@\input texinfo @c -*-texinfo-*-
@setfilename gcj.info
@settitle Guide to GNU gcj
@include gcc-common.texi
@c Note: When reading this manual you'll find lots of strange
@c circumlocutions like ``compiler for the Java language''.
@c This is necessary due to Sun's restrictions on the use of
@c the word ``Java'.
@c When this manual is copyrighted.
@set copyrights-gcj 2001, 2002
@c Versions
@set which-gcj GCC-@value{version-GCC}
@copying
@c man begin COPYRIGHT
Copyright @copyright{} @value{copyrights-gcj} 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
@c man end
section entitled
``GNU Free Documentation License''.
@ignore
@c man begin COPYRIGHT
man page gfdl(7).
@c man end
@end ignore
@c man begin COPYRIGHT
(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.
@c man end
@end copying
@ifinfo
@format
@dircategory Programming
@direntry
* Gcj: (gcj). Ahead-of-time compiler for the Java language
@end direntry
@dircategory Individual utilities
@direntry
* gcjh: (gcj)Invoking gcjh.
Generate header files from Java class files
* jv-scan: (gcj)Invoking jv-scan.
Print information about Java source files
* jcf-dump: (gcj)Invoking jcf-dump.
Print information about Java class files
* gij: (gcj)Invoking gij. GNU interpreter for Java bytecode
* jv-convert: (gcj)Invoking jv-convert.
Convert file from one encoding to another
* rmic: (gcj)Invoking rmic.
Generate stubs for Remote Method Invocation.
* rmiregistry: (gcj)Invoking rmiregistry.
The remote object registry.
@end direntry
@end format
@insertcopying
@end ifinfo
@titlepage
@title GNU gcj
@author Tom Tromey
@page
@vskip 0pt plus 1filll
For the @value{which-gcj} Version*
@sp 1
Published by the Free Software Foundation @*
59 Temple Place - Suite 330@*
Boston, MA 02111-1307, USA@*
@sp 1
@insertcopying
@end titlepage
@contents
@page
@node Top
@top Introduction
This manual describes how to use @command{gcj}, the GNU compiler for the
Java programming language. @command{gcj} can generate both @file{.class}
files and object files, and it can read both Java source code and
@file{.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 @command{gcj}
* Compatibility:: Compatibility between gcj and other tools for Java
* Invoking gcjh:: Generate header files from class files
* Invoking jv-scan:: Print information about source files
* Invoking jcf-dump:: Print information about class files
* Invoking gij:: Interpreting Java bytecodes
* Invoking jv-convert:: Converting from one encoding to another
* Invoking rmic:: Generate stubs for Remote Method Invocation.
* Invoking rmiregistry:: The remote object registry.
* About CNI:: Description of the Cygnus Native Interface
* System properties:: Modifying runtime behavior of the libgcj library
* Resources:: Where to look for more information
@end menu
@include gpl.texi
@include fdl.texi
@node Invoking gcj
@chapter Invoking gcj
@c man title gcj Ahead-of-time compiler for the Java language
@ignore
@c man begin SYNOPSIS gcj
gcj [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
[@option{--CLASSPATH}=@var{path}] [@option{--classpath}=@var{path}]
[@option{-f}@var{option}@dots{}] [@option{--encoding}=@var{name}]
[@option{--main}=@var{classname}] [@option{-D}@var{name}[=@var{value}]@dots{}]
[@option{-C}] [@option{--resource} @var{resource-name}] [@option{-d} @var{directory}]
[@option{-W}@var{warn}@dots{}]
@var{sourcefile}@dots{}
@c man end
@c man begin SEEALSO gcj
gcc(1), gcjh(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
and the Info entries for @file{gcj} and @file{gcc}.
@c man end
@end ignore
@c man begin DESCRIPTION gcj
As @command{gcj} is just another front end to @command{gcc}, it supports many
of the same options as gcc. @xref{Option Summary, , Option Summary,
gcc, Using the GNU Compiler Collection (GCC)}. This manual only documents the
options specific to @command{gcj}.
@c man end
@menu
* Input and output files::
* Input Options:: How gcj finds files
* Encodings:: Options controlling source file encoding
* Warnings:: Options controlling warnings specific to gcj
* Code Generation:: Options controlling the output of gcj
* Configure-time Options:: Options you won't use
@end menu
@c man begin OPTIONS gcj
@node Input and output files
@section Input and output files
A @command{gcj} command is like a @command{gcc} command, in that it
consists of a number of options and file names. The following kinds
of input file names are supported:
@table @gcctabopt
@item @var{file}.java
Java source files.
@item @var{file}.class
Java bytecode files.
@item @var{file}.zip
@itemx @var{file}.jar
An archive containing one or more @code{.class} files, all of
which are compiled. The archive may be compressed.
@item @@@var{file}
A file containing a whitespace-separated list of input file names.
(Currently, these must all be @code{.java} source files, but that
may change.)
Each named file is compiled, just as if it had been on the command line.
@item @var{library}.a
@itemx @var{library}.so
@itemx -l@var{libname}
Libraries to use when linking. See the @command{gcc} manual.
@end table
You can specify more than one input file on the @command{gcj} command line,
in which case they will all be compiled. If you specify a
@code{-o @var{FILENAME}}
option, all the input files will be compiled together, producing a
single output file, named @var{FILENAME}.
This is allowed even when using @code{-S} or @code{-c},
but not when using @code{-C} or @code{--resource}.
(This is an extension beyond the what plain @command{gcc} allows.)
(If more than one input file is specified, all must currently
be @code{.java} files, though we hope to fix this.)
@node Input Options
@section Input Options
@cindex class path
@command{gcj} has options to control where it looks to find files it needs.
For instance, @command{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, @command{gcj} has a notion of a @dfn{class path}. There are
several options and environment variables which can be used to
manipulate the class path. When @command{gcj} looks for a given class, it
searches the class path looking for matching @file{.class} or
@file{.java} file. @command{gcj} comes with a built-in class path which
points at the installed @file{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 @file{.zip} or @file{.jar}
file, which @command{gcj} will search as if it is a directory.
@table @gcctabopt
@item -I@var{dir}
All directories specified by @code{-I} are kept in order and prepended
to the class path constructed from all the other options. Unless
compatibility with tools like @code{javac} is important, we recommend
always using @code{-I} instead of the other options for manipulating the
class path.
