![\"JOGL](\"https:\/\/www.fitzsim.org\/screenshots\/jogl-demo-2005-08-31.png\")
<\/a><\/p>\n\nAs you can see, it’s hard to tell which is which!\n<\/p>\n
Big Merge<\/strong><\/p>\n The screenshot represents several efforts coming together. First, the demo is running on natively-compiled GNU Classpath HEAD. Tom Tromey<\/a> recently finished the Big Merge, making Classpath a sub-directory of gcc\/libjava. To develop this demo, I replaced gcc\/libjava\/classpath with a checkout of Classpath HEAD and rebuilt libgcj. After patching a few places where divergences crept in I have a GCJ HEAD + Classpath HEAD development environment setup. Developing in this way is much more pleasant than the previous merge-every-patch approach; now I use the same source checkout for libgcj and JamVM+Classpath.<\/p>\n java-gcjHEAD-compat<\/strong><\/p>\n The screenshot shows me running the java command in the exact same way in both windows, thanks to java-gcjHEAD-compat<\/a>. It is an RPM spec file that allows you to create a first-class JPackage-style JVM alternative that points to a GCJ installation in a non-standard prefix. For example, I’ve installed GCJ HEAD in \/home\/fitzsim\/install. java-gcjHEAD-compat points \/usr\/bin\/java (through alternatives) to \/home\/fitzsim\/install\/bin\/gij.<\/p>\n This is very convenient for developing libgcj as a JPackage JVM replacement since all JPackages assume the JVM is installed in \/usr\/lib\/jvm. It’s also useful for building RPMs against libgcj HEAD, which I’ve done with JOGL in the screenshot. Now, JOGL is a heavy user of the AWT Native Interface which is implemented by a small library, libjawt.so. That brings me to my next topic.<\/p>\n libjawt.so Binary Compatibility<\/strong><\/p>\n It has proved quite challenging to create a drop-in replacement for Sun’s libjawt.so. The jawt.h and jawt_md.h headers that Sun includes in the AWT Native Interface document<\/a> are marked confidential and proprietary so I didn’t want to refer to them when creating an implementation for GNU Classpath. My initial libjawt.so attempt was not binary-compatible and so created problems for packages like JOGL that need to run on either libgcj or a proprietary JVM.<\/p>\n Using gdb and ptype I was able to determine the layout of JAWT types, so now libgcj’s libgcjawt.so is binary-compatible, except that its SONAME is versioned by libtool. But the SONAME of Sun’s libjawt.so is unversioned, simply “libjawt.so”, so even naming our library the same would not produce a binary-compatible result (e.g. “libjawt.so.6”) — and besides, doing so would cause confusion since libgcj’s “libjawt” is always present in the default library path, whereas Sun’s libjawt is not. So, what to do?<\/p>\n The last step in drop-in replaceability turned out to be a clever trick that Jakub Jelinek suggested:<\/p>\n That’s a line from java-gcj-compat’s Makefile.am. It creates an empty library with the proper SONAME that links to the versioned libgcjawt.so. That’s it! java-gcj-compat’s libjawt.so gets installed in $JAVA_HOME\/jre\/lib\/i386 just like Sun’s and apps needing libjawt.so to build or run don’t know the difference! I was relieved that this step was so simple — I was expecting linker script madness.<\/p>\n The proof that this all works is in the screenshot; I built Anthony Green<\/a>‘s JOGL RPMs<\/a> using java-gcjHEAD-compat and its libjawt.so and I’m running the JOGL demo on both Sun and java-gcjHEAD-compat. lsof shows the correct libjawt.so being loaded in each case:<\/p>\n Now I want to see JOGL submitted to Fedora Extras and all this binary-compatibility work land in Fedora Core 5!<\/p>\n","protected":false},"excerpt":{"rendered":" Here’s a screenshot of a JOGL demo running on Sun and libgcj: As you can see, it’s hard to tell which is which! Big Merge The screenshot represents several efforts coming together. First, the demo is running on natively-compiled GNU Classpath HEAD. Tom Tromey recently finished the Big Merge, making Classpath a sub-directory of gcc\/libjava. … <\/p>\n\r\nlibjawt.so:\r\n\techo | $(GCJ_BIN_DIR)\/gcc$(gcc_suffix) -shared -O2 -fpic -o libjawt.so -Wl,-soname,libjawt.so -xc - -lgcjawt\r\n<\/pre>\n
\r\n$ \/usr\/sbin\/lsof | grep libjawt.so\r\njava 32435 fitzsim mem REG 3,2 2680 470785 \/usr\/lib\/jvm\/java-1.4.2-gcjHEAD-1.4.2.0\/jre\/lib\/i386\/libjawt.so\r\njava 32460 fitzsim mem REG 3,2 3824 812009 \/usr\/java\/jdk1.5.0_01\/jre\/lib\/i386\/libjawt.so\r\n<\/pre>\n