1 \input texinfo @c -*-texinfo-*-
3 @settitle Guide to GNU gcj
5 @include gcc-common.texi
7 @c Note: When reading this manual you'll find lots of strange
8 @c circumlocutions like ``compiler for the Java language''.
9 @c This is necessary due to Sun's restrictions on the use of
12 @c When this manual is copyrighted.
13 @set copyrights-gcj 2001, 2002
16 @set which-gcj GCC-@value{version-GCC}
19 @c man begin COPYRIGHT
20 Copyright @copyright{} @value{copyrights-gcj} Free Software Foundation, Inc.
22 Permission is granted to copy, distribute and/or modify this document
23 under the terms of the GNU Free Documentation License, Version 1.2 or
24 any later version published by the Free Software Foundation; with the
25 Invariant Sections being ``GNU General Public License'', the Front-Cover
26 texts being (a) (see below), and with the Back-Cover Texts being (b)
27 (see below). A copy of the license is included in the
30 ``GNU Free Documentation License''.
32 @c man begin COPYRIGHT
37 @c man begin COPYRIGHT
39 (a) The FSF's Front-Cover Text is:
43 (b) The FSF's Back-Cover Text is:
45 You have freedom to copy and modify this GNU Manual, like GNU
46 software. Copies published by the Free Software Foundation raise
47 funds for GNU development.
53 @dircategory Programming
55 * Gcj: (gcj). Ahead-of-time compiler for the Java language
58 @dircategory Individual utilities
60 * gcjh: (gcj)Invoking gcjh.
61 Generate header files from Java class files
62 * jv-scan: (gcj)Invoking jv-scan.
63 Print information about Java source files
64 * jcf-dump: (gcj)Invoking jcf-dump.
65 Print information about Java class files
66 * gij: (gcj)Invoking gij. GNU interpreter for Java bytecode
67 * jv-convert: (gcj)Invoking jv-convert.
68 Convert file from one encoding to another
69 * rmic: (gcj)Invoking rmic.
70 Generate stubs for Remote Method Invocation.
71 * rmiregistry: (gcj)Invoking rmiregistry.
72 The remote object registry.
84 @vskip 0pt plus 1filll
85 For the @value{which-gcj} Version*
87 Published by the Free Software Foundation @*
88 59 Temple Place - Suite 330@*
89 Boston, MA 02111-1307, USA@*
100 This manual describes how to use @command{gcj}, the GNU compiler for the
101 Java programming language. @command{gcj} can generate both @file{.class}
102 files and object files, and it can read both Java source code and
106 * Copying:: The GNU General Public License
107 * GNU Free Documentation License::
108 How you can share and copy this manual
109 * Invoking gcj:: Compiler options supported by @command{gcj}
110 * Compatibility:: Compatibility between gcj and other tools for Java
111 * Invoking gcjh:: Generate header files from class files
112 * Invoking jv-scan:: Print information about source files
113 * Invoking jcf-dump:: Print information about class files
114 * Invoking gij:: Interpreting Java bytecodes
115 * Invoking jv-convert:: Converting from one encoding to another
116 * Invoking rmic:: Generate stubs for Remote Method Invocation.
117 * Invoking rmiregistry:: The remote object registry.
118 * About CNI:: Description of the Compiled Native Interface
119 * System properties:: Modifying runtime behavior of the libgcj library
120 * Resources:: Where to look for more information
130 @chapter Invoking gcj
132 @c man title gcj Ahead-of-time compiler for the Java language
135 @c man begin SYNOPSIS gcj
136 gcj [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
137 [@option{--CLASSPATH}=@var{path}] [@option{--classpath}=@var{path}]
138 [@option{-f}@var{option}@dots{}] [@option{--encoding}=@var{name}]
139 [@option{--main}=@var{classname}] [@option{-D}@var{name}[=@var{value}]@dots{}]
140 [@option{-C}] [@option{--resource} @var{resource-name}] [@option{-d} @var{directory}]
141 [@option{-W}@var{warn}@dots{}]
142 @var{sourcefile}@dots{}
144 @c man begin SEEALSO gcj
145 gcc(1), gcjh(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
146 and the Info entries for @file{gcj} and @file{gcc}.
150 @c man begin DESCRIPTION gcj
152 As @command{gcj} is just another front end to @command{gcc}, it supports many
153 of the same options as gcc. @xref{Option Summary, , Option Summary,
154 gcc, Using the GNU Compiler Collection (GCC)}. This manual only documents the
155 options specific to @command{gcj}.
160 * Input and output files::
161 * Input Options:: How gcj finds files
162 * Encodings:: Options controlling source file encoding
163 * Warnings:: Options controlling warnings specific to gcj
164 * Code Generation:: Options controlling the output of gcj
165 * Configure-time Options:: Options you won't use
168 @c man begin OPTIONS gcj
170 @node Input and output files
171 @section Input and output files
173 A @command{gcj} command is like a @command{gcc} command, in that it
174 consists of a number of options and file names. The following kinds
175 of input file names are supported:
178 @item @var{file}.java
180 @item @var{file}.class
183 @itemx @var{file}.jar
184 An archive containing one or more @code{.class} files, all of
185 which are compiled. The archive may be compressed. Files in
186 an archive which don't end with @samp{.class} are treated as
187 resource files; they are copmiled into the resulting object file
188 as @samp{core:} URLs.
190 A file containing a whitespace-separated list of input file names.
191 (Currently, these must all be @code{.java} source files, but that
193 Each named file is compiled, just as if it had been on the command line.
194 @item @var{library}.a
195 @itemx @var{library}.so
196 @itemx -l@var{libname}
197 Libraries to use when linking. See the @command{gcc} manual.
200 You can specify more than one input file on the @command{gcj} command line,
201 in which case they will all be compiled. If you specify a
202 @code{-o @var{FILENAME}}
203 option, all the input files will be compiled together, producing a
204 single output file, named @var{FILENAME}.
205 This is allowed even when using @code{-S} or @code{-c},
206 but not when using @code{-C} or @code{--resource}.
207 (This is an extension beyond the what plain @command{gcc} allows.)
208 (If more than one input file is specified, all must currently
209 be @code{.java} files, though we hope to fix this.)
212 @section Input Options
216 @command{gcj} has options to control where it looks to find files it needs.
217 For instance, @command{gcj} might need to load a class that is referenced
218 by the file it has been asked to compile. Like other compilers for the
219 Java language, @command{gcj} has a notion of a @dfn{class path}. There are
220 several options and environment variables which can be used to
221 manipulate the class path. When @command{gcj} looks for a given class, it
222 searches the class path looking for matching @file{.class} or
223 @file{.java} file. @command{gcj} comes with a built-in class path which
224 points at the installed @file{libgcj.jar}, a file which contains all the
227 In the below, a directory or path component can refer either to an
228 actual directory on the filesystem, or to a @file{.zip} or @file{.jar}
229 file, which @command{gcj} will search as if it is a directory.
233 All directories specified by @code{-I} are kept in order and prepended
234 to the class path constructed from all the other options. Unless
235 compatibility with tools like @code{javac} is important, we recommend
236 always using @code{-I} instead of the other options for manipulating the
239 @item --classpath=@var{path}
240 This sets the class path to @var{path}, a colon-separated list of paths
241 (on Windows-based systems, a semicolon-separate list of paths).
242 This does not override the builtin (``boot'') search path.
244 @item --CLASSPATH=@var{path}
245 Deprecated synonym for @code{--classpath}.
247 @item --bootclasspath=@var{path}
248 Where to find the standard builtin classes, such as @code{java.lang.String}.
250 @item --extdirs=@var{path}
251 For each directory in the @var{path}, place the contents of that
252 directory at the end of the class path.
255 This is an environment variable which holds a list of paths.
258 The final class path is constructed like so:
262 First come all directories specified via @code{-I}.
265 If @option{--classpath} is specified, its value is appended.
266 Otherwise, if the @code{CLASSPATH} environment variable is specified,
267 then its value is appended.
268 Otherwise, the current directory (@code{"."}) is appended.
271 If @code{--bootclasspath} was specified, append its value.
272 Otherwise, append the built-in system directory, @file{libgcj.jar}.
275 Finally, if @code{--extdirs} was specified, append the contents of the
276 specified directories at the end of the class path. Otherwise, append
277 the contents of the built-in extdirs at @code{$(prefix)/share/java/ext}.
280 The classfile built by @command{gcj} for the class @code{java.lang.Object}
281 (and placed in @code{libgcj.jar}) contains a special zero length
282 attribute @code{gnu.gcj.gcj-compiled}. The compiler looks for this
283 attribute when loading @code{java.lang.Object} and will report an error
284 if it isn't found, unless it compiles to bytecode (the option
285 @code{-fforce-classes-archive-check} can be used to override this
286 behavior in this particular case.)