@item --classpath=@var{path}
This sets the class path to @var{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.
@item --CLASSPATH=@var{path}
Deprecated synonym for @code{--classpath}.
@item --bootclasspath=@var{path}
Where to find the standard builtin classes, such as @code{java.lang.String}.
@item --extdirs=@var{path}
For each directory in the @var{path}, place the contents of that
directory at the end of the class path.
@item CLASSPATH
This is an environment variable which holds a list of paths.
@end table
The final class path is constructed like so:
@itemize @bullet
@item
First come all directories specified via @code{-I}.
@item
If @option{--classpath} is specified, its value is appended.
Otherwise, if the @code{CLASSPATH} environment variable is specified,
then its value is appended.
Otherwise, the current directory (@code{"."}) is appended.
@item
If @code{--bootclasspath} was specified, append its value.
Otherwise, append the built-in system directory, @file{libgcj.jar}.
@item
Finally, if @code{--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 @code{$(prefix)/share/java/ext}.
@end itemize
The classfile built by @command{gcj} for the class @code{java.lang.Object}
(and placed in @code{libgcj.jar}) contains a special zero length
attribute @code{gnu.gcj.gcj-compiled}. The compiler looks for this
attribute when loading @code{java.lang.Object} and will report an error
if it isn't found, unless it compiles to bytecode (the option
@code{-fforce-classes-archive-check} can be used to override this
behavior in this particular case.)
@table @gcctabopt
@item -fforce-classes-archive-check
This forces the compiler to always check for the special zero length
attribute @code{gnu.gcj.gcj-compiled} in @code{java.lang.Object} and
issue an error if it isn't found.
@end table
@node Encodings
@section Encodings
The Java programming language uses Unicode throughout. In an effort to
integrate well with other locales, @command{gcj} allows @file{.java} files
to be written using almost any encoding. @command{gcj} knows how to
convert these encodings into its internal encoding at compile time.
You can use the @code{--encoding=@var{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
@command{gcj} assumes the default encoding to be the @samp{UTF-8} encoding
of Unicode.
To implement @code{--encoding}, @command{gcj} simply uses the host
platform's @code{iconv} conversion routine. This means that in practice
@command{gcj} is limited by the capabilities of the host platform.
The names allowed for the argument @code{--encoding} vary from platform
to platform (since they are not standardized anywhere). However,
@command{gcj} implements the encoding named @samp{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.
@node Warnings
@section Warnings
@command{gcj} implements several warnings. As with other generic
@command{gcc} warnings, if an option of the form @code{-Wfoo} enables a
warning, then @code{-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.
@table @gcctabopt
@item -Wredundant-modifiers
With this flag, @command{gcj} will warn about redundant modifiers. For
instance, it will warn if an interface method is declared @code{public}.
@item -Wextraneous-semicolon
This causes @command{gcj} to warn about empty statements. Empty statements
have been deprecated.
@item -Wno-out-of-date
This option will cause @command{gcj} not to warn when a source file is
newer than its matching class file. By default @command{gcj} will warn
about this.
@item -Wunused
This is the same as @command{gcc}'s @code{-Wunused}.
@item -Wall
This is the same as @code{-Wredundant-modifiers -Wextraneous-semicolon
-Wunused}.
@end table
@node Code Generation
@section Code Generation
In addition to the many @command{gcc} options controlling code generation,
@command{gcj} has several options specific to itself.
@table @gcctabopt
@item --main=@var{CLASSNAME}
This option is used when linking to specify the name of the class whose
@code{main} method should be invoked when the resulting executable is
run. @footnote{The linker by default looks for a global function named
@code{main}. Since Java does not have global functions, and a
collection of Java classes may have more than one class with a
@code{main} method, you need to let the linker know which of those
@code{main} methods it should invoke when starting the application.}
@item -D@var{name}[=@var{value}]
This option can only be used with @code{--main}. It defines a system
property named @var{name} with value @var{value}. If @var{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 @code{java.lang.System.getProperty} method.
@item -C
This option is used to tell @command{gcj} to generate bytecode
(@file{.class} files) rather than object code.
@item --resource @var{resource-name}
This option is used to tell @command{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 @samp{core:/@var{resource-name}}. Note that
@var{resource-name} is the name of the resource as found at runtime; for
instance, it could be used in a call to @code{ResourceBundle.getBundle}.
The actual file name to be compiled this way must be specified
separately.
@item -d @var{directory}
When used with @code{-C}, this causes all generated @file{.class} files
to be put in the appropriate subdirectory of @var{directory}. By
default they will be put in subdirectories of the current working
directory.
@item -fno-bounds-check
By default, @command{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 @code{ArrayIndexOutOfBoundsException}.
@item -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
@code{ArrayStoreException}.
@item -fjni
With @command{gcj} there are two options for writing native methods: CNI
and JNI@. By default @command{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 @code{-fjni}. This option causes
@command{gcj} to generate stubs which will invoke the underlying JNI
methods.
@item -fno-assert
Don't recognize the @code{assert} keyword. This is for compatibility
with older versions of the language specification.
@item -fno-optimize-static-class-initialization
When the optimization level is greater or equal to @code{-O2},
@command{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 @code{-C} was specified.) When compiling to native
code, @code{-fno-optimize-static-class-initialization} will turn this
optimization off, regardless of the optimization level in use.
@end table
@node Configure-time Options
@section Configure-time Options
Some @command{gcj} code generations options affect the resulting ABI, and
so can only be meaningfully given when @code{libgcj}, the runtime
package, is configured. @code{libgcj} puts the appropriate options from
this group into a @samp{spec} file which is read by @command{gcj}. These
options are listed here for completeness; if you are using @code{libgcj}
then you won't want to touch these options.
@table @gcctabopt
@item -fuse-boehm-gc
This enables the use of the Boehm GC bitmap marking code. In particular
this causes @command{gcj} to put an object marking descriptor into each
vtable.
@item -fhash-synchronization
By default, synchronization data (the data used for @code{synchronize},
@code{wait}, and @code{notify}) is pointed to by a word in each object.
With this option @command{gcj} assumes that this information is stored in a
hash table and not in the object itself.
@item -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.
@item -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.
