...from our main FAQ, plus the GCJ FAQ to begin with. Applied.
Gerald Index: java/faq.html =================================================================== RCS file: java/faq.html diff -N java/faq.html --- java/faq.html 29 Jun 2014 19:39:16 -0000 1.74 +++ /dev/null 1 Jan 1970 00:00:00 -0000 @@ -1,443 +0,0 @@ -<html> -<head> -<title>The GCJ FAQ</title> -</head> - -<body> - - <h1>The GCJ FAQ</h1> - - <ol> - <li><a href="#1_0">General Questions</a> - <ol> - <li><a href="#1_1">What license is used for libgcj?</a></li> - <li><a href="#1_6">How can I debug my Java program?</a></li> - <li><a href="#1_7">Can I interface byte-compiled and native java code?</a></li> - </ol> - </li> - <li><a href="#2_0">Java Feature Support</a> - <ol> - <li><a href="#2_1">What Java API's are supported? How complete is - the support? </a></li> - <li><a href="#2_2">Does GCJ support using straight C native methods - ala JNI? </a></li> - <li><a href="#2_3">Why does GCJ use CNI? </a></li> - <li><a href="#2_4">What is the state of AWT support?</a></li> - <li><a href="#2_5">How about support for Swing ?</a></li> - <li><a href="#2_6">What support is there for RMI ?</a></li> - <li><a href="#2_7">Can I use any code from other projects - to supplement libgcj's current features?</a></li> - <li><a href="#2_8">What features of the Java language are/arn't supported</a></li> - </ol> - </li> - <li><a href="#4_0">Gcj Compile/Link Questions</a> - <ol> - <li><a href="#4_1">Why do I get <tt>undefined reference to `main'</tt> - errors?</a></li> - <li><a href="#4_2">Can GCJ only handle source code? </a></li> - <li><a href="#4_3">"gcj -C" Doesn't seem to work like javac/jikes. - Whats going on? </a></li> - <li><a href="#4_4">Where does GCJ look for files? </a></li> - <li><a href="#4_5">How does gcj resolve wether to compile .class or - .java files? </a></li> - </ol> - </li> - <li><a href="#5_0">Runtime Questions</a> - <ol> - <li><a href="#5_1">My program is dumping core! What's going on?</a></li> - <li><a href="#5_2">When I run the debugger I get a SEGV in the GC! - What's going on?</a></li> - <li><a href="#5_3">I have just compiled and benchmarked my Java application - and it seems to be running slower than than XXX JIT JVM. Is there - anything I can do to make it go faster?</a></li> - <li><a href="#5_4">Can I profile Garbage Collection? </a></li> - <li><a href="#5_5">How do I increase the runtime's initial and maximum - heap sizes?</a></li> - <li><a href="#5_6">How can I profile my application?</a></li> - </ol> - </li> - <li><a href="#6_0">Programming Issues</a> - <ol> - <li><a href="#6_1">Are there any examples of how to use CNI?</a></li> - <li><a href="#6_2">Is it possible to invoke GCJ compiled Java code from a C++ application?</a></li> - </ol> - </li> - </ol> - - <h2><a name="1_0">General Questions</a></h2> - - <h3><a name="1_1">1.1 What license is used for libgcj?</a></h3> - <dl> - <dd> - libgcj is distributed under the GPL, with the 'libgcc exception'. - This means that linking with libgcj does not by itself cause - your program to fall under the GPL. See LIBGCJ_LICENSE in - the source tree for more details. - </dd> - </dl> - - <hr /> - <h3><a name="1_6">1.6 How can I debug my Java program?</a></h3> - <dl> - <dd> - <a href="ftp://ftp.gnu.org/pub/gnu/gdb/";>gdb 5.0</a> - includes support for debugging gcj-compiled Java programs. For more - information please read <a href="gdb.html">Java Debugging with gdb</a>. - </dd> - </dl> - - <hr /> - <h3><a name="1_7">1.7 Can I interface byte-compiled and native java code</a></h3> - <dl> - <dd> - <p>libgcj has a bytecode interpreter that allows you to mix .class files with - compiled code. It works pretty transparently: if a compiled version of a class is - not found in the application binary or linked shared libraries, the class loader - will search for a bytecode version in your classpath, much like a VM would. Be - sure to build libgcj with the --enable-interpreter option to enable this - functionality.</p> - - <p>The program "gij" provides a front end to the interpreter that behaves - much like a traditional virtual machine. You can even use "gij" to run a shared library - which is compiled from java code and contains a main method:</p> - <pre> -$ gcj -shared -o lib-HelloWorld.so HelloWorld.java -$ gij HelloWorld - </pre> - This works because gij uses Class.forName, which knows how to load shared objects. - </dd> - </dl> - - <h2><a name="2_0">Java Feature Support</a></h2> - - <h3><a name="2_1">2.1 What Java API's are supported?