert the addresses to function names when the runtime is unable to do it through dladdr.

gnu.gcj.runtime.VMClassLoader.library_control
     This controls how shared libraries are automatically loaded by the built-in class loader.  By default, or 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, then lookups are never done.  For more information, see Extensions.

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 http://java.sun.com/.

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://sources.redhat.com/mauve/ for more information.