@end table
@c man end
@node Compatibility
@chapter Compatibility with the Java Platform
As we believe it is important that the Java platform not be fragmented,
@command{gcj} and @code{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
@command{gcj}.
@menu
* Limitations::
* Extensions::
@end menu
@node Limitations
@section Standard features not yet supported
This list of compatibility issues is by no means complete.
@itemize @bullet
@item
@command{gcj} implements the JDK 1.2 language. It supports inner classes
and the new 1.4 @code{assert} keyword. It does not yet support the Java 2
@code{strictfp} keyword (it recognizes the keyword but ignores it).
@item
@code{libgcj} is largely compatible with the JDK 1.2 libraries.
However, @code{libgcj} is missing many packages, most notably
@code{java.awt}. There are also individual missing classes and methods.
We currently do not have a list showing differences between
@code{libgcj} and the Java 2 platform.
@item
Sometimes the @code{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.
@item
@command{gcj} does not currently allow for piecemeal replacement of
components within @code{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 @code{org.w3c.dom} and @code{org.xml.sax} packages into
their own libraries, separate from @code{libgcj}. If you intend to
use these classes, you must link them explicitly with
@code{-l-org-w3c-dom} and @code{-l-org-xml-sax}. Future versions of
@command{gcj} may not have this restriction.
@end itemize
@node Extensions
@section Extra features unique to gcj
The main feature of @command{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.
@itemize @bullet
@item
@command{gcj} makes it easy and efficient to mix code written in Java and C++.
@xref{About CNI}, for more info on how to use this in your programs.
@item
When you compile your classes into a shared library they can be automatically
loaded by the @code{libgcj} system classloader. When trying to load a class
@code{gnu.pkg.SomeClass} the system classloader will first try to load the
shared library @file{lib-gnu-pkg-SomeClass.so}, if that fails to load the
class then it will try to load @file{lib-gnu-pkg.so} and finally when the
class is still not loaded it will try to load @file{lib-gnu.so}. Note that
all @samp{.}s will be transformed into @samp{-}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 @code{libgcj} bytecode interpreter to load the class from the standard
classpath.
@end itemize
@node Invoking gcjh
@chapter Invoking gcjh
@c man title gcjh generate header files from Java class files
@c man begin DESCRIPTION gcjh
The @code{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.
@c man end
@ignore
@c man begin SYNOPSIS gcjh
gcjh [@option{-stubs}] [@option{-jni}]
[@option{-add} @var{text}] [@option{-append} @var{text}] [@option{-friend} @var{text}]
[@option{-preprend} @var{text}]
[@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
[@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
[@option{-o} @var{file}] [@option{-td} @var{dir}]
[@option{-M}] [@option{-MM}] [@option{-MD}] [@option{-MMD}]
[@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
@var{classname}@dots{}
@c man end
@c man begin SEEALSO gcjh
gcc(1), gcj(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
and the Info entries for @file{gcj} and @file{gcc}.
@c man end
@end ignore
@c man begin OPTIONS gcjh
@table @gcctabopt
@item -stubs
This causes @code{gcjh} to generate stub files instead of header files.
By default the stub file will be named after the class, with a suffix of
@samp{.cc}. In JNI mode, the default output file will have the suffix
@samp{.c}.
@item -jni
This tells @code{gcjh} to generate a JNI header or stub. By default,
CNI headers are generated.
@item -add @var{text}
Inserts @var{text} into the class body. This is ignored in JNI mode.
@item -append @var{text}
Inserts @var{text} into the header file after the class declaration.
This is ignored in JNI mode.
@item -friend @var{text}
Inserts @var{text} into the class as a @code{friend} declaration.
This is ignored in JNI mode.
@item -prepend @var{text}
Inserts @var{text} into the header file before the class declaration.
This is ignored in JNI mode.
@item --classpath=@var{path}
@itemx --CLASSPATH=@var{path}
@itemx -I@var{directory}
@itemx -d @var{directory}
@itemx -o @var{file}
These options are all identical to the corresponding @command{gcj} options.
@item -o @var{file}
Sets the output file name. This cannot be used if there is more than
one class on the command line.
@item -td @var{directory}
Sets the name of the directory to use for temporary files.
@item -M
Print all dependencies to stdout; suppress ordinary output.
@item -MM
Print non-system dependencies to stdout; suppress ordinary output.
@item -MD
Print all dependencies to stdout.
@item -MMD
Print non-system dependencies to stdout.
@item --help
Print help about @code{gcjh} and exit. No further processing is done.
@item --version
Print version information for @code{gcjh} and exit. No further
processing is done.
@item -v, --verbose
Print extra information while running.
@end table
All remaining options are considered to be names of classes.
@c man end
@node Invoking jv-scan
@chapter Invoking jv-scan
@c man title jv-scan print information about Java source file
@c man begin DESCRIPTION jv-scan
The @code{jv-scan} program can be used to print information about a Java
source file (@file{.java} file).
@c man end
@ignore
@c man begin SYNOPSIS jv-scan
jv-scan [@option{--no-assert}] [@option{--complexity}]
[@option{--encoding}=@var{name}] [@option{--print-main}]
[@option{--list-class}] [@option{--list-filename}]
[@option{--version}] [@option{--help}]
[@option{-o} @var{file}] @var{inputfile}@dots{}
@c man end
@c man begin SEEALSO jv-scan
gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
and the Info entries for @file{gcj} and @file{gcc}.
@c man end
@end ignore
@c man begin OPTIONS jv-scan
@table @gcctabopt
@item --no-assert
Don't recognize the @code{assert} keyword, for backwards compatibility
with older versions of the language specification.
@item --complexity
This prints a complexity measure, related to cyclomatic complexity, for
each input file.
@item --encoding=@var{name}
This works like the corresponding @command{gcj} option.
@item --print-main
This prints the name of the class in this file containing a @code{main}
method.
@item --list-class
This lists the names of all classes defined in the input files.
@item --list-filename
If @code{--list-class} is given, this option causes @code{jv-scan} to
also print the name of the file in which each class was found.
@item -o @var{file}
Print output to the named file.
@item --help
Print help, then exit.
@item --version
Print version number, then exit.
