4
The latest version of this document is always available at
5
http://gcc.gnu.org/java/faq.html.
9
What license is used for libgcj?
10
How can I report a bug in libgcj?
11
How can I contribute to libgcj
12
Is libgcj part of GCC?
13
Will gcj and libgcj work on my machine?
14
How can I debug my Java program?
15
Can I interface byte-compiled and native java code?
20
What Java API's are supported? How complete is
22
Does GCJ support using straight C native methods
25
What is the state of AWT support?
26
How about support for Swing ?
27
What support is there for RMI ?
28
Can I use any code from other OpenSource projects
29
to supplement libgcj's current features ?
30
What features of the Java language are/arn't supported
35
I need something more recent than the last release; how
38
Can I configure/build in the source tree?
39
My libgcj build fails with "invalid use of undefined type
40
struct sigcontext_struct"
43
Gcj Compile/Link Questions
45
Why do I get undefined reference to `main' errors?
46
Can GCJ only handle source code?
47
"gcj -C" Doesn't seem to work like javac/jikes. Whats going on?
48
Where does GCJ look for files?
49
How does gcj resolve wether to compile .class or .java files?
50
I'm getting link errors!
51
I'm getting 'undefined symbol: __dso_handle'
56
My program is dumping core! What's going on?
57
When I run the debugger I get a SEGV in the GC! What's going on?
58
I have just compiled and benchmarked my Java application
59
and it seems to be running slower than than XXX JIT JVM. Is there
60
anything I can do to make it go faster?
61
Can I profile Garbage Collection?
62
How do I increase the runtime's initial and maximum heap sizes?
63
How can I profile my application?
64
My program seems to hang and doesn't produce any output
69
Are there any examples of how to use CNI?
70
Is it possible to invoke GCJ compiled Java code from a
76
1.1 What license is used for libgcj?
78
libgcj is distributed under the GPL, with the 'libgcc exception'.
79
This means that linking with libgcj does not by itself cause
80
your program to fall under the GPL. See LIBGCJ_LICENSE in
81
the source tree for more details.
83
1.2 How can I report a bug in libgcj?
85
libgcj has a corresponding Gnats bug database which you can
86
browse. You can also submit new bug reports from the Gnats
89
1.3 How can I contribute to libgcj?
91
You can send simple bug fixes in as patches. Please follow
92
the GCC guidelines for submitting patches. For more complex
93
changes, you must sign copyright over to the Free Software
94
Foundation. See the contribution page for details.
96
1.4 Is libgcj part of GCC?
98
Yes, libgcj is now part of GCC. It can be downloaded,
99
configured and built as one single tree.
101
1.5 Will gcj and libgcj work on my machine?
103
Gcj and libgcj are known to work more or less with IA-32 and
104
Sparc Solaris, Tru64 Unix, as well as IA-32, IA-64, Alpha,
105
and PowerPC Linux. They might work on other
106
systems. Generally speaking, porting to a new system should
107
not be hard. This would be a good way to volunteer.
109
1.6 How can I debug my Java program?
111
gdb 5.0 includes support for debugging gcj-compiled Java
112
programs. For more information please read Java Debugging
115
1.7 Can I interface byte-compiled and native java code
117
libgcj has a bytecode interpreter that allows you to mix
118
.class files with compiled code. It works pretty
119
transparently: if a compiled version of a class is not found
120
in the application binary or linked shared libraries, the
121
class loader will search for a bytecode version in your
122
classpath, much like a VM would. Be sure to build libgcj
123
with the --enable-interpreter option to enable this
126
The program "gij" provides a front end to the interpreter
127
that behaves much like a traditional virtual machine. You
128
can even use "gij" to run a shared library which is compiled
129
from java code and contains a main method:
131
$ gcj -shared -o lib-HelloWorld.so HelloWorld.java
134
This works because gij uses Class.forName, which knows how
135
to load shared objects.
140
2.1 What Java API's are supported? How complete is
145
Just look in the 'libjava' directory of libgcj and see
146
what classes are there. Most GUI stuff isn't there yet,
147
that's true, but many of the other classes are easy to add
148
if they don't yet exist.