289 @item -fforce-classes-archive-check
290 This forces the compiler to always check for the special zero length
291 attribute @code{gnu.gcj.gcj-compiled} in @code{java.lang.Object} and
292 issue an error if it isn't found.
298 The Java programming language uses Unicode throughout. In an effort to
299 integrate well with other locales, @command{gcj} allows @file{.java} files
300 to be written using almost any encoding. @command{gcj} knows how to
301 convert these encodings into its internal encoding at compile time.
303 You can use the @code{--encoding=@var{NAME}} option to specify an
304 encoding (of a particular character set) to use for source files. If
305 this is not specified, the default encoding comes from your current
306 locale. If your host system has insufficient locale support, then
307 @command{gcj} assumes the default encoding to be the @samp{UTF-8} encoding
310 To implement @code{--encoding}, @command{gcj} simply uses the host
311 platform's @code{iconv} conversion routine. This means that in practice
312 @command{gcj} is limited by the capabilities of the host platform.
314 The names allowed for the argument @code{--encoding} vary from platform
315 to platform (since they are not standardized anywhere). However,
316 @command{gcj} implements the encoding named @samp{UTF-8} internally, so if
317 you choose to use this for your source files you can be assured that it
318 will work on every host.
324 @command{gcj} implements several warnings. As with other generic
325 @command{gcc} warnings, if an option of the form @code{-Wfoo} enables a
326 warning, then @code{-Wno-foo} will disable it. Here we've chosen to
327 document the form of the warning which will have an effect -- the
328 default being the opposite of what is listed.
331 @item -Wredundant-modifiers
332 With this flag, @command{gcj} will warn about redundant modifiers. For
333 instance, it will warn if an interface method is declared @code{public}.
335 @item -Wextraneous-semicolon
336 This causes @command{gcj} to warn about empty statements. Empty statements
337 have been deprecated.
339 @item -Wno-out-of-date
340 This option will cause @command{gcj} not to warn when a source file is
341 newer than its matching class file. By default @command{gcj} will warn
344 @item -Wno-deprecated
345 Warn if a deprecated class, method, or field is referred to.
348 This is the same as @command{gcc}'s @code{-Wunused}.
351 This is the same as @code{-Wredundant-modifiers -Wextraneous-semicolon
356 @node Code Generation
357 @section Code Generation
359 In addition to the many @command{gcc} options controlling code generation,
360 @command{gcj} has several options specific to itself.
363 @item --main=@var{CLASSNAME}
364 This option is used when linking to specify the name of the class whose
365 @code{main} method should be invoked when the resulting executable is
366 run. @footnote{The linker by default looks for a global function named
367 @code{main}. Since Java does not have global functions, and a
368 collection of Java classes may have more than one class with a
369 @code{main} method, you need to let the linker know which of those
370 @code{main} methods it should invoke when starting the application.}
372 @item -D@var{name}[=@var{value}]
373 This option can only be used with @code{--main}. It defines a system
374 property named @var{name} with value @var{value}. If @var{value} is not
375 specified then it defaults to the empty string. These system properties
376 are initialized at the program's startup and can be retrieved at runtime
377 using the @code{java.lang.System.getProperty} method.
380 This option is used to tell @command{gcj} to generate bytecode
381 (@file{.class} files) rather than object code.
383 @item --resource @var{resource-name}
384 This option is used to tell @command{gcj} to compile the contents of a
385 given file to object code so it may be accessed at runtime with the core
386 protocol handler as @samp{core:/@var{resource-name}}. Note that
387 @var{resource-name} is the name of the resource as found at runtime; for
388 instance, it could be used in a call to @code{ResourceBundle.getBundle}.
389 The actual file name to be compiled this way must be specified
392 @item -d @var{directory}
393 When used with @code{-C}, this causes all generated @file{.class} files
394 to be put in the appropriate subdirectory of @var{directory}. By
395 default they will be put in subdirectories of the current working
398 @item -fno-bounds-check
399 By default, @command{gcj} generates code which checks the bounds of all
400 array indexing operations. With this option, these checks are omitted, which
401 can improve performance for code that uses arrays extensively. Note that this
402 can result in unpredictable behavior if the code in question actually does
403 violate array bounds constraints. It is safe to use this option if you are
404 sure that your code will never throw an @code{ArrayIndexOutOfBoundsException}.
406 @item -fno-store-check
407 Don't generate array store checks. When storing objects into arrays, a runtime
408 check is normally generated in order to ensure that the object is assignment
409 compatible with the component type of the array (which may not be known
410 at compile-time). With this option, these checks are omitted. This can
411 improve performance for code which stores objects into arrays frequently.
412 It is safe to use this option if you are sure your code will never throw an
413 @code{ArrayStoreException}.
416 With @command{gcj} there are two options for writing native methods: CNI
417 and JNI@. By default @command{gcj} assumes you are using CNI@. If you are
418 compiling a class with native methods, and these methods are implemented
419 using JNI, then you must use @code{-fjni}. This option causes
420 @command{gcj} to generate stubs which will invoke the underlying JNI
424 Don't recognize the @code{assert} keyword. This is for compatibility
425 with older versions of the language specification.
427 @item -fno-optimize-static-class-initialization
428 When the optimization level is greater or equal to @code{-O2},
429 @command{gcj} will try to optimize the way calls into the runtime are made
430 to initialize static classes upon their first use (this optimization
431 isn't carried out if @code{-C} was specified.) When compiling to native
432 code, @code{-fno-optimize-static-class-initialization} will turn this
433 optimization off, regardless of the optimization level in use.
437 @node Configure-time Options
438 @section Configure-time Options
440 Some @command{gcj} code generations options affect the resulting ABI, and
441 so can only be meaningfully given when @code{libgcj}, the runtime
442 package, is configured. @code{libgcj} puts the appropriate options from
443 this group into a @samp{spec} file which is read by @command{gcj}. These
444 options are listed here for completeness; if you are using @code{libgcj}
445 then you won't want to touch these options.
449 This enables the use of the Boehm GC bitmap marking code. In particular
450 this causes @command{gcj} to put an object marking descriptor into each
453 @item -fhash-synchronization
454 By default, synchronization data (the data used for @code{synchronize},
455 @code{wait}, and @code{notify}) is pointed to by a word in each object.
456 With this option @command{gcj} assumes that this information is stored in a
457 hash table and not in the object itself.
459 @item -fuse-divide-subroutine
460 On some systems, a library routine is called to perform integer
461 division. This is required to get exception handling correct when
464 @item -fcheck-references
465 On some systems it's necessary to insert inline checks whenever
466 accessing an object via a reference. On other systems you won't need
467 this because null pointer accesses are caught automatically by the
474 @chapter Compatibility with the Java Platform
476 As we believe it is important that the Java platform not be fragmented,
477 @command{gcj} and @code{libgcj} try to conform to the relevant Java
478 specifications. However, limited manpower and incomplete and unclear
479 documentation work against us. So, there are caveats to using
488 @section Standard features not yet supported
490 This list of compatibility issues is by no means complete.
494 @command{gcj} implements the JDK 1.2 language. It supports inner classes
495 and the new 1.4 @code{assert} keyword. It does not yet support the Java 2
496 @code{strictfp} keyword (it recognizes the keyword but ignores it).
499 @code{libgcj} is largely compatible with the JDK 1.2 libraries.
500 However, @code{libgcj} is missing many packages, most notably
501 @code{java.awt}. There are also individual missing classes and methods.
502 We currently do not have a list showing differences between
503 @code{libgcj} and the Java 2 platform.
506 Sometimes the @code{libgcj} implementation of a method or class differs
507 from the JDK implementation. This is not always a bug. Still, if it
508 affects you, it probably makes sense to report it so that we can discuss
509 the appropriate response.
512 @command{gcj} does not currently allow for piecemeal replacement of
513 components within @code{libgcj}. Unfortunately, programmers often want
514 to use newer versions of certain packages, such as those provided by
515 the Apache Software Foundation's Jakarta project. This has forced us
516 to place the @code{org.w3c.dom} and @code{org.xml.sax} packages into
517 their own libraries, separate from @code{libgcj}. If you intend to
518 use these classes, you must link them explicitly with
519 @code{-l-org-w3c-dom} and @code{-l-org-xml-sax}. Future versions of
520 @command{gcj} may not have this restriction.