@end table
@c man end
@node Invoking jcf-dump
@chapter Invoking jcf-dump
@c man title jcf-dump print information about Java class files
@ignore
@c man begin SYNOPSIS jcf-dump
jcf-dump [@option{-c}] [@option{--javap}]
[@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
[@option{-I}@var{dir}@dots{}] [@option{-o} @var{file}]
[@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
@var{classname}@dots{}
@c man end
@c man begin SEEALSO jcf-dump
gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
and the Info entries for @file{gcj} and @file{gcc}.
@c man end
@end ignore
@c man begin DESCRIPTION jcf-dump
This is a class file examiner, similar to @code{javap}. It will print
information about a number of classes, which are specified by class name
or file name.
@c man end
@c man begin OPTIONS jcf-dump
@table @gcctabopt
@item -c
Disassemble method bodies. By default method bodies are not printed.
@item --javap
Generate output in @code{javap} format. The implementation of this
feature is very incomplete.
@item --classpath=@var{path}
@itemx --CLASSPATH=@var{path}
@itemx -I@var{directory}
@itemx -o @var{file}
These options as the same as the corresponding @command{gcj} options.
@item --help
Print help, then exit.
@item --version
Print version number, then exit.
@item -v, --verbose
Print extra information while running.
@end table
@c man end
@node Invoking gij
@chapter Invoking gij
@c man title gij GNU interpreter for Java bytecode
@ignore
@c man begin SYNOPSIS gij
gij [@option{OPTION}] @dots{} @var{JARFILE} [@var{ARGS}@dots{}]
gij [@option{-jar}] [@option{OPTION}] @dots{} @var{CLASS} [@var{ARGS}@dots{}]
[@option{-cp} @var{path}] [@option{-classpath} @var{path}]
[@option{-D}@var{name}[=@var{value}]@dots{}]
[@option{-ms=}@var{number}] [@option{-mx=}@var{number}]
[@option{--showversion}] [@option{--version}] [@option{--help}]
@c man end
@c man begin SEEALSO gij
gcc(1), gcj(1), gcjh(1), jv-scan(1), jcf-dump(1), gfdl(7),
and the Info entries for @file{gcj} and @file{gcc}.
@c man end
@end ignore
@c man begin DESCRIPTION gij
@code{gij} is a Java bytecode interpreter included with @code{libgcj}.
@code{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 @command{gcj}.
The primary argument to @code{gij} is the name of a class or, with
@code{-jar}, a jar file. Options before this argument are interpreted
by @code{gij}; remaining options are passed to the interpreted program.
If a class name is specified and this class does not have a @code{main}
method with the appropriate signature (a @code{static void} method with
a @code{String[]} as its sole argument), then @code{gij} will print an
error and exit.
If a jar file is specified then @code{gij} will use information in it to
determine which class' @code{main} method will be invoked.
@code{gij} will invoke the @code{main} method with all the remaining
command-line options.
Note that @code{gij} is not limited to interpreting code. Because
@code{libgcj} includes a class loader which can dynamically load shared
objects, it is possible to give @code{gij} the name of a class which has
been compiled and put into a shared library on the class path.
@c man end
@c man begin OPTIONS gij
@table @gcctabopt
@item -cp @var{path}
@itemx -classpath @var{path}
Set the initial class path. The class path is used for finding
class and resource files. If specified, this option overrides the
@code{CLASSPATH} environment variable. Note that this option is
ignored if @code{-jar} is used.
@item -D@var{name}[=@var{value}]
This defines a system property named @var{name} with value @var{value}.
If @var{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 @code{java.lang.System.getProperty}
method.
@item -ms=@var{number}
This sets the initial heap size.
@item -mx=@var{number}
This sets the maximum heap size.
@item -jar
This indicates that the name passed to @code{gij} should be interpreted
as the name of a jar file, not a class.
@item --help
Print help, then exit.
@item --showversion
Print version number and continue.
@item --version
Print version number, then exit.
@end table
@c man end
@node Invoking jv-convert
@chapter Invoking jv-convert
@c man title jv-convert Convert file from one encoding to another
@c man begin SYNOPSIS jv-convert
@command{jv-convert} [@option{OPTION}] @dots{} [@var{INPUTFILE} [@var{OUTPUTFILE}]]
@ignore
[@option{--encoding} @var{name}]
[@option{--from} @var{name}]
[@option{--to} @var{name}]
[@option{-i} @var{file}] [@option{-o} @var{file}]
[@option{--reverse}] [@option{--help}] [@option{--version}]
@end ignore
@c man end
@c man begin DESCRIPTION jv-convert
@command{jv-convert} is a utility included with @code{libgcj} which
converts a file from one encoding to another. It is similar to the Unix
@command{iconv} utility.
The encodings supported by @command{jv-convert} are platform-dependent.
Currently there is no way to get a list of all supported encodings.
@c man end
@c man begin OPTIONS jv-convert
@table @gcctabopt
@item --encoding @var{name}
@itemx --from @var{name}
Use @var{name} as the input encoding. The default is the current
locale's encoding.
@item --to @var{name}
Use @var{name} as the output encoding. The default is the
@code{JavaSrc} encoding; this is ASCII with @samp{\u} escapes for
non-ASCII characters.
@item -i @var{file}
Read from @var{file}. The default is to read from standard input.
@item -o @var{file}
Write to @var{file}. The default is to write to standard output.
@item --reverse
Swap the input and output encodings.
@item --help
Print a help message, then exit.
@item --version
Print version information, then exit.
@end table
@c man end
@node Invoking rmic
@chapter Invoking rmic
@c man title rmic Generate stubs for Remote Method Invocation
@c man begin SYNOPSIS rmic
@command{rmic} [@option{OPTION}] @dots{} @var{class} @dots{}
@ignore
[@option{-keep}]
[@option{-keepgenerated}]
[@option{-v1.1}]
[@option{-vcompat}]
[@option{-v1.2}]
[@option{-nocompile}]
[@option{-verbose}]
[@option{-d} @var{directory}]
[@option{-help}]
[@option{-version}]
@end ignore
@c man end
@c man begin DESCRIPTION rmic
@command{rmic} is a utility included with @code{libgcj} which generates
stubs for remote objects.
@c FIXME: Add real information here.
@c This really isn't much more than the --help output.
Note that this program isn't yet fully compatible with the JDK
@command{rmic}. Some options, such as @option{-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 @samp{--}. For
instance, @option{--help} is accepted.