150
I think it's important to stress that there is a big
151
difference between Java and the many libraries which Java
152
supports. Unfortunately, Sun's promise of "write once, run
153
everywhere" assumes much more than a JVM: you also need
154
the full set of JDK libraries. Considering that new Java
155
APIs come out every week, it's going to be impossible to
158
To make things worse, you can't simply run Sun's JDK
159
classes on any old JVM -- they assume that a bunch of
160
native methods are also defined. Since this native method
161
requirement isn't defined by the JDK specs, you're
162
effectively constrained to using Sun's JVMs if you want to
163
use Sun's JDK libraries. Oh yes -- you could also
164
reimplement all of those native methods yourself, and make
165
sure they behave exactly as Sun's do. Note that they're
168
2.2 Does GCJ support using straight C native methods
171
Yes. libgcj now has experimental support for JNI, in
172
addition to its native Compiled Native Interface (CNI). gcjh
173
will generate JNI stubs and headers using the "-jni"
174
option. However, we do prefer CNI: it is more efficient,
175
easier to write, and (at least potentially) easier to debug.
177
2.3 Why does GCJ use CNI?
179
Per Bothner explains:
181
We use CNI because we think it is a better solution,
182
especially for a Java implementation that is based on the
183
idea that Java is just another programming language that
184
can be implemented using standard compilation
185
techniques. Given that, and the idea that languages
186
implemented using Gcc should be compatible where it makes
187
sense, it follows that the Java calling convention should
188
be as similar as practical to that used for other
189
languages, especially C++, since we can think of Java as a
190
subset of C++. CNI is just a set of helper functions and
191
conventions built on the idea that C++ and Java have the
192
*same* calling convention and object layout; they are
193
binary compatible. (This is a simplification, but close
196
2.4 What is the state of AWT support?
198
Work is in progress to implement AWT and Java2D. We intend
199
to support both GTK and xlib peers written using CNI. Some
200
components are already working atop the xlib peers.
202
2.5 How about support for Swing?
204
Once AWT support is working then Swing support can be
205
considered. There is at least one free-software partial
206
implementations of Swing that may be usable.
208
2.6 What support is there for RMI?
210
RMI code exists on the CVS trunk (aka gcc 3.1), but it has
211
not been heavily tested. This code was donated by
212
Transvirtual Technologies.
214
2.7 Can I use any code from other OpenSource
215
projects to supplement libgcj's current features?
217
Certainly. However, in many cases, if you wanted to
218
contribute the code back into the official libgcj
219
distribution, we would require that the original author(s)
220
assign copyright to the Free Software Foundation. As of
221
March 6, 2000, libgcj has been relicenced, and copyright
222
has been assigned to the FSF. This allows us to share and
223
merge much of the libgcj codebase with the Classpath
224
project. Our eventual goal is for Classpath to be an
225
upstream source provider for libgcj, however it will be
226
some time before this becomes reality: libgcj and Classpath
227
have different implementations of many core java
228
classes. In order to merge them, we need to select the best
229
(most efficient, cleanest) implementation of each
230
method/class/package, resolve any conflicts created by the
231
merge, and test the final result. Needless to say, this is
232
a lot of work. If you can help out, please let us know!
234
2.8 What features of the Java language are/aren't supported.
236
GCJ supports all Java language constructs as per the Java
237
language Specification. Recent GCJ snapshots have added
238
support for most JDK1.1 (and beyond) language features,
239
including inner classes.
244
3.1 I need something more recent than the last release.
245
How should I build it?
247
Please read here: http://gcc.gnu.org/java/build-snapshot.html
249
3.2 Linker bug on Solaris
251
There is a known problem with the native Solaris linker when
252
using gcc/gcj. A good indication you've run into this
253
problem is if you get an error that looks like the following
254
when building libgcj:
256
ld: warning: option -o appears more than once, first setting taken
257
ld: fatal: file libfoo.so: cannot open file: No such file or directory
258
ld: fatal: File processing errors. No output written to .libs/libfoo.so
259
collect2: ld returned 1 exit status
261
A known workaround for this and other reported link problems
262
on the various releases of Solaris is to build gcc/gcj with
263
the latest GNU binutils instead of the native Solaris
264
ld. The most straightforward way to do this is to build and
265
install binutils, and then reference it in the configure for
266
gcc via --with-ld=/path_to_binutils_install/bin/ld
267
(--with-as may also be similarly specified but is not
268
believed to be required).