524 @section Extra features unique to gcj
526 The main feature of @command{gcj} is that it can compile programs written in
527 the Java programming language to native code. Most extensions that have been
528 added are to facilitate this functionality.
532 @command{gcj} makes it easy and efficient to mix code written in Java and C++.
533 @xref{About CNI}, for more info on how to use this in your programs.
536 When you compile your classes into a shared library they can be automatically
537 loaded by the @code{libgcj} system classloader. When trying to load a class
538 @code{gnu.pkg.SomeClass} the system classloader will first try to load the
539 shared library @file{lib-gnu-pkg-SomeClass.so}, if that fails to load the
540 class then it will try to load @file{lib-gnu-pkg.so} and finally when the
541 class is still not loaded it will try to load @file{lib-gnu.so}. Note that
542 all @samp{.}s will be transformed into @samp{-}s and that searching
543 for inner classes starts with their outermost outer class. If the class
544 cannot be found this way the system classloader tries to use
545 the @code{libgcj} bytecode interpreter to load the class from the standard
546 classpath. This process can be controlled to some degree via the
547 @code{gnu.gcj.runtime.VMClassLoader.library_control} property;
548 @xref{libgcj Runtime Properties}.
552 @code{libgcj} includes a special @samp{gcjlib} URL type. A URL of
553 this form is like a @code{jar} URL, and looks like
554 @samp{gcjlib:/path/to/shared/library.so!/path/to/resource}. An access
555 to one of these URLs causes the shared library to be @code{dlopen()}d,
556 and then the resource is looked for in that library. These URLs are
557 most useful when used in conjunction with @code{java.net.URLClassLoader}.
558 Note that, due to implementation limitations, currently any such URL
559 can be accessed by only one class loader, and libraries are never
560 unloaded. This means some care must be exercised to make sure that
561 a @code{gcjlib} URL is not accessed by more than one class loader at once.
562 In a future release this limitation will be lifted, and such
563 libraries will be mapped privately.
567 @chapter Invoking gcjh
569 @c man title gcjh generate header files from Java class files
571 @c man begin DESCRIPTION gcjh
573 The @code{gcjh} program is used to generate header files from class
574 files. It can generate both CNI and JNI header files, as well as stub
575 implementation files which can be used as a basis for implementing the
576 required native methods.
581 @c man begin SYNOPSIS gcjh
582 gcjh [@option{-stubs}] [@option{-jni}]
583 [@option{-add} @var{text}] [@option{-append} @var{text}] [@option{-friend} @var{text}]
584 [@option{-preprend} @var{text}]
585 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
586 [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
587 [@option{-o} @var{file}] [@option{-td} @var{dir}]
588 [@option{-M}] [@option{-MM}] [@option{-MD}] [@option{-MMD}]
589 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
590 @var{classname}@dots{}
592 @c man begin SEEALSO gcjh
593 gcc(1), gcj(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
594 and the Info entries for @file{gcj} and @file{gcc}.
598 @c man begin OPTIONS gcjh
602 This causes @code{gcjh} to generate stub files instead of header files.
603 By default the stub file will be named after the class, with a suffix of
604 @samp{.cc}. In JNI mode, the default output file will have the suffix
608 This tells @code{gcjh} to generate a JNI header or stub. By default,
609 CNI headers are generated.
611 @item -add @var{text}
612 Inserts @var{text} into the class body. This is ignored in JNI mode.
614 @item -append @var{text}
615 Inserts @var{text} into the header file after the class declaration.
616 This is ignored in JNI mode.
618 @item -friend @var{text}
619 Inserts @var{text} into the class as a @code{friend} declaration.
620 This is ignored in JNI mode.
622 @item -prepend @var{text}
623 Inserts @var{text} into the header file before the class declaration.
624 This is ignored in JNI mode.
626 @item --classpath=@var{path}
627 @itemx --CLASSPATH=@var{path}
628 @itemx -I@var{directory}
629 @itemx -d @var{directory}
631 These options are all identical to the corresponding @command{gcj} options.
634 Sets the output file name. This cannot be used if there is more than
635 one class on the command line.
637 @item -td @var{directory}
638 Sets the name of the directory to use for temporary files.
641 Print all dependencies to stdout; suppress ordinary output.
644 Print non-system dependencies to stdout; suppress ordinary output.
647 Print all dependencies to stdout.
650 Print non-system dependencies to stdout.
653 Print help about @code{gcjh} and exit. No further processing is done.
656 Print version information for @code{gcjh} and exit. No further
660 Print extra information while running.
663 All remaining options are considered to be names of classes.
667 @node Invoking jv-scan
668 @chapter Invoking jv-scan
670 @c man title jv-scan print information about Java source file
672 @c man begin DESCRIPTION jv-scan
674 The @code{jv-scan} program can be used to print information about a Java
675 source file (@file{.java} file).
680 @c man begin SYNOPSIS jv-scan
681 jv-scan [@option{--no-assert}] [@option{--complexity}]
682 [@option{--encoding}=@var{name}] [@option{--print-main}]
683 [@option{--list-class}] [@option{--list-filename}]
684 [@option{--version}] [@option{--help}]
685 [@option{-o} @var{file}] @var{inputfile}@dots{}
687 @c man begin SEEALSO jv-scan
688 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
689 and the Info entries for @file{gcj} and @file{gcc}.
693 @c man begin OPTIONS jv-scan
697 Don't recognize the @code{assert} keyword, for backwards compatibility
698 with older versions of the language specification.
701 This prints a complexity measure, related to cyclomatic complexity, for
704 @item --encoding=@var{name}
705 This works like the corresponding @command{gcj} option.
708 This prints the name of the class in this file containing a @code{main}
712 This lists the names of all classes defined in the input files.
714 @item --list-filename
715 If @code{--list-class} is given, this option causes @code{jv-scan} to
716 also print the name of the file in which each class was found.
719 Print output to the named file.
722 Print help, then exit.
725 Print version number, then exit.
730 @node Invoking jcf-dump
731 @chapter Invoking jcf-dump
733 @c man title jcf-dump print information about Java class files
736 @c man begin SYNOPSIS jcf-dump
737 jcf-dump [@option{-c}] [@option{--javap}]
738 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
739 [@option{-I}@var{dir}@dots{}] [@option{-o} @var{file}]
740 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
741 @var{classname}@dots{}
743 @c man begin SEEALSO jcf-dump
744 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
745 and the Info entries for @file{gcj} and @file{gcc}.
749 @c man begin DESCRIPTION jcf-dump
751 This is a class file examiner, similar to @code{javap}. It will print
752 information about a number of classes, which are specified by class name
757 @c man begin OPTIONS jcf-dump
761 Disassemble method bodies. By default method bodies are not printed.
764 Generate output in @code{javap} format. The implementation of this
765 feature is very incomplete.
767 @item --classpath=@var{path}
768 @itemx --CLASSPATH=@var{path}
769 @itemx -I@var{directory}
771 These options as the same as the corresponding @command{gcj} options.
774 Print help, then exit.
777 Print version number, then exit.
780 Print extra information while running.
786 @chapter Invoking gij
788 @c man title gij GNU interpreter for Java bytecode
791 @c man begin SYNOPSIS gij
792 gij [@option{OPTION}] @dots{} @var{JARFILE} [@var{ARGS}@dots{}]
794 gij [@option{-jar}] [@option{OPTION}] @dots{} @var{CLASS} [@var{ARGS}@dots{}]
795 [@option{-cp} @var{path}] [@option{-classpath} @var{path}]
796 [@option{-D}@var{name}[=@var{value}]@dots{}]
797 [@option{-ms=}@var{number}] [@option{-mx=}@var{number}]
798 [@option{-X@var{argument}]
799 [@option{--showversion}] [@option{--version}] [@option{--help}][@option{-?}]
801 @c man begin SEEALSO gij
802 gcc(1), gcj(1), gcjh(1), jv-scan(1), jcf-dump(1), gfdl(7),
803 and the Info entries for @file{gcj} and @file{gcc}.
807 @c man begin DESCRIPTION gij
809 @code{gij} is a Java bytecode interpreter included with @code{libgcj}.
810 @code{gij} is not available on every platform; porting it requires a
811 small amount of assembly programming which has not been done for all the
812 targets supported by @command{gcj}.
814 The primary argument to @code{gij} is the name of a class or, with
815 @code{-jar}, a jar file. Options before this argument are interpreted
816 by @code{gij}; remaining options are passed to the interpreted program.
818 If a class name is specified and this class does not have a @code{main}
819 method with the appropriate signature (a @code{static void} method with
820 a @code{String[]} as its sole argument), then @code{gij} will print an
823 If a jar file is specified then @code{gij} will use information in it to
824 determine which class' @code{main} method will be invoked.