@c man end
@c man begin OPTIONS rmic
@table @gcctabopt
@item -keep
@itemx -keepgenerated
By default, @command{rmic} deletes intermediate files. Either of these
options causes it not to delete such files.
@item -v1.1
Cause @command{rmic} to create stubs and skeletons for the 1.1
protocol version.
@item -vcompat
Cause @command{rmic} to create stubs and skeletons compatible with both
the 1.1 and 1.2 protocol versions. This is the default.
@item -v1.2
Cause @command{rmic} to create stubs and skeletons for the 1.2
protocol version.
@item -nocompile
Don't compile the generated files.
@item -verbose
Print information about what @command{rmic} is doing.
@item -d @var{directory}
Put output files in @var{directory}. By default the files are put in
the current working directory.
@item -help
Print a help message, then exit.
@item -version
Print version information, then exit.
@end table
@c man end
@node Invoking rmiregistry
@chapter Invoking rmiregistry
@c man title rmiregistry Remote object registry
@c man begin SYNOPSIS rmiregistry
@command{rmic} [@option{OPTION}] @dots{} [@var{port}]
@ignore
[@option{--help}]
[@option{--version}]
@end ignore
@c man end
@c man begin DESCRIPTION rmiregistry
@command{rmiregistry} starts a remote object registry on the current
host. If no port number is specified, then port 1099 is used.
@c FIXME: Add real information here.
@c This really isn't much more than the --help output.
@c man end
@c man begin OPTIONS rmiregistry
@table @gcctabopt
@item --help
Print a help message, then exit.
@item --version
Print version information, then exit.
@end table
@c man end
@node About CNI
@chapter About CNI
This documents CNI, the Cygnus 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 Java 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++.
* 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++.
@end menu
@node Basic concepts
@section 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
@command{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++.)
@noindent A CNI C++ source file must have:
@example
#include <gcj/cni.h>
@end example
@noindent and then must include one header file for each Java class it uses, e.g.:
@example
#include <java/lang/Character.h>
#include <java/util/Date.h>
#include <java/lang/IndexOutOfBoundsException.h>
@end example
@noindent These header files are automatically generated by @code{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 @code{Jv} prefix, for example the function
@code{JvNewObjectArray}. This convention is used to avoid conflicts
with other libraries. Internal functions in CNI start with the prefix
@code{_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. (This
manual lists @code{_Jv_AllocBytes} as an example; CNI should instead
provide a @code{JvAllocBytes} function.)
@subsection Limitations
Whilst a Java class is just a C++ class that doesn't mean that you are
freed from the shackles of Java, a @acronym{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 (@code{char*}) as an argument, or to declare a
member variable of some non-Java datatype.
@node Packages
@section Packages
The only global names in Java are class names, and packages. A
@dfn{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++ @dfn{namespace}. The Java class
@code{java.lang.String} is in the package @code{java.lang}, which is a
sub-package of @code{java}. The C++ equivalent is the class
@code{java::lang::String}, which is in the namespace @code{java::lang}
which is in the namespace @code{java}.
@noindent Here is how you could express this:
@example
(// @r{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
@{
...
@};
@end example
@noindent The @code{gcjh} tool automatically generates the necessary namespace
declarations.
@subsection 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 @code{package} declaration
specifies that the following class declarations are in the named
package, without having to explicitly name the full package
qualifiers. The @code{package} declaration can be
followed by zero or more @code{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
@code{using} declaration and directive.
@noindent In Java:
@example
import @var{package-name}.@var{class-name};
@end example
@noindent allows the program text to refer to @var{class-name} as a shorthand for
the fully qualified name: @code{@var{package-name}.@var{class-name}}.
@noindent To achieve the same effect C++, you have to do this:
@example
using @var{package-name}::@var{class-name};
@end example
@noindent Java can also cause imports on demand, like this:
@example
import @var{package-name}.*;
@end example
@noindent Doing this allows any class from the package @var{package-name} to be
referred to only by its class-name within the program text.
@noindent The same effect can be achieved in C++ like this:
@example
using namespace @var{package-name};
@end example
@node Primitive types
@section Primitive types
Java provides 8 @dfn{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:
@multitable @columnfractions .20 .25 .60
@item @strong{Java type} @tab @strong{C/C++ typename} @tab @strong{Description}
@item @code{char} @tab @code{jchar} @tab 16 bit Unicode character
@item @code{boolean} @tab @code{jboolean} @tab logical (true or false) values
@item @code{byte} @tab @code{jbyte} @tab 8-bit signed integer
@item @code{short} @tab @code{jshort} @tab 16 bit signed integer
@item @code{int} @tab @code{jint} @tab 32 bit signed integer
@item @code{long} @tab @code{jlong} @tab 64 bit signed integer
@item @code{float} @tab @code{jfloat} @tab 32 bit IEEE floating point number
@item @code{double} @tab @code{jdouble} @tab 64 bit IEEE floating point number
@item @code{void} @tab @code{void} @tab no value
@end multitable
When referring to a Java type You should always use these C++ typenames (e.g.: @code{jint})
to avoid disappointment.
@subsection Reference types associated with primitive types
In Java each primitive type has an associated reference type,
e.g.: @code{boolean} has an associated @code{java.lang.Boolean} class.
In order to make working with such classes easier GCJ provides the macro
@code{JvPrimClass}:
@deffn macro JvPrimClass type
Return a pointer to the @code{Class} object corresponding to the type supplied.
@example
JvPrimClass(void) @result{} java.lang.Void.TYPE
@end example
@end deffn
@node Interfaces
@section Interfaces
A Java class can @dfn{implement} zero or more
@dfn{interfaces}, in addition to inheriting from
a single base class.
@acronym{CNI} allows CNI code to implement methods of interfaces.
You can also call methods through interface references, with some
limitations.
@acronym{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:
@example
interface A
@{
void a();
@}
interface B extends A
@{
void b();
@}
@end example
and declare a variable of type @code{B} in C++, you can't call
@code{a()} unless you cast it to an @code{A} first.
@node Objects and Classes
@section Objects and Classes
@subsection Classes
All Java classes are derived from @code{java.lang.Object}. C++ does
not have a unique root class, but we use the C++ class
@code{java::lang::Object} as the C++ version of the
@code{java.lang.Object} Java class. All other Java classes are mapped
into corresponding C++ classes derived from @code{java::lang::Object}.