270
Please note, gcc/gcj must be built using GNU ld prior to
271
doing a clean build of libgcj!
273
3.3 Can I configure/build in the source tree?
275
No. You cannot configure/build in the source tree. If you
276
try, you'll see something like:
279
Configuring for a i686-pc-linux-gnu host.
280
*** Cannot currently configure in source tree.
282
Instead, you must build in another directory. E.g.:
288
3.4 My libgcj build fails with "invalid use of undefined type
289
struct sigcontext_struct"
291
If you're using Linux, this probably means you need to
292
upgrade to a newwer, glibc (libc6) based Linux
293
distribution. libgcj does not support the older linux libc5.
294
It might be possible to get a working libgcj by changing
295
occurances of "sigcontext_struct" to "sigcontext", however
296
this has not been tested. Even if it works, it is likely
297
that there are other issues with older libc versions that
298
would prevent libgcj from working correctly (threads bugs,
301
Gcj Compile/Link Questions
302
==========================
304
4.1 Why do I get undefined reference to `main' errors?
306
When using gcj to link a Java program, you must use the --main=
307
option to indicate the class that has the desired main method.
308
This is because every Java class can have a main method, thus
309
you have to tell gcj which one to use.
311
4.2 Can GCJ only handle source code?
313
GCJ will compile both source (.java) and bytecode (.class)
314
files. However, in many cases the native code produced by
315
compiling from source is better optimized than that compiled
318
Per Bothner explains:
320
The reason is that when you compile to bytecode you lose a
321
lot of information about program structure etc. That
322
information helps in generating better code. We can in
323
theory recover the information we need by analysing the
324
structure of the bytecodes, but it is sometimes difficult
325
- or sometimes it just that no-one has gotten around to
326
it. Specific examples include loop structure (gcc
327
generates better code with explicit loops rather than with
328
the equivalent spaghetti code), array initializers, and
329
the JDK 1.1 `CLASS.class' syntax, all of which are
330
represented using more low-level constructs in bytecode.
332
4.3 "gcj -C" Doesn't seem to work like javac/jikes. Whats going on?
334
The behavior of "gcj -C" is not at all like javac or jikes,
335
which will compile (not just scan) all .java's which are out
336
of date with regard to their .class's.
338
4.4 Where does GCJ look for files?
340
GCJ looks for classes to compile based on the CLASSPATH
341
environment variable. libgcj.jar and other files are found
342
relative to the path of the compiler itself, so it is safe
343
to move the entire compiler tree to a different path, and
344
there is no need to include libgcj.jar in your CLASSPATH.
346
4.5 How does gcj resolve whether to compile .class or .java files?
348
GCJ compiles only the files presented to it on the command
349
line. However, it also needs to scan other files in order to
350
determine the layout of other classes and check for errors
351
in your code. For these dependencies, GCJ will favour
352
.class files if they are available because it is faster to
353
parse a class file than source code.
355
4.6 I'm getting link errors
357
If you get errors at link time that refer to 'undefined
358
reference to `java::lang::Object type_info function', verify
359
that you have compiled any CNI C++ files with the -fno-rtti
360
option. This is only required for versions of GCJ earlier
363
4.7 I'm getting 'undefined symbol: __dso_handle'
365
Some versions of the GNU linker have broken support for the
366
'.hidden' directive, which results in problems with shared
367
libraries built with recent versions of gcc.
369
There are three solutions:
371
- downgrade to binutils that don't support .hidden at all,
372
- upgrade to a recent binutils, or
373
- undef the HAVE_GAS_HIDDEN definition in gcc's auto-host.h
379
5.1 My program is dumping core! What's going on?
381
It could be any number of things. One common mistake is
382
having your CLASSPATH environment variable pointing at a
383
third party's java.lang and friends. Either unset CLASSPATH,
384
or make sure it does not refer to core libraries other than
385
those found in libgcj.jar.Note that newwer versions of GCJ
386
will reject the core class library if it wasn't generated by
389
5.2 When I run the debugger I get a SEGV in the GC! What's going on?
391
This is "normal"; the Garbage Collector (GC) uses it to
392
determine stack boundaries. It is ordinarily caught and
393
handled by the GC -- you can see this in the debugger by
394
using cont to continue to the "real" segv.
