There have been a few AOT compilers for Java for some time. Two of the better known examples are GCJ and Excelsior JET. Even though Excelsior JET also has a JIT, the main focus was on the AOT aspect.

There have been many suggestions that HotSpot should cache the JIT-generated code to improve start-up performance, avoid the need for a warm-up phase on every invocation of the application and possibly share more data between applications. This is particularly relevant for desktop applications. HotSpot engineers claim that this is a complex task and that the benefits might not be as great as expected because AOT-generated code would be slower than JIT-generated one, so they implemented a simpler solution in JDK 5, Class Data Sharing. It’s pretty limited because it only works with the Client VM and the serial garbage collector and also because it only loads a set of classes from the system jar.

Interestingly, IBM did some work in this area in the IBM JRE for Java 5 and improved it further in the one for Java 6. This is described in some detail in a developerWorks article. It’s worth reading if you’re interested in this sort of thing, but I’ll list some points that I found interesting:

  • AOT code can be stored into a shared class cache allowing another JVM to use it to reduce startup time.
  • Class compression is used to increase the amount of classes that are stored in the cache.
  • AOT code is also subject to JIT compilation if it’s invoked often in order to optimise it further.
  • AOT code executed by a JVM is copied from the shared class cache so there is no direct footprint benefit but there are memory and CPU savings from being able to reuse this code rather than repeat the compilation.
  • AOT compilation is based on heuristics that select methods that are likely to improve startup time.
  • Eclipse without any additional plugins took 3.313 to start with AOT and shared classes versus 4.204 seconds without. Larger improvements could take place if there were more plugins installed.
  • Tomcat startup time improved from 1138ms to 851ms. Using shared classes without AOT caused the time to be 950ms, which means that both AOT and shared classes contributed to the improvement.

Downloading the IBM JRE/JDK requires registration and in case you need the Windows version, you must download a bundle that includes Eclipse. Links to the various downloads can be found in the developerWorks article.

These are interesting results, and it would be interesting to find out if HotSpot would also benefit from similar enhancements.

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