826 @code{gij} will invoke the @code{main} method with all the remaining
827 command-line options.
829 Note that @code{gij} is not limited to interpreting code. Because
830 @code{libgcj} includes a class loader which can dynamically load shared
831 objects, it is possible to give @code{gij} the name of a class which has
832 been compiled and put into a shared library on the class path.
836 @c man begin OPTIONS gij
840 @itemx -classpath @var{path}
841 Set the initial class path. The class path is used for finding
842 class and resource files. If specified, this option overrides the
843 @code{CLASSPATH} environment variable. Note that this option is
844 ignored if @code{-jar} is used.
846 @item -D@var{name}[=@var{value}]
847 This defines a system property named @var{name} with value @var{value}.
848 If @var{value} is not specified then it defaults to the empty string.
849 These system properties are initialized at the program's startup and can
850 be retrieved at runtime using the @code{java.lang.System.getProperty}
853 @item -ms=@var{number}
854 This sets the initial heap size.
856 @item -mx=@var{number}
857 This sets the maximum heap size.
860 @itemx -X@var{argument}
861 Supplying @code{-X} by itself will cause @code{gij} to list all the
862 supported @code{-X} options. Currently there are none. Unrecognized
863 @code{-X} options are ignored, for compatibility with other runtimes.
866 This indicates that the name passed to @code{gij} should be interpreted
867 as the name of a jar file, not a class.
871 Print help, then exit.
874 Print version number and continue.
877 Print version number, then exit.
882 @node Invoking jv-convert
883 @chapter Invoking jv-convert
885 @c man title jv-convert Convert file from one encoding to another
887 @c man begin SYNOPSIS jv-convert
888 @command{jv-convert} [@option{OPTION}] @dots{} [@var{INPUTFILE} [@var{OUTPUTFILE}]]
891 [@option{--encoding} @var{name}]
892 [@option{--from} @var{name}]
893 [@option{--to} @var{name}]
894 [@option{-i} @var{file}] [@option{-o} @var{file}]
895 [@option{--reverse}] [@option{--help}] [@option{--version}]
899 @c man begin DESCRIPTION jv-convert
901 @command{jv-convert} is a utility included with @code{libgcj} which
902 converts a file from one encoding to another. It is similar to the Unix
903 @command{iconv} utility.
905 The encodings supported by @command{jv-convert} are platform-dependent.
906 Currently there is no way to get a list of all supported encodings.
910 @c man begin OPTIONS jv-convert
913 @item --encoding @var{name}
914 @itemx --from @var{name}
915 Use @var{name} as the input encoding. The default is the current
918 @item --to @var{name}
919 Use @var{name} as the output encoding. The default is the
920 @code{JavaSrc} encoding; this is ASCII with @samp{\u} escapes for
921 non-ASCII characters.
924 Read from @var{file}. The default is to read from standard input.
927 Write to @var{file}. The default is to write to standard output.
930 Swap the input and output encodings.
933 Print a help message, then exit.
936 Print version information, then exit.
942 @chapter Invoking rmic
944 @c man title rmic Generate stubs for Remote Method Invocation
946 @c man begin SYNOPSIS rmic
947 @command{rmic} [@option{OPTION}] @dots{} @var{class} @dots{}
950 [@option{-keepgenerated}]
954 [@option{-nocompile}]
956 [@option{-d} @var{directory}]
962 @c man begin DESCRIPTION rmic
964 @command{rmic} is a utility included with @code{libgcj} which generates
965 stubs for remote objects.
967 @c FIXME: Add real information here.
968 @c This really isn't much more than the --help output.
970 Note that this program isn't yet fully compatible with the JDK
971 @command{rmic}. Some options, such as @option{-classpath}, are
972 recognized but currently ignored. We have left these options
973 undocumented for now.
975 Long options can also be given with a GNU-style leading @samp{--}. For
976 instance, @option{--help} is accepted.
980 @c man begin OPTIONS rmic
984 @itemx -keepgenerated
985 By default, @command{rmic} deletes intermediate files. Either of these
986 options causes it not to delete such files.
989 Cause @command{rmic} to create stubs and skeletons for the 1.1
993 Cause @command{rmic} to create stubs and skeletons compatible with both
994 the 1.1 and 1.2 protocol versions. This is the default.
997 Cause @command{rmic} to create stubs and skeletons for the 1.2
1001 Don't compile the generated files.
1004 Print information about what @command{rmic} is doing.
1006 @item -d @var{directory}
1007 Put output files in @var{directory}. By default the files are put in
1008 the current working directory.
1011 Print a help message, then exit.
1014 Print version information, then exit.
1020 @node Invoking rmiregistry
1021 @chapter Invoking rmiregistry
1023 @c man title rmiregistry Remote object registry
1025 @c man begin SYNOPSIS rmiregistry
1026 @command{rmic} [@option{OPTION}] @dots{} [@var{port}]
1029 [@option{--version}]
1033 @c man begin DESCRIPTION rmiregistry
1035 @command{rmiregistry} starts a remote object registry on the current
1036 host. If no port number is specified, then port 1099 is used.
1038 @c FIXME: Add real information here.
1039 @c This really isn't much more than the --help output.
1043 @c man begin OPTIONS rmiregistry
1047 Print a help message, then exit.
1050 Print version information, then exit.
1059 This documents CNI, the Compiled Native Interface,
1060 which is is a convenient way to write Java native methods using C++.
1061 This is a more efficient, more convenient, but less portable
1062 alternative to the standard JNI (Java Native Interface).
1065 * Basic concepts:: Introduction to using CNI@.
1066 * Packages:: How packages are mapped to C++.
1067 * Primitive types:: Handling Java types in C++.
1068 * Interfaces:: How Java interfaces map to C++.
1069 * Objects and Classes:: C++ and Java classes.
1070 * Class Initialization:: How objects are initialized.
1071 * Object allocation:: How to create Java objects in C++.
1072 * Arrays:: Dealing with Java arrays in C++.
1073 * Methods:: Java methods in C++.
1074 * Strings:: Information about Java Strings.
1075 * Mixing with C++:: How CNI can interoperate with C++.
1076 * Exception Handling:: How exceptions are handled.
1077 * Synchronization:: Synchronizing between Java and C++.
1078 * Invocation:: Starting the Java runtime from C++.
1079 * Reflection:: Using reflection from C++.
1083 @node Basic concepts
1084 @section Basic concepts
1086 In terms of languages features, Java is mostly a subset
1087 of C++. Java has a few important extensions, plus a powerful standard
1088 class library, but on the whole that does not change the basic similarity.
1089 Java is a hybrid object-oriented language, with a few native types,
1090 in addition to class types. It is class-based, where a class may have
1091 static as well as per-object fields, and static as well as instance methods.
1092 Non-static methods may be virtual, and may be overloaded. Overloading is
1093 resolved at compile time by matching the actual argument types against
1094 the parameter types. Virtual methods are implemented using indirect calls
1095 through a dispatch table (virtual function table). Objects are
1096 allocated on the heap, and initialized using a constructor method.
1097 Classes are organized in a package hierarchy.
1099 All of the listed attributes are also true of C++, though C++ has
1100 extra features (for example in C++ objects may be allocated not just
1101 on the heap, but also statically or in a local stack frame). Because
1102 @command{gcj} uses the same compiler technology as G++ (the GNU
1103 C++ compiler), it is possible to make the intersection of the two
1104 languages use the same ABI (object representation and calling
1105 conventions). The key idea in CNI is that Java objects are C++
1106 objects, and all Java classes are C++ classes (but not the other way
1107 around). So the most important task in integrating Java and C++ is to
1108 remove gratuitous incompatibilities.
1110 You write CNI code as a regular C++ source file. (You do have to use
1111 a Java/CNI-aware C++ compiler, specifically a recent version of G++.)
1113 @noindent A CNI C++ source file must have:
1116 #include <gcj/cni.h>
1119 @noindent and then must include one header file for each Java class it uses, e.g.:
1122 #include <java/lang/Character.h>
1123 #include <java/util/Date.h>
1124 #include <java/lang/IndexOutOfBoundsException.h>
1127 @noindent These header files are automatically generated by @code{gcjh}.
1130 CNI provides some functions and macros to make using Java objects and
1131 primitive types from C++ easier. In general, these CNI functions and
1132 macros start with the @code{Jv} prefix, for example the function
1133 @code{JvNewObjectArray}. This convention is used to avoid conflicts
1134 with other libraries. Internal functions in CNI start with the prefix
1135 @code{_Jv_}. You should not call these; if you find a need to, let us
1136 know and we will try to come up with an alternate solution. (This
1137 manual lists @code{_Jv_AllocBytes} as an example; CNI should instead
1138 provide a @code{JvAllocBytes} function.)