396
5.3 I have just compiled and benchmarked my Java application
397
and it seems to be running slower than than XXX JIT JVM. Is there
398
anything I can do to make it go faster?
402
- If your programs allocate many small, short lived objects,
403
the heap could be filling and triggering GC too
404
regularly. Try increasing the initial and maximum heap sizes
405
as per 5.5 How do I increase the runtime's initial and
407
- RE - array accesses. We have sub-optimal runtime checking
408
code, and the compiler is still not so smart about
409
automatically removing array checks. If your code is ready,
410
and it doesn't rely on them, try compiling with
412
- Try static linking. On many platforms, dynamic (PIC)
413
function calls are more expensive than static ones. In
414
particular, the interaction with boehm-gc seems to incur
415
extra overhead when shared libraries are used.
416
- If your Java application doesn't need threads, try
417
building libgcj using --enable-threads=none. Portions of the
418
libgcj runtime are still more efficient when
421
5.4 Can I profile Garbage Collection?
423
It is possible to turn on verbose GC output by supressing
424
the -DSILENT flag during build. One way to do this is to
425
comment out the line with #define SILENT 1 from
426
boehm-gc/configure before configuring libgcj. The GC will
427
print collection statistics to stdout. (Rebuilding boehm-gc
428
alone without this flag doesn't seem to work.)
430
5.5 How do I increase the runtime's initial and maximum heap sizes?
432
Some programs that allocate many small, short-lived objects
433
can cause the default-sized heap to fill quickly and GC
434
often. With the 2.95.1 release there is no means to adjust
435
the heap at runtime. Recent snapshots provide the -ms and
436
-mx arguments to gij to specify the initial and maximum heap
439
5.6 How can I profile my application?
441
Currently, only single threaded Java code may be used by the
442
profiler (gprof). POSIX threads seem to be incompatible with
443
the gmon stuff. A couple of other tools that have been
444
mentioned on the GCJ mailing list are sprof and cprof. The
445
former is part of GNU libc.
447
5.7 My program seems to hang and doesn't produce any output
449
Some versions had a bug in the iconv support. You can work
450
around it by setting LANG=en_US.UTF-8 at runtime, or give
451
the following option during compile time
452
-Dfile.encoding=UTF-8. This problem should no longer occur
453
as of November 1, 2000.
458
6.1 Are there any examples of how to use CNI?
460
Glenn Chambers has created a couple of trivial examples for
461
version 2.95 and version 3.0. As a comparison, here is the
462
same example as a JNI application using Kaffe. The same
463
code will work with GCJ, as shown here.
465
Note that for version 2.95, you must compile the C++ files
466
used for CNI with the -fno-rtti option. This constraint
467
does not apply in version 3.0 and later.
469
The primary source of documentation for CNI is at
470
http://gcc.gnu.org/java/papers/cni/t1.html
472
6.2 Is it possible to invoke GCJ compiled Java code from a
475
Yes, GCJ 3.1 supports a CNI-based invocation interface as
476
well as the traditional JNI invocation API. See the GCJ
477
Manual for more details on how to use the CNI interface.
479
Please send FSF & GNU inquiries & questions tognu@gnu.org.There are
480
also other waysto contact the FSF.
482
These pages are maintained by The GCC team.
484
Please send comments on these web pages and GCC to our publicmailing
485
list at gcc@gnu.org orgcc@gcc.gnu.org, send other questions to
488
Copyright (C) Free Software Foundation, Inc.,
489
59 Temple Place - Suite 330, Boston, MA 02111, USA.
491
Verbatim copying and distribution of this entire article is permitted
492
in any medium, provided this notice is preserved.
494
Last modified 2003-04-30
added
removed
debian/FAQ.gcj