Interface inheritance (the @code{implements} keyword) is currently not
reflected in the C++ mapping.
@subsection 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 @emph{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 (@code{byte},
@code{short}, @code{char}, and @code{boolean}) are @emph{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 @code{gcjh}-generated header for a
class, you can access fields of Java classes in the @emph{natural}
way. For example, given the following Java class:
@example
public class Int
@{
public int i;
public Integer (int i) @{ this.i = i; @}
public static zero = new Integer(0);
@}
@end example
you can write:
@example
#include <gcj/cni.h>;
#include <Int>;
Int*
mult (Int *p, jint k)
@{
if (k == 0)
return Int::zero; // @r{Static member access.}
return new Int(p->i * k);
@}
@end example
@subsection 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.
@node Class Initialization
@section 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
@code{String} objects corresponding to string literals in
the code.
The GCJ compiler inserts calls to @code{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, @code{libgcj} will make sure a class is initialized before an
instance of that object is created. This is one of the
responsibilities of the @code{new} operation. This is taken care of
both in Java code, and in C++ code. When G++ sees a @code{new} of a
Java class, it will call a routine in @code{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 @code{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 @code{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++.
@node Object allocation
@section Object allocation
New Java objects are allocated using a
@dfn{class instance creation expression}, e.g.:
@example
new @var{Type} ( ... )
@end example
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 @acronym{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.
@noindent For example:
@example
java::util::Hashtable *ht = new java::util::Hashtable(120);
@end example
@deftypefun void* _Jv_AllocBytes (jsize @var{size})
Allocates @var{size} bytes from the heap. The memory is not scanned
by the garbage collector but it freed if no references to it are discovered.
@end deftypefun
@node Arrays
@section 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
@code{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
@code{JArray} template, which as defined as follows:
@example
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]; @}
@};
@end example
There are a number of @code{typedef}s which correspond to @code{typedef}s
from the @acronym{JNI}. Each is the type of an array holding objects
of the relevant type:
@example
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;
@end example
@deftypemethod {template<class T>} T* elements (JArray<T> @var{array})
This template function can be used to get a pointer to the elements of
the @code{array}. For instance, you can fetch a pointer to the
integers that make up an @code{int[]} like so:
@example
extern jintArray foo;
jint *intp = elements (foo);
@end example
The name of this function may change in the future.
@end deftypemethod
@deftypefun jobjectArray JvNewObjectArray (jsize @var{length}, jclass @var{klass}, jobject @var{init})
Here @code{klass} is the type of elements of the array and
@code{init} is the initial value put into every slot in the array.
@end deftypefun
@subsection 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:
@example
JvNew@var{Type}Array
@end example
@noindent For example:
@example
JvNewBooleanArray
@end example
@noindent can be used to create an array of Java primitive boolean types.
@noindent The following function definition is the template for all such functions:
@deftypefun jbooleanArray JvNewBooleanArray (jint @var{length})
Create's an array @var{length} indices long.
@end deftypefun
@deftypefun jsize JvGetArrayLength (jarray @var{array})
Returns the length of the @var{array}.
@end deftypefun
@node Methods
@section Methods
Java methods are mapped directly into C++ methods.
The header files generated by @code{gcjh}
include the appropriate method definitions.
Basically, the generated methods have the same names and
@emph{corresponding} types as the Java methods,
and are called in the natural manner.
@subsection 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 @code{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 @dfn{mangling},
and the encoded name is the @dfn{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 @emph{same} encoding scheme.
@subsection Static methods
Static Java methods are invoked in @acronym{CNI} using the standard
C++ syntax, using the @code{::} operator rather
than the @code{.} operator.
@noindent For example:
@example
jint i = java::lang::Math::round((jfloat) 2.3);
@end example
@noindent C++ method definition syntax is used to define a static native method.
For example:
@example
#include <java/lang/Integer>
java::lang::Integer*
java::lang::Integer::getInteger(jstring str)
@{
...
@}
@end example
@subsection Object Constructors
Constructors are called implicitly as part of object allocation
using the @code{new} operator.
@noindent For example:
@example
java::lang::Integer *x = new java::lang::Integer(234);
@end example
Java does not allow a constructor to be a native method.
This limitation can be coded round however because a constructor
can @emph{call} a native method.
@subsection Instance methods
Calling a Java instance method from a C++ @acronym{CNI} method is done
using the standard C++ syntax, e.g.:
@example
// @r{First create the Java object.}
java::lang::Integer *x = new java::lang::Integer(234);
// @r{Now call a method.}
jint prim_value = x->intValue();
if (x->longValue == 0)
...
@end example
@noindent Defining a Java native instance method is also done the natural way:
@example
#include <java/lang/Integer.h>
jdouble
java::lang:Integer::doubleValue()
@{
return (jdouble) value;
@}
@end example
@subsection Interface methods
In Java you can call a method using an interface reference. This is
supported, but not completely. @xref{Interfaces}.
@node Strings
@section Strings
@acronym{CNI} provides a number of utility functions for
working with Java Java @code{String} objects.
The names and interfaces are analogous to those of @acronym{JNI}.
@deftypefun jstring JvNewString (const char* @var{chars}, jsize @var{len})
Returns a Java @code{String} object with characters from the C string
@var{chars} up to the index @var{len} in that array.
@end deftypefun
@deftypefun jstring JvNewStringLatin1 (const char* @var{bytes}, jsize @var{len})
Returns a Java @code{String} made up of @var{len} bytes from @var{bytes}.
@end deftypefun
@deftypefun jstring JvNewStringLatin1 (const char* @var{bytes})
As above but the length of the @code{String} is @code{strlen(@var{bytes})}.
@end deftypefun
@deftypefun jstring JvNewStringUTF (const char* @var{bytes})
Returns a @code{String} which is made up of the UTF encoded characters
present in the C string @var{bytes}.
@end deftypefun
@deftypefun jchar* JvGetStringChars (jstring @var{str})
Returns a pointer to an array of characters making up the @code{String} @var{str}.
@end deftypefun
@deftypefun int JvGetStringUTFLength (jstring @var{str})
Returns the number of bytes required to encode the contents of the
@code{String} @var{str} in UTF-8.