1141 @subsection Limitations
1143 Whilst a Java class is just a C++ class that doesn't mean that you are
1144 freed from the shackles of Java, a @acronym{CNI} C++ class must adhere to the
1145 rules of the Java programming language.
1147 For example: it is not possible to declare a method in a CNI class
1148 that will take a C string (@code{char*}) as an argument, or to declare a
1149 member variable of some non-Java datatype.
1155 The only global names in Java are class names, and packages. A
1156 @dfn{package} can contain zero or more classes, and also zero or more
1157 sub-packages. Every class belongs to either an unnamed package or a
1158 package that has a hierarchical and globally unique name.
1160 A Java package is mapped to a C++ @dfn{namespace}. The Java class
1161 @code{java.lang.String} is in the package @code{java.lang}, which is a
1162 sub-package of @code{java}. The C++ equivalent is the class
1163 @code{java::lang::String}, which is in the namespace @code{java::lang}
1164 which is in the namespace @code{java}.
1166 @noindent Here is how you could express this:
1169 (// @r{Declare the class(es), possibly in a header file:}
1178 class java::lang::String : public java::lang::Object
1184 @noindent The @code{gcjh} tool automatically generates the necessary namespace
1188 @subsection Leaving out package names
1190 Always using the fully-qualified name of a java class can be
1191 tiresomely verbose. Using the full qualified name also ties the code
1192 to a single package making code changes necessary should the class
1193 move from one package to another. The Java @code{package} declaration
1194 specifies that the following class declarations are in the named
1195 package, without having to explicitly name the full package
1196 qualifiers. The @code{package} declaration can be
1197 followed by zero or more @code{import} declarations, which
1198 allows either a single class or all the classes in a package to be
1199 named by a simple identifier. C++ provides something similar with the
1200 @code{using} declaration and directive.
1205 import @var{package-name}.@var{class-name};
1208 @noindent allows the program text to refer to @var{class-name} as a shorthand for
1209 the fully qualified name: @code{@var{package-name}.@var{class-name}}.
1212 @noindent To achieve the same effect C++, you have to do this:
1215 using @var{package-name}::@var{class-name};
1219 @noindent Java can also cause imports on demand, like this:
1222 import @var{package-name}.*;
1225 @noindent Doing this allows any class from the package @var{package-name} to be
1226 referred to only by its class-name within the program text.
1229 @noindent The same effect can be achieved in C++ like this:
1232 using namespace @var{package-name};
1236 @node Primitive types
1237 @section Primitive types
1239 Java provides 8 @dfn{primitives} types which represent integers, floats,
1240 characters and booleans (and also the void type). C++ has its own
1241 very similar concrete types. Such types in C++ however are not always
1242 implemented in the same way (an int might be 16, 32 or 64 bits for example)
1243 so CNI provides a special C++ type for each primitive Java type:
1245 @multitable @columnfractions .20 .25 .60
1246 @item @strong{Java type} @tab @strong{C/C++ typename} @tab @strong{Description}
1247 @item @code{char} @tab @code{jchar} @tab 16 bit Unicode character
1248 @item @code{boolean} @tab @code{jboolean} @tab logical (true or false) values
1249 @item @code{byte} @tab @code{jbyte} @tab 8-bit signed integer
1250 @item @code{short} @tab @code{jshort} @tab 16 bit signed integer
1251 @item @code{int} @tab @code{jint} @tab 32 bit signed integer
1252 @item @code{long} @tab @code{jlong} @tab 64 bit signed integer
1253 @item @code{float} @tab @code{jfloat} @tab 32 bit IEEE floating point number
1254 @item @code{double} @tab @code{jdouble} @tab 64 bit IEEE floating point number
1255 @item @code{void} @tab @code{void} @tab no value
1258 When referring to a Java type You should always use these C++ typenames (e.g.: @code{jint})
1259 to avoid disappointment.
1262 @subsection Reference types associated with primitive types
1264 In Java each primitive type has an associated reference type,
1265 e.g.: @code{boolean} has an associated @code{java.lang.Boolean} class.
1266 In order to make working with such classes easier GCJ provides the macro
1269 @deffn macro JvPrimClass type
1270 Return a pointer to the @code{Class} object corresponding to the type supplied.
1273 JvPrimClass(void) @result{} java.lang.Void.TYPE
1282 A Java class can @dfn{implement} zero or more
1283 @dfn{interfaces}, in addition to inheriting from
1284 a single base class.
1286 @acronym{CNI} allows CNI code to implement methods of interfaces.
1287 You can also call methods through interface references, with some
1290 @acronym{CNI} doesn't understand interface inheritance at all yet. So,
1291 you can only call an interface method when the declared type of the
1292 field being called matches the interface which declares that
1293 method. The workaround is to cast the interface reference to the right
1296 For example if you have:
1304 interface B extends A
1310 and declare a variable of type @code{B} in C++, you can't call
1311 @code{a()} unless you cast it to an @code{A} first.
1313 @node Objects and Classes
1314 @section Objects and Classes
1318 All Java classes are derived from @code{java.lang.Object}. C++ does
1319 not have a unique root class, but we use the C++ class
1320 @code{java::lang::Object} as the C++ version of the
1321 @code{java.lang.Object} Java class. All other Java classes are mapped
1322 into corresponding C++ classes derived from @code{java::lang::Object}.
1324 Interface inheritance (the @code{implements} keyword) is currently not
1325 reflected in the C++ mapping.
1328 @subsection Object fields
1330 Each object contains an object header, followed by the instance fields
1331 of the class, in order. The object header consists of a single
1332 pointer to a dispatch or virtual function table. (There may be extra
1333 fields @emph{in front of} the object, for example for memory
1334 management, but this is invisible to the application, and the
1335 reference to the object points to the dispatch table pointer.)
1337 The fields are laid out in the same order, alignment, and size as in
1338 C++. Specifically, 8-bite and 16-bit native types (@code{byte},
1339 @code{short}, @code{char}, and @code{boolean}) are @emph{not} widened
1340 to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types
1341 to 32 bits when on the VM stack or temporary registers.
1343 If you include the @code{gcjh}-generated header for a
1344 class, you can access fields of Java classes in the @emph{natural}
1345 way. For example, given the following Java class:
1351 public Integer (int i) @{ this.i = i; @}
1352 public static zero = new Integer(0);
1359 #include <gcj/cni.h>;
1363 mult (Int *p, jint k)
1366 return Int::zero; // @r{Static member access.}
1367 return new Int(p->i * k);
1372 @subsection Access specifiers
1374 CNI does not strictly enforce the Java access
1375 specifiers, because Java permissions cannot be directly mapped
1376 into C++ permission. Private Java fields and methods are mapped
1377 to private C++ fields and methods, but other fields and methods
1378 are mapped to public fields and methods.
1382 @node Class Initialization
1383 @section Class Initialization
1385 Java requires that each class be automatically initialized at the time
1386 of the first active use. Initializing a class involves
1387 initializing the static fields, running code in class initializer
1388 methods, and initializing base classes. There may also be
1389 some implementation specific actions, such as allocating
1390 @code{String} objects corresponding to string literals in
1393 The GCJ compiler inserts calls to @code{JvInitClass} at appropriate
1394 places to ensure that a class is initialized when required. The C++
1395 compiler does not insert these calls automatically---it is the
1396 programmer's responsibility to make sure classes are initialized.
1397 However, this is fairly painless because of the conventions assumed by
1400 First, @code{libgcj} will make sure a class is initialized before an
1401 instance of that object is created. This is one of the
1402 responsibilities of the @code{new} operation. This is taken care of
1403 both in Java code, and in C++ code. When G++ sees a @code{new} of a
1404 Java class, it will call a routine in @code{libgcj} to allocate the
1405 object, and that routine will take care of initializing the class.
1406 Note however that this does not happen for Java arrays; you must
1407 allocate those using the appropriate CNI function. It follows that
1408 you can access an instance field, or call an instance (non-static)
1409 method and be safe in the knowledge that the class and all of its base
1410 classes have been initialized.
1412 Invoking a static method is also safe. This is because the
1413 Java compiler adds code to the start of a static method to make sure
1414 the class is initialized. However, the C++ compiler does not
1415 add this extra code. Hence, if you write a native static method
1416 using CNI, you are responsible for calling @code{JvInitClass}
1417 before doing anything else in the method (unless you are sure
1418 it is safe to leave it out).
