I am trying to compile an application server with JDK9's new AOT feature, and am facing a number of challenges.
The appserver consists of ~180 MB jars; compiling that together overflows integer so I've tried to compile each module to one (.so) library. These modules have dependencies on other modules, so I had to put them on classpath using -J-cp -Jdependencies. This has resulted in 4.4 GB of libs - as AOT is supposed to speed up server boot, you can imagine that loading this from the disk does not really help. (It is possible to strip those libraries of their debug info, but we're still talking about an order of scale growth compared to the jars.)
I was rather disappointed that jaotc actually class-loads compiled classes, which triggers static constructors (and this gives me errors at times). Also, the compiler cannot handle missing referenced classes, and sometimes this is just a runtime-dependency - the server operates without issues even without them. So I had to provide empty mock classes to satisfy the compiler.
However, when running the server with AOT tracing (-Xlog:aot+class+load=trace:file=/tmp/aot.txt:none, not the stdout -XX:+PrintAOT) I have found that the libs contain some part of the dependencies as well:
found java.lang.Object in /home/user/aot/common/libjava.base-coop.so for classloader 0x2b5745e6ac80 tid=0x00002b574401e800
found java.lang.Object in /home/user/aot/appserver/lib/libcom.example.module1.so for classloader 0x2b5745e6ac80 tid=0x00002b574401e800
found java.lang.Object in /home/user/aot/appserver/lib/libcom.example.module2.so for classloader 0x2b5745e6ac80 tid=0x00002b574401e800
This confirmed my doubts that the lib contains more than just the code from the jars I gave the compiler to compile but at the very least the code for the superclasses as well. I am also unsure how the JVM behaves when it finds the same class in multiple libs.
Is it possible to strip the duplicities? What's the recommended approach to big/multi-lib projects?
This feature has been removed by JEP410. Code related to AOT are mostly removed in JDK 17, you should not be using this feature. If you still want this feature, you may consider to use GraalVM.