</a> How complete is - the support?</h3> - <dl> - <dd> - <a href="mailto:m...@cs.berkeley.edu";>Matt Welsh</a> writes: - <blockquote> - <p>Just look in the 'libjava' directory of libgcj and see what classes - are there. Most GUI stuff isn't there yet, that's true, but many of - the other classes are easy to add if they don't yet exist. </p> - - <p>I think it's important to stress that there is a big difference - between Java and the many libraries which Java supports. Unfortunately, - Sun's promise of "write once, run everywhere" assumes much - more than a JVM: you also need the full set of JDK libraries. Considering - that new Java APIs come out every week, it's going to be impossible - to track everything. </p> - - <p>To make things worse, you can't simply run Sun's JDK classes on - any old JVM -- they assume that a bunch of native methods are also - defined. Since this native method requirement isn't defined by the - JDK specs, you're effectively constrained to using Sun's JVMs if you - want to use Sun's JDK libraries. Oh yes -- you could also reimplement - all of those native methods yourself, and make sure they behave exactly - as Sun's do. Note that they're undocumented! </p> - </blockquote> - </dd> - </dl> - - <hr /> - <h3><a name="2_2">2.2 </a>Does GCJ support using straight C native methods - ala JNI? </h3> - <dl> - <dd> - Yes. libgcj now has experimental support for JNI, in addition to - its native Compiled Native Interface (CNI). gcjh will generate JNI - stubs and headers using the "-jni" option. However, we do - prefer CNI: it is more efficient, easier to write, and (at least - potentially) easier to debug. - </dd> - </dl> - - <hr /> - <h3><a name="2_3">2.3 </a>Why does GCJ use CNI? </h3> - <dl> - <dd> - <a href="mailto:p...@bothner.com";>Per Bothner</a> explains: - <blockquote> - <p>We use CNI because we think it is a better solution, especially - for a Java implementation that is based on the idea that Java is - just another programming language that can be implemented using - standard compilation techniques. Given that, and the idea that languages - implemented using Gcc should be compatible where it makes sense, - it follows that the Java calling convention should be as similar - as practical to that used for other languages, especially C++, since - we can think of Java as a subset of C++. CNI is just a set of helper - functions and conventions built on the idea that C++ and Java have - the *same* calling convention and object layout; they are binary - compatible. (This is a simplification, but close enough.)</p> - </blockquote> - </dd> - </dl> - - <hr /> - <h3><a name="2_4">2.4 What is the state of AWT support</a>?</h3> - <dl> - <dd> - Work is in progress to implement AWT and Java2D. We - intend to support both GTK and xlib peers written using CNI. Some - components are already working atop the - <a href="http://www.ii.uib.no/~rolfwr/jcnix/";>xlib peers</a>. - </dd> - </dl> - - <hr /> - <h3><a name="2_5">2.5 How about support for Swing</a>?</h3> - <dl> - <dd> - Once AWT support is working then Swing support can be considered. There - is at least one free-software partial implementations of Swing that may - be usable. - </dd> - </dl> - - <hr /> - <h3><a name="2_6">2.6 </a>What support is there for RMI?</h3> - <dl> - <dd> RMI code exists on the CVS trunk (aka gcc 3.1), but it - has not been heavily tested. This code was donated by - Transvirtual Technologies. - </dd> - </dl> - - <hr /> - <h3><a name="2_7">2.7 Can I use any code from other - projects to supplement libgcj's current features?</a></h3> - <dl> - <dd> - <p>Certainly, provided the licenses are compatible. - However, in many cases, if you wanted to contribute the - code back into the official libgcj distribution, we would require that the - original author(s) assign copyright to the Free Software Foundation. </p> - <p> As of March 6, 2000, libgcj has been relicenced, and copyright has - been assigned to the <a href="http://www.gnu.org/";>FSF</a>. This allows - us to share and merge much of the libgcj codebase with the - <a href="http://www.gnu.org/software/classpath/";>Classpath</a> project. - Our eventual goal is for Classpath to be an upstream source provider for libgcj, - however it will be some time before this becomes reality: libgcj and Classpath - have different implementations of - many core java classes. In order to merge them, we need to select the - best (most efficient, cleanest) implementation of each method/class/package, - resolve any conflicts created by the merge, and test the final result. - Needless to say, this is a lot of work. If you can help out, please - <a href="mailto:j...