1420 Accessing a static field also requires the class of the
1421 field to be initialized. The Java compiler will generate code
1422 to call @code{Jv_InitClass} before getting or setting the field.
1423 However, the C++ compiler will not generate this extra code,
1424 so it is your responsibility to make sure the class is
1425 initialized before you access a static field from C++.
1428 @node Object allocation
1429 @section Object allocation
1431 New Java objects are allocated using a
1432 @dfn{class instance creation expression}, e.g.:
1435 new @var{Type} ( ... )
1438 The same syntax is used in C++. The main difference is that
1439 C++ objects have to be explicitly deleted; in Java they are
1440 automatically deleted by the garbage collector.
1441 Using @acronym{CNI}, you can allocate a new Java object
1442 using standard C++ syntax and the C++ compiler will allocate
1443 memory from the garbage collector. If you have overloaded
1444 constructors, the compiler will choose the correct one
1445 using standard C++ overload resolution rules.
1447 @noindent For example:
1450 java::util::Hashtable *ht = new java::util::Hashtable(120);
1453 @deftypefun void* _Jv_AllocBytes (jsize @var{size})
1454 Allocates @var{size} bytes from the heap. The memory is not scanned
1455 by the garbage collector but it freed if no references to it are discovered.
1462 While in many ways Java is similar to C and C++, it is quite different
1463 in its treatment of arrays. C arrays are based on the idea of pointer
1464 arithmetic, which would be incompatible with Java's security
1465 requirements. Java arrays are true objects (array types inherit from
1466 @code{java.lang.Object}). An array-valued variable is one that
1467 contains a reference (pointer) to an array object.
1469 Referencing a Java array in C++ code is done using the
1470 @code{JArray} template, which as defined as follows:
1473 class __JArray : public java::lang::Object
1480 class JArray : public __JArray
1484 T& operator[](jint i) @{ return data[i]; @}
1489 There are a number of @code{typedef}s which correspond to @code{typedef}s
1490 from the @acronym{JNI}. Each is the type of an array holding objects
1491 of the relevant type:
1494 typedef __JArray *jarray;
1495 typedef JArray<jobject> *jobjectArray;
1496 typedef JArray<jboolean> *jbooleanArray;
1497 typedef JArray<jbyte> *jbyteArray;
1498 typedef JArray<jchar> *jcharArray;
1499 typedef JArray<jshort> *jshortArray;
1500 typedef JArray<jint> *jintArray;
1501 typedef JArray<jlong> *jlongArray;
1502 typedef JArray<jfloat> *jfloatArray;
1503 typedef JArray<jdouble> *jdoubleArray;
1507 @deftypemethod {template<class T>} T* elements (JArray<T> @var{array})
1508 This template function can be used to get a pointer to the elements of
1509 the @code{array}. For instance, you can fetch a pointer to the
1510 integers that make up an @code{int[]} like so:
1513 extern jintArray foo;
1514 jint *intp = elements (foo);
1517 The name of this function may change in the future.
1521 @deftypefun jobjectArray JvNewObjectArray (jsize @var{length}, jclass @var{klass}, jobject @var{init})
1522 Here @code{klass} is the type of elements of the array and
1523 @code{init} is the initial value put into every slot in the array.
1527 @subsection Creating arrays
1529 For each primitive type there is a function which can be used to
1530 create a new array of that type. The name of the function is of the
1534 JvNew@var{Type}Array
1537 @noindent For example:
1543 @noindent can be used to create an array of Java primitive boolean types.
1545 @noindent The following function definition is the template for all such functions:
1547 @deftypefun jbooleanArray JvNewBooleanArray (jint @var{length})
1548 Create's an array @var{length} indices long.
1551 @deftypefun jsize JvGetArrayLength (jarray @var{array})
1552 Returns the length of the @var{array}.
1559 Java methods are mapped directly into C++ methods.
1560 The header files generated by @code{gcjh}
1561 include the appropriate method definitions.
1562 Basically, the generated methods have the same names and
1563 @emph{corresponding} types as the Java methods,
1564 and are called in the natural manner.
1566 @subsection Overloading
1568 Both Java and C++ provide method overloading, where multiple
1569 methods in a class have the same name, and the correct one is chosen
1570 (at compile time) depending on the argument types.
1571 The rules for choosing the correct method are (as expected) more complicated
1572 in C++ than in Java, but given a set of overloaded methods
1573 generated by @code{gcjh} the C++ compiler will choose
1576 Common assemblers and linkers are not aware of C++ overloading,
1577 so the standard implementation strategy is to encode the
1578 parameter types of a method into its assembly-level name.
1579 This encoding is called @dfn{mangling},
1580 and the encoded name is the @dfn{mangled name}.
1581 The same mechanism is used to implement Java overloading.
1582 For C++/Java interoperability, it is important that both the Java
1583 and C++ compilers use the @emph{same} encoding scheme.
1585 @subsection Static methods
1587 Static Java methods are invoked in @acronym{CNI} using the standard
1588 C++ syntax, using the @code{::} operator rather
1589 than the @code{.} operator.
1591 @noindent For example:
1594 jint i = java::lang::Math::round((jfloat) 2.3);
1597 @noindent C++ method definition syntax is used to define a static native method.
1601 #include <java/lang/Integer>
1602 java::lang::Integer*
1603 java::lang::Integer::getInteger(jstring str)
1610 @subsection Object Constructors
1612 Constructors are called implicitly as part of object allocation
1613 using the @code{new} operator.
1615 @noindent For example:
1618 java::lang::Integer *x = new java::lang::Integer(234);
1621 Java does not allow a constructor to be a native method.
1622 This limitation can be coded round however because a constructor
1623 can @emph{call} a native method.
1626 @subsection Instance methods
1628 Calling a Java instance method from a C++ @acronym{CNI} method is done
1629 using the standard C++ syntax, e.g.:
1632 // @r{First create the Java object.}
1633 java::lang::Integer *x = new java::lang::Integer(234);
1634 // @r{Now call a method.}
1635 jint prim_value = x->intValue();
1636 if (x->longValue == 0)
1640 @noindent Defining a Java native instance method is also done the natural way:
1643 #include <java/lang/Integer.h>
1646 java::lang:Integer::doubleValue()
1648 return (jdouble) value;
1653 @subsection Interface methods
1655 In Java you can call a method using an interface reference. This is
1656 supported, but not completely. @xref{Interfaces}.
1664 @acronym{CNI} provides a number of utility functions for
1665 working with Java Java @code{String} objects.
1666 The names and interfaces are analogous to those of @acronym{JNI}.
1669 @deftypefun jstring JvNewString (const char* @var{chars}, jsize @var{len})
1670 Returns a Java @code{String} object with characters from the C string
1671 @var{chars} up to the index @var{len} in that array.
1674 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes}, jsize @var{len})
1675 Returns a Java @code{String} made up of @var{len} bytes from @var{bytes}.
1679 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes})
1680 As above but the length of the @code{String} is @code{strlen(@var{bytes})}.
1683 @deftypefun jstring JvNewStringUTF (const char* @var{bytes})
1684 Returns a @code{String} which is made up of the UTF encoded characters
1685 present in the C string @var{bytes}.
1688 @deftypefun jchar* JvGetStringChars (jstring @var{str})
1689 Returns a pointer to an array of characters making up the @code{String} @var{str}.
1692 @deftypefun int JvGetStringUTFLength (jstring @var{str})
1693 Returns the number of bytes required to encode the contents of the
1694 @code{String} @var{str} in UTF-8.
1697 @deftypefun jsize JvGetStringUTFRegion (jstring @var{str}, jsize @var{start}, jsize @var{len}, char* @var{buf})
1698 Puts the UTF-8 encoding of a region of the @code{String} @var{str} into
1699 the buffer @code{buf}. The region to fetch is marked by @var{start} and @var{len}.
1701 Note that @var{buf} is a buffer, not a C string. It is @emph{not}
1706 @node Mixing with C++
1707 @section Interoperating with C/C++
1709 Because @acronym{CNI} is designed to represent Java classes and methods it
1710 cannot be mixed readily with C/C++ types.
1712 One important restriction is that Java classes cannot have non-Java
1713 type instance or static variables and cannot have methods which take
1714 non-Java types as arguments or return non-Java types.
1716 @noindent None of the following is possible with CNI:
1720 class ::MyClass : public java::lang::Object
1722 char* variable; // @r{char* is not a valid Java type.}
1727 ::SomeClass::someMethod (char *arg)
1732 @} // @r{@code{uint} is not a valid Java type, neither is @code{char*}}
1735 @noindent Of course, it is ok to use C/C++ types within the scope of a method:
1740 ::SomeClass::otherMethod (jstring str)
1749 But this restriction can cause a problem so @acronym{CNI} includes the
1750 @code{gnu.gcj.RawData} class. The @code{RawData} class is a
1751 @dfn{non-scanned reference} type. In other words variables declared
1752 of type @code{RawData} can contain any data and are not checked by the
1753 compiler in any way.