@end deftypefun
@deftypefun jsize JvGetStringUTFRegion (jstring @var{str}, jsize @var{start}, jsize @var{len}, char* @var{buf})
Puts the UTF-8 encoding of a region of the @code{String} @var{str} into
the buffer @code{buf}. The region to fetch is marked by @var{start} and @var{len}.
Note that @var{buf} is a buffer, not a C string. It is @emph{not}
null terminated.
@end deftypefun
@node Mixing with C++
@section Interoperating with C/C++
Because @acronym{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.
@noindent None of the following is possible with CNI:
@example
class ::MyClass : public java::lang::Object
@{
char* variable; // @r{char* is not a valid Java type.}
@}
uint
::SomeClass::someMethod (char *arg)
@{
.
.
.
@} // @r{@code{uint} is not a valid Java type, neither is @code{char*}}
@end example
@noindent Of course, it is ok to use C/C++ types within the scope of a method:
@example
jint
::SomeClass::otherMethod (jstring str)
@{
char *arg = ...
.
.
.
@}
@end example
But this restriction can cause a problem so @acronym{CNI} includes the
@code{gnu.gcj.RawData} class. The @code{RawData} class is a
@dfn{non-scanned reference} type. In other words variables declared
of type @code{RawData} can contain any data and are not checked by the
compiler in any way.
This means that you can put C/C++ data structures (including classes)
in your @acronym{CNI} classes, as long as you use the appropriate cast.
@noindent Here are some examples:
@example
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);
@}
@end example
@node Exception Handling
@section 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 @code{throw} construct, and this
exception can be caught by Java code. Similarly, you can catch an
exception thrown from Java using the C++ @code{catch}
construct.
@noindent Here is an example:
@example
if (i >= count)
throw new java::lang::IndexOutOfBoundsException();
@end example
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:
@example
struct S @{ ~S(); @};
extern void bar(); // @r{Is implemented in Java and may throw exceptions.}
void foo()
@{
S s;
bar();
@}
@end example
The usual effect of an incorrect guess is a link failure, complaining of
a missing routine called @code{__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
@code{#pragma GCC java_exceptions} at the head of the
file. This @code{#pragma} must appear before any
functions that throw or catch exceptions, or run destructors when
exceptions are thrown through them.
@node Synchronization
@section Synchronization
Each Java object has an implicit monitor.
The Java VM uses the instruction @code{monitorenter} to acquire
and lock a monitor, and @code{monitorexit} to release it.
The corresponding CNI macros are @code{JvMonitorEnter} and
@code{JvMonitorExit} (JNI has similar methods @code{MonitorEnter}
and @code{MonitorExit}).
The Java source language does not provide direct access to these primitives.
Instead, there is a @code{synchronized} statement that does an
implicit @code{monitorenter} before entry to the block,
and does a @code{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
@code{try finally} surrounding synchronization locks.
From C++, it makes sense to use a destructor to release a lock.
@acronym{CNI} defines the following utility class:
@example
class JvSynchronize() @{
jobject obj;
JvSynchronize(jobject o) @{ obj = o; JvMonitorEnter(o); @}
~JvSynchronize() @{ JvMonitorExit(obj); @}
@};
@end example
So this Java code:
@example
synchronized (OBJ)
@{
CODE
@}
@end example
@noindent might become this C++ code:
@example
@{
JvSynchronize dummy (OBJ);
CODE;
@}
@end example
Java also has methods with the @code{synchronized} attribute.
This is equivalent to wrapping the entire method body in a
@code{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 @command{gcj}
the @code{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 @code{JvSynchronize}
in a @code{native synchronized} method.
@node Invocation
@section 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
@dfn{invocation API}, are provided to support this.
@deftypefun jint JvCreateJavaVM (void* @var{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 @code{main()} function, before any other Java or CNI calls are made.
It is safe, but not recommended, to call @code{JvCreateJavaVM()} more than
once provided it is only called from a single thread.
The @var{vmargs} parameter can be used to specify initialization parameters
for the Java runtime. It may be @code{NULL}.
This function returns @code{0} upon success, or @code{-1} if the runtime is
already initialized.
@emph{Note:} In GCJ 3.1, the @code{vm_args} parameter is ignored. It may be
used in a future release.
@end deftypefun
@deftypefun java::lang::Thread* JvAttachCurrentThread (jstring @var{name}, java::lang::ThreadGroup* @var{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 @code{JvCreateJavaVM}.
@var{name} specifies a name for the thread. It may be @code{NULL}, in which
case a name will be generated.
@var{group} is the ThreadGroup in which this thread will be a member. If it
is @code{NULL}, the thread will be a member of the main thread group.
The return value is the Java @code{Thread} object that represents the thread.
It is safe to call @code{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.
@end deftypefun
@deftypefun jint JvDetachCurrentThread ()
Unregisters a thread from the Java runtime. This should be called by threads
that were attached using @code{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 @code{0} upon success, or @code{-1} if the current thread
is not attached.
@end deftypefun
@subsection 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.
@subsection 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 @code{java.lang.System} class is initialized in order to
access its @code{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 @command{c++ test.cc -lgcj}.
@example
// 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();
@}
@}
@end example
@node Reflection
@section Reflection
Reflection is possible with CNI code, it functions similarly to how it
functions with JNI@.
@c clean this up... I mean, what are the types jfieldID and jmethodID in JNI?
The types @code{jfieldID} and @code{jmethodID}
are as in JNI@.
@noindent The functions:
@itemize
@item @code{JvFromReflectedField},
@item @code{JvFromReflectedMethod},
@item @code{JvToReflectedField}
@item @code{JvToFromReflectedMethod}
@end itemize
@noindent will be added shortly, as will other functions corresponding to JNI@.
@node System properties
@chapter System properties
The runtime behavior of the @code{libgcj} library can be modified by setting
certain system properties. These properties can be compiled into the program
using the @code{-D@var{name}[=@var{value}]} option to @command{gcj} or by
setting them explicitly in the program by calling the
@code{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
@code{java.lang.System.getProperty()} method.
@menu
* Standard Properties:: Standard properties supported by @code{libgcj}
* GNU Classpath Properties:: Properties found in Classpath based libraries
* libgcj Runtime Properties:: Properties specific to @code{libgcj}
@end menu
@node Standard Properties
@section Standard Properties
The following properties are normally found in all implementations of the core
libraries for the Java language.
