IBM contributed thing which is called class sharing. The thing is simple, you generate class cache which is then used when Java is starting up (and also shared across JVMs). Startups are faster.

How to use this (pretty old) feature? You re loking for -Xshare option:

$ java -X 2>&1 | grep share
-Xshare:off       do not attempt to use shared class data
-Xshare:auto      use shared class data if possible (default)
-Xshare:on        require using shared class data, otherwise fail.

To generate the cache do this:

# java -Xshare:dump

It creates about 25 MB file: /usr/lib/jvm/java-1.7.0-openjdk-

Although help screen shows OpenJDK should automatically use shared class cache when it is present, this does not work on my Fedora 17. So you need to make sure your app has -Xshare:on in the JVM options. This is a bug perhaps.

How much fast is it? Well, I did not test Eclipse or something like that as I dont use it, but it is measurable even for Hello World:

$