1755 This means that you can put C/C++ data structures (including classes)
1756 in your @acronym{CNI} classes, as long as you use the appropriate cast.
1758 @noindent Here are some examples:
1762 class ::MyClass : public java::lang::Object
1764 gnu.gcj.RawData string;
1767 gnu.gcj.RawData getText ();
1771 ::MyClass::MyClass ()
1778 ::MyClass::getText ()
1784 ::MyClass::printText ()
1786 printf("%s\n", (char*) string);
1791 @node Exception Handling
1792 @section Exception Handling
1794 While C++ and Java share a common exception handling framework,
1795 things are not yet perfectly integrated. The main issue is that the
1796 run-time type information facilities of the two
1797 languages are not integrated.
1799 Still, things work fairly well. You can throw a Java exception from
1800 C++ using the ordinary @code{throw} construct, and this
1801 exception can be caught by Java code. Similarly, you can catch an
1802 exception thrown from Java using the C++ @code{catch}
1805 @noindent Here is an example:
1809 throw new java::lang::IndexOutOfBoundsException();
1812 Normally, G++ will automatically detect when you are writing C++
1813 code that uses Java exceptions, and handle them appropriately.
1814 However, if C++ code only needs to execute destructors when Java
1815 exceptions are thrown through it, GCC will guess incorrectly. Sample
1819 struct S @{ ~S(); @};
1821 extern void bar(); // @r{Is implemented in Java and may throw exceptions.}
1830 The usual effect of an incorrect guess is a link failure, complaining of
1831 a missing routine called @code{__gxx_personality_v0}.
1833 You can inform the compiler that Java exceptions are to be used in a
1834 translation unit, irrespective of what it might think, by writing
1835 @code{#pragma GCC java_exceptions} at the head of the
1836 file. This @code{#pragma} must appear before any
1837 functions that throw or catch exceptions, or run destructors when
1838 exceptions are thrown through them.
1840 @node Synchronization
1841 @section Synchronization
1843 Each Java object has an implicit monitor.
1844 The Java VM uses the instruction @code{monitorenter} to acquire
1845 and lock a monitor, and @code{monitorexit} to release it.
1847 The corresponding CNI macros are @code{JvMonitorEnter} and
1848 @code{JvMonitorExit} (JNI has similar methods @code{MonitorEnter}
1849 and @code{MonitorExit}).
1852 The Java source language does not provide direct access to these primitives.
1853 Instead, there is a @code{synchronized} statement that does an
1854 implicit @code{monitorenter} before entry to the block,
1855 and does a @code{monitorexit} on exit from the block.
1856 Note that the lock has to be released even when the block is abnormally
1857 terminated by an exception, which means there is an implicit
1858 @code{try finally} surrounding synchronization locks.
1860 From C++, it makes sense to use a destructor to release a lock.
1861 @acronym{CNI} defines the following utility class:
1864 class JvSynchronize() @{
1866 JvSynchronize(jobject o) @{ obj = o; JvMonitorEnter(o); @}
1867 ~JvSynchronize() @{ JvMonitorExit(obj); @}
1880 @noindent might become this C++ code:
1884 JvSynchronize dummy (OBJ);
1889 Java also has methods with the @code{synchronized} attribute.
1890 This is equivalent to wrapping the entire method body in a
1891 @code{synchronized} statement.
1892 (Alternatively, an implementation could require the caller to do
1893 the synchronization. This is not practical for a compiler, because
1894 each virtual method call would have to test at run-time if
1895 synchronization is needed.) Since in @command{gcj}
1896 the @code{synchronized} attribute is handled by the
1897 method implementation, it is up to the programmer
1898 of a synchronized native method to handle the synchronization
1899 (in the C++ implementation of the method).
1900 In other words, you need to manually add @code{JvSynchronize}
1901 in a @code{native synchronized} method.
1906 CNI permits C++ applications to make calls into Java classes, in addition to
1907 allowing Java code to call into C++. Several functions, known as the
1908 @dfn{invocation API}, are provided to support this.
1910 @deftypefun jint JvCreateJavaVM (void* @var{vm_args})
1911 Initializes the Java runtime. This function performs essential initialization
1912 of the threads interface, garbage collector, exception handling and other key
1913 aspects of the runtime. It must be called once by an application with
1914 a non-Java @code{main()} function, before any other Java or CNI calls are made.
1915 It is safe, but not recommended, to call @code{JvCreateJavaVM()} more than
1916 once provided it is only called from a single thread.
1917 The @var{vmargs} parameter can be used to specify initialization parameters
1918 for the Java runtime. It may be @code{NULL}.
1919 This function returns @code{0} upon success, or @code{-1} if the runtime is
1920 already initialized.
1922 @emph{Note:} In GCJ 3.1, the @code{vm_args} parameter is ignored. It may be
1923 used in a future release.
1926 @deftypefun java::lang::Thread* JvAttachCurrentThread (jstring @var{name}, java::lang::ThreadGroup* @var{group})
1927 Registers an existing thread with the Java runtime. This must be called once
1928 from each thread, before that thread makes any other Java or CNI calls. It
1929 must be called after @code{JvCreateJavaVM}.
1930 @var{name} specifies a name for the thread. It may be @code{NULL}, in which
1931 case a name will be generated.
1932 @var{group} is the ThreadGroup in which this thread will be a member. If it
1933 is @code{NULL}, the thread will be a member of the main thread group.
1934 The return value is the Java @code{Thread} object that represents the thread.
1935 It is safe to call @code{JvAttachCurrentThread()} more than once from the same
1936 thread. If the thread is already attached, the call is ignored and the current
1937 thread object is returned.
1940 @deftypefun jint JvDetachCurrentThread ()
1941 Unregisters a thread from the Java runtime. This should be called by threads
1942 that were attached using @code{JvAttachCurrentThread()}, after they have
1943 finished making calls to Java code. This ensures that any resources associated
1944 with the thread become eligible for garbage collection.
1945 This function returns @code{0} upon success, or @code{-1} if the current thread
1949 @subsection Handling uncaught exceptions
1951 If an exception is thrown from Java code called using the invocation API, and
1952 no handler for the exception can be found, the runtime will abort the
1953 application. In order to make the application more robust, it is recommended
1954 that code which uses the invocation API be wrapped by a top-level try/catch
1955 block that catches all Java exceptions.
1959 The following code demonstrates the use of the invocation API. In this
1960 example, the C++ application initializes the Java runtime and attaches
1961 itself. The @code{java.lang.System} class is initialized in order to
1962 access its @code{out} field, and a Java string is printed. Finally, the thread
1963 is detached from the runtime once it has finished making Java calls. Everything
1964 is wrapped with a try/catch block to provide a default handler for any uncaught
1967 The example can be compiled with @command{c++ test.cc -lgcj}.
1971 #include <gcj/cni.h>
1972 #include <java/lang/System.h>
1973 #include <java/io/PrintStream.h>
1974 #include <java/lang/Throwable.h>
1976 int main(int argc, char *argv)
1978 using namespace java::lang;
1982 JvCreateJavaVM(NULL);
1983 JvAttachCurrentThread(NULL, NULL);
1985 String *message = JvNewStringLatin1("Hello from C++");
1986 JvInitClass(&System::class$);
1987 System::out->println(message);
1989 JvDetachCurrentThread();
1991 catch (Throwable *t)
1993 System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
1994 t->printStackTrace();
2002 Reflection is possible with CNI code, it functions similarly to how it
2003 functions with JNI@.
2005 @c clean this up... I mean, what are the types jfieldID and jmethodID in JNI?
2006 The types @code{jfieldID} and @code{jmethodID}
2009 @noindent The functions:
2012 @item @code{JvFromReflectedField},
2013 @item @code{JvFromReflectedMethod},
2014 @item @code{JvToReflectedField}
2015 @item @code{JvToFromReflectedMethod}
2018 @noindent will be added shortly, as will other functions corresponding to JNI@.
2021 @node System properties
2022 @chapter System properties
2024 The runtime behavior of the @code{libgcj} library can be modified by setting
2025 certain system properties. These properties can be compiled into the program
2026 using the @code{-D@var{name}[=@var{value}]} option to @command{gcj} or by
2027 setting them explicitly in the program by calling the
2028 @code{java.lang.System.setProperty()} method. Some system properties are only
2029 used for informational purposes (like giving a version number or a user name).