@table @gcctabopt
@item java.version
The @code{libgcj} version number.
@item java.vendor
Set to @samp{The Free Software Foundation, Inc.}
@item java.vendor.url
Set to @uref{http://gcc.gnu.org/java/}.
@item java.home
The directory where @code{gcj} was installed. Taken from the @code{--prefix}
option given to @command{configure}.
@item java.class.version
The class format version number supported by the libgcj byte code interpreter.
(Currently @samp{46.0})
@item java.vm.specification.version
The Virtual Machine Specification version implemented by @code{libgcj}.
(Currently @samp{1.0})
@item java.vm.specification.vendor
The name of the Virtual Machine specification designer.
@item java.vm.specification.name
The name of the Virtual Machine specification
(Set to @samp{Java Virtual Machine Specification}).
@item java.vm.version
The @command{gcj} version number.
@item java.vm.vendor
Set to @samp{The Free Software Foundation, Inc.}
@item java.vm.name
Set to @samp{GNU libgcj}.
@item java.specification.version
The Runtime Environment specification version implemented by @code{libgcj}.
(Currently set to @samp{1.3})
@item java.specification.vendor
The Runtime Environment specification designer.
@item java.specification.name
The name of the Runtime Environment specification
(Set to @samp{Java Platform API Specification}).
@item java.class.path
The paths (jar files, zip files and directories) used for finding class files.
@item java.library.path
Directory path used for finding native libraries.
@item java.io.tmpdir
The directory used to put temporary files in.
@item java.compiler
Name of the Just In Time compiler to use by the byte code interpreter.
Currently not used in @code{libgcj}.
@item java.ext.dirs
Directories containing jar files with extra libraries. Will be used when
resolving classes. Currently not used in @code{libgcj}.
@item java.protocol.handler.pkgs
A @samp{|} separated list of package names that is used to find classes that
implement handlers for @code{java.net.URL}.
@item java.rmi.server.codebase
A list of URLs that is used by the @code{java.rmi.server.RMIClassLoader}
to load classes from.
@item jdbc.drivers
A list of class names that will be loaded by the @code{java.sql.DriverManager}
when it starts up.
@item file.separator
The separator used in when directories are included in a filename
(normally @samp{/} or @samp{\} ).
@item file.encoding
The default character encoding used when converting platform native files to
Unicode (usually set to @samp{8859_1}).
@item path.separator
The standard separator used when a string contains multiple paths
(normally @samp{:} or @samp{;}), the string is usually not a valid character
to use in normal directory names.)
@item line.separator
The default line separator used on the platform (normally @samp{\n}, @samp{\r}
or a combination of those two characters).
@item policy.provider
The class name used for the default policy provider returned by
@code{java.security.Policy.getPolicy}.
@item user.name
The name of the user running the program. Can be the full name, the login name
or empty if unknown.
@item user.home
The default directory to put user specific files in.
@item user.dir
The current working directory from which the program was started.
@item user.language
The default language as used by the @code{java.util.Locale} class.
@item user.region
The default region as used by the @code{java.util.Local} class.
@item user.variant
The default variant of the language and region local used.
@item user.timezone
The default timezone as used by the @code{java.util.TimeZone} class.
@item os.name
The operating system/kernel name that the program runs on.
@item os.arch
The hardware that we are running on.
@item os.version
The version number of the operating system/kernel.
@item awt.appletWarning
The string to display when an untrusted applet is displayed.
Returned by @code{java.awt.Window.getWarningString()} when the window is
``insecure''.
@item awt.toolkit
The class name used for initializing the default @code{java.awt.Toolkit}.
Defaults to @code{gnu.awt.gtk.GtkToolkit}.
@item http.proxyHost
Name of proxy host for http connections.
@item http.proxyPort
Port number to use when a proxy host is in use.
@end table
@node GNU Classpath Properties
@section GNU Classpath Properties
@code{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.
@table @gcctabopt
@item gcj.dumpobject
Enables printing serialization debugging by the @code{java.io.ObjectInput} and
@code{java.io.ObjectOutput} classes when set to something else then the empty
string. Only used when running a debug build of the library.
@item gnu.classpath.vm.shortname
This is a succint name of the virtual machine. For @code{libgcj},
this will always be @samp{libgcj}.
@item gnu.classpath.home.url
A base URL used for finding system property files (e.g.,
@file{classpath.security}). By default this is a @samp{file:} URL
pointing to the @file{lib} directory under @samp{java.home}.
@end table
@node libgcj Runtime Properties
@section libgcj Runtime Properties
The following properties are specific to the @code{libgcj} runtime and will
normally not be found in other core libraries for the java language.
@table @gcctabopt
@item java.fullversion
The combination of @code{java.vm.name} and @code{java.vm.version}.
@item java.vm.info
Same as @code{java.fullversion}.
@item impl.prefix
Used by the @code{java.net.DatagramSocket} class when set to something else
then the empty string. When set all newly created @code{DatagramSocket}s will
try to load a class @code{java.net.[impl.prefix]DatagramSocketImpl} instead of
the normal @code{java.net.PlainDatagramSocketImpl}.
@item gnu.gcj.progname
The name that was used to invoked the program.
@item gnu.gcj.runtime.NameFinder.demangle
Whether names in a stack trace should be demangled. Defaults to @code{true}.
@item 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 @code{true}.
@item 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 @code{true}.
@item gnu.gcj.runtime.NameFinder.remove_interpreter
Whether runtime interpreter calls (methods in the @code{_Jv_InterpMethod} class
and functions starting with @samp{ffi_}) should be removed from the stack
trace. Only done when the stack is sanitized. Defaults to @code{true}.
@item gnu.gcj.runtime.NameFinder.use_addr2line
Whether an external process (@command{addr2line} or @command{addr2name.awk})
should be used as fallback to convert the addresses to function names when
the runtime is unable to do it through @code{dladdr}.
@end table
@node Resources
@chapter Resources
While writing @command{gcj} and @code{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 @uref{http://java.sun.com/}.
The current @command{gcj} home page is
@uref{http://gcc.gnu.org/java/}.
For more information on gcc, see @uref{http://gcc.gnu.org/}.
Some @code{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
@uref{http://sources.redhat.com/mauve/} for more information.
@bye