@gcc.gnu.org";>let us know</a>!</p> - </dd> - </dl> - - <hr /> - <h3><a name="2_8">2.8 What features of the Java language are/aren't supported.</a></h3> - <dl> - <dd> - GCJ supports all Java language constructs as per the - <a href="http://docs.oracle.com/javase/specs/";>Java language specification</a>. - </dd> - </dl> - - - <h2><a name="4_0">Gcj Compile/Link Questions</a></h2> - - <h3><a name="4_1">4.1 Why do I get <tt>undefined reference to `main'</tt> - errors?</a></h3> - <dl> - <dd> - When using <tt>gcj</tt> to link a Java program, you must use the <tt>--main=</tt> - option to indicate the class that has the desired <tt>main</tt> method. - This is because every Java class can have a <tt>main</tt> method, thus - you have to tell gcj which one to use. - </dd> - </dl> - - <hr /> - <h3><a name="4_2">4.2 Can GCJ only handle source code?</a></h3> - <dl> - <dd> - <p>GCJ will compile both source <code>(.java)</code> and bytecode - <code>(.class)</code> files. However, in many cases the native code produced - by compiling from source is better optimized than that compiled from .class - files.</p> - - <p><a href="mailto:p...@bothner.com";>Per Bothner</a> explains: </p> - <blockquote> - <p>The reason is that when you compile to bytecode you lose a lot - of information about program structure etc. That information helps - in generating better code. We can in theory recover the information - we need by analysing the structure of the bytecodes, but it is sometimes - difficult - or sometimes it just that no-one has gotten around to - it.</p> - <p>Specific examples include loop structure (gcc generates better - code with explicit loops rather than with the equivalent spaghetti - code), array initializers, and the JDK 1.1 `CLASS.class' syntax, - all of which are represented using more low-level constructs in - bytecode.</p> - </blockquote> - </dd> - </dl> - - <hr /> - <h3><a name="4_3">4.3 "gcj -C"</a> Doesn't seem to work like javac/jikes. - Whats going on?</h3> - <dl> - <dd> - <p>The behavior of "gcj -C" is not at all like javac or jikes, which - will compile (not just scan) all .java's which are out of date with - regard to their .class's.</p> - </dd> - </dl> - - <hr /> - <h3><a name="4_4">4.4 Where does GCJ look for files?</a></h3> - <dl> - <dd> - <p>GCJ looks for classes to compile based on the CLASSPATH environment - variable. <code>libgcj.jar</code> and other files are found relative to the - path of the compiler itself, so it is safe to move the entire compiler tree - to a different path, and there is no need to include <code>libgcj.jar</code> - in your CLASSPATH. </p> - </dd> - </dl> - - <hr /> - <h3><a name="4_5">4.5 How does gcj resolve whether to compile .class - or .java files?</a></h3> - <dl> - <dd> - <p>GCJ compiles only the files presented to it on the command line. However, - it also needs to scan other files in order to determine the layout of other - classes and check for errors in your code. For these dependencies, GCJ will - favour .class files if they are available because it is faster to parse a - class file than source code.</p> - </dd> - </dl> - - - <h2><a name="5_0">Runtime Questions</a></h2> - - <h3><a name="5_1">5.1 My program is dumping core! What's going on?</a></h3> - <dl> - <dd> - It could be any number of things. One common mistake is - having your <tt>CLASSPATH</tt> environment variable pointing - at a third party's java.lang and friends. Either unset - <tt>CLASSPATH</tt>, or make sure it does not refer to core - libraries other than those found in - <tt>libgcj.jar</tt>.Note that newwer versions of - GCJ will reject the core class library if it wasn't - <a href="https://gcc.gnu.org/ml/java/2001-03/msg00383.html";>generated</a> - by GCJ itself. - </dd> - </dl> - - <hr /> - <h3><a name="5_2">5.2 When I run the debugger I get a SEGV in the GC! - What's going on?</a></h3> - <dl> - <dd> - This is "normal"; the Garbage Collector (GC) uses it to determine - stack boundaries. It is ordinarily caught and handled by the GC -- you - can see this in the debugger by using <tt>cont</tt> to continue to the - "real" segv. - </dd> - </dl> - - <hr /> - <h3><a name="5_3">5.3 I have just compiled and benchmarked my Java application - and it seems to be running slower than than XXX JIT JVM. Is there - anything I can do to make it go faster?</a></h3> - <dl> - <dd>A few things: - <ul> - <li>If your programs allocate many small, short lived objects, the - heap could be filling and triggering GC too regularly. Try increasing - the initial and maximum heap sizes as per <i>5.5 How do I increase - the runtime's initial and maximum heap size?</i></li> - <li>RE - array accesses. We have sub-optimal runtime checking code, - and the compiler is still not so smart about automatically removing - array checks. If your code is ready, and it doesn't rely on them, - try compiling with --no-bounds-check. </li> - <li>Try static linking. On many platforms, dynamic (PIC) function - calls are more expensive than static ones. In particular, the - interaction with boehm-gc seems to incur extra overhead when shared - libraries are used. </li> - <li>If your Java application doesn't need threads, try building libgcj - using --enable-threads=none. Portions of the libgcj runtime are - still more efficient when single-threaded.</li> - </ul> - </dd> - </dl> - - <hr /> - <h3><a name="5_4">5.4 Can I profile Garbage Collection?</a></h3> - <dl> - <dd> - <p>It is possible to turn on verbose GC output by suppressing the -DSILENT - flag during build. One way to do this is to comment out the line with - #define SILENT 1 from boehm-gc/configure before configuring libgcj. </p> - <p>The GC will print collection statistics to stdout. (Rebuilding boehm-gc - alone without this flag doesn't seem to work.)</p> - </dd> - </dl> - - <hr /> - <h3><a name="5_5">5.5 How do I increase the runtime's initial and maximum - heap sizes?</a></h3> - <dl> - <dd> - <p>Some programs that allocate many small, short-lived objects can cause - the default-sized heap to fill quickly and GC often. With the 2.95.1 - release there is no means to adjust the heap at runtime. Recent snapshots - provide the -ms and -mx arguments to gij to specify the initial and - maximum heap sizes, respectively.</p> - </dd> - </dl> - - <hr /> - <h3><a name="5_6">5.6 How can I profile my application?</a></h3> - <dl> - <dd> - <p>Currently, only single threaded Java code may be used by the profiler - (gprof). POSIX threads seem to be incompatible with the gmon stuff.</p> - - <p>A couple of other tools that have been mentioned on the GCJ mailing list are - sprof and cprof. The former is part of GNU libc.</p> - </dd> - </dl> - - <h2><a name="6_0"></a>Programming Issues</h2> - - <h3><a name="6_1"></a>6.1 Are there any examples of how to use CNI?</h3> - <dl> - <dd> - <p>Glenn Chambers has created a couple - of trivial examples for - <a href="cni-1.txt">version 2.95</a> - and <a href="cni-2.txt">version 3.0</a>. - As a comparison, <a href="jni-kaffe.txt">here</a> - is the same example as a JNI application using - <a href="http://www.kaffe.org";>Kaffe</a>. The same code will - work with GCJ, as shown <a href="jni-comp.txt">here</a>.</p> - - <p>Note that for version 2.95, you must compile the C++ files - used for CNI with the -fno-rtti option. - This constraint does not apply in version 3.0 and later.</p> - - <p>The primary source of documentation for CNI is - <a href="papers/cni/t1.html">this paper</a>.</p> - </dd> - </dl> - - <hr /> - <h3><a name="6_2">6.2 Is it possible to invoke GCJ compiled Java code from a C++ application?</a></h3> - <dl> - <dd> - <p>Yes, GCJ 3.1 supports a CNI-based invocation interface as well as - the traditional JNI invocation API. - - See the <a href="https://gcc.gnu.org/onlinedocs/gcj/Invocation.html";>GCJ Manual</a> - for more details on how to use the CNI interface. </p> - </dd> - </dl> - -</body> -</html> Index: faq.html =================================================================== RCS file: /cvs/gcc/wwwdocs/htdocs/faq.html,v retrieving revision 1.222 diff -u -r1.222 faq.html --- faq.html 26 Sep 2016 10:23:16 -0000 1.222 +++ faq.html 29 Dec 2016 21:04:45 -0000 @@ -17,8 +17,7 @@ <p>Other GCC-related FAQs: <a href="https://gcc.gnu.org/onlinedocs/libstdc++/faq.html";> - libstdc++-v3</a>, and - <a href="java/faq.html">GCJ</a>.</p> + libstdc++-v3</a>.</p> <hr /> <h1>Questions</h1>