2030 A program can inspect the current value of a property by calling the
2031 @code{java.lang.System.getProperty()} method.
2034 * Standard Properties:: Standard properties supported by @code{libgcj}
2035 * GNU Classpath Properties:: Properties found in Classpath based libraries
2036 * libgcj Runtime Properties:: Properties specific to @code{libgcj}
2039 @node Standard Properties
2040 @section Standard Properties
2042 The following properties are normally found in all implementations of the core
2043 libraries for the Java language.
2048 The @code{libgcj} version number.
2051 Set to @samp{The Free Software Foundation, Inc.}
2053 @item java.vendor.url
2054 Set to @uref{http://gcc.gnu.org/java/}.
2057 The directory where @code{gcj} was installed. Taken from the @code{--prefix}
2058 option given to @command{configure}.
2060 @item java.class.version
2061 The class format version number supported by the libgcj byte code interpreter.
2062 (Currently @samp{46.0})
2064 @item java.vm.specification.version
2065 The Virtual Machine Specification version implemented by @code{libgcj}.
2066 (Currently @samp{1.0})
2068 @item java.vm.specification.vendor
2069 The name of the Virtual Machine specification designer.
2071 @item java.vm.specification.name
2072 The name of the Virtual Machine specification
2073 (Set to @samp{Java Virtual Machine Specification}).
2075 @item java.vm.version
2076 The @command{gcj} version number.
2078 @item java.vm.vendor
2079 Set to @samp{The Free Software Foundation, Inc.}
2082 Set to @samp{GNU libgcj}.
2084 @item java.specification.version
2085 The Runtime Environment specification version implemented by @code{libgcj}.
2086 (Currently set to @samp{1.3})
2088 @item java.specification.vendor
2089 The Runtime Environment specification designer.
2091 @item java.specification.name
2092 The name of the Runtime Environment specification
2093 (Set to @samp{Java Platform API Specification}).
2095 @item java.class.path
2096 The paths (jar files, zip files and directories) used for finding class files.
2098 @item java.library.path
2099 Directory path used for finding native libraries.
2101 @item java.io.tmpdir
2102 The directory used to put temporary files in.
2105 Name of the Just In Time compiler to use by the byte code interpreter.
2106 Currently not used in @code{libgcj}.
2109 Directories containing jar files with extra libraries. Will be used when
2110 resolving classes. Currently not used in @code{libgcj}.
2112 @item java.protocol.handler.pkgs
2113 A @samp{|} separated list of package names that is used to find classes that
2114 implement handlers for @code{java.net.URL}.
2116 @item java.rmi.server.codebase
2117 A list of URLs that is used by the @code{java.rmi.server.RMIClassLoader}
2118 to load classes from.
2121 A list of class names that will be loaded by the @code{java.sql.DriverManager}
2124 @item file.separator
2125 The separator used in when directories are included in a filename
2126 (normally @samp{/} or @samp{\} ).
2129 The default character encoding used when converting platform native files to
2130 Unicode (usually set to @samp{8859_1}).
2132 @item path.separator
2133 The standard separator used when a string contains multiple paths
2134 (normally @samp{:} or @samp{;}), the string is usually not a valid character
2135 to use in normal directory names.)
2137 @item line.separator
2138 The default line separator used on the platform (normally @samp{\n}, @samp{\r}
2139 or a combination of those two characters).
2141 @item policy.provider
2142 The class name used for the default policy provider returned by
2143 @code{java.security.Policy.getPolicy}.
2146 The name of the user running the program. Can be the full name, the login name
2147 or empty if unknown.
2150 The default directory to put user specific files in.
2153 The current working directory from which the program was started.
2156 The default language as used by the @code{java.util.Locale} class.
2159 The default region as used by the @code{java.util.Local} class.
2162 The default variant of the language and region local used.
2165 The default timezone as used by the @code{java.util.TimeZone} class.
2168 The operating system/kernel name that the program runs on.
2171 The hardware that we are running on.
2174 The version number of the operating system/kernel.
2176 @item awt.appletWarning
2177 The string to display when an untrusted applet is displayed.
2178 Returned by @code{java.awt.Window.getWarningString()} when the window is
2182 The class name used for initializing the default @code{java.awt.Toolkit}.
2183 Defaults to @code{gnu.awt.gtk.GtkToolkit}.
2185 @item http.proxyHost
2186 Name of proxy host for http connections.
2188 @item http.proxyPort
2189 Port number to use when a proxy host is in use.
2193 @node GNU Classpath Properties
2194 @section GNU Classpath Properties
2196 @code{libgcj} is based on the GNU Classpath (Essential Libraries for Java) a
2197 GNU project to create free core class libraries for use with virtual machines
2198 and compilers for the Java language. The following properties are common to
2199 libraries based on GNU Classpath.
2203 @item gcj.dumpobject
2204 Enables printing serialization debugging by the @code{java.io.ObjectInput} and
2205 @code{java.io.ObjectOutput} classes when set to something else then the empty
2206 string. Only used when running a debug build of the library.
2208 @item gnu.classpath.vm.shortname
2209 This is a succint name of the virtual machine. For @code{libgcj},
2210 this will always be @samp{libgcj}.
2212 @item gnu.classpath.home.url
2213 A base URL used for finding system property files (e.g.,
2214 @file{classpath.security}). By default this is a @samp{file:} URL
2215 pointing to the @file{lib} directory under @samp{java.home}.
2219 @node libgcj Runtime Properties
2220 @section libgcj Runtime Properties
2222 The following properties are specific to the @code{libgcj} runtime and will
2223 normally not be found in other core libraries for the java language.
2227 @item java.fullversion
2228 The combination of @code{java.vm.name} and @code{java.vm.version}.
2231 Same as @code{java.fullversion}.
2234 Used by the @code{java.net.DatagramSocket} class when set to something else
2235 then the empty string. When set all newly created @code{DatagramSocket}s will
2236 try to load a class @code{java.net.[impl.prefix]DatagramSocketImpl} instead of
2237 the normal @code{java.net.PlainDatagramSocketImpl}.
2239 @item gnu.gcj.progname
2240 The name that was used to invoked the program.
2242 @item gnu.gcj.runtime.NameFinder.demangle
2243 Whether names in a stack trace should be demangled. Defaults to @code{true}.
2245 @item gnu.gcj.runtime.NameFinder.sanitize
2246 Whether calls to initialize exceptions and starting the runtime system
2247 should be removed from the stack trace. Only done when names are
2248 demangled. Defaults to @code{true}.
2250 @item gnu.gcj.runtime.NameFinder.remove_unknown
2251 Whether calls to unknown functions (class and method names are unknown)
2252 should be removed from the stack trace. Only done when the stack is
2253 sanitized. Ignored if this means no stack trace information would be
2254 available anymore. Defaults to @code{true}.
2256 @item gnu.gcj.runtime.NameFinder.remove_interpreter
2257 Whether runtime interpreter calls (methods in the @code{_Jv_InterpMethod} class
2258 and functions starting with @samp{ffi_}) should be removed from the stack
2259 trace. Only done when the stack is sanitized. Defaults to @code{true}.
2262 @item gnu.gcj.runtime.NameFinder.use_addr2line
2263 Whether an external process (@command{addr2line} or @command{addr2name.awk})
2264 should be used as fallback to convert the addresses to function names when
2265 the runtime is unable to do it through @code{dladdr}.
2267 @item gnu.gcj.runtime.VMClassLoader.library_control
2268 This controls how shared libraries are automatically loaded by the
2269 built-in class loader. By default, or if this property is set to
2270 @samp{full}, a full search is done for each requested class. If this
2271 property is set to @samp{cache}, then any failed lookups are cached
2272 and not tried again. If this property is set to @samp{never}, then
2273 lookups are never done. For more information, @xref{Extensions}.
2281 While writing @command{gcj} and @code{libgcj} we have, of course, relied
2282 heavily on documentation from Sun Microsystems. In particular we have
2283 used The Java Language Specification (both first and second editions),
2284 the Java Class Libraries (volumes one and two), and the Java Virtual
2285 Machine Specification. In addition we've used the online documentation
2286 at @uref{http://java.sun.com/}.
2288 The current @command{gcj} home page is
2289 @uref{http://gcc.gnu.org/java/}.
2291 For more information on gcc, see @uref{http://gcc.gnu.org/}.
2293 Some @code{libgcj} testing is done using the Mauve test suite. This is
2294 a free software Java class library test suite which is being written
2295 because the JCK is not free. See
2296 @uref{http://sources.redhat.com/mauve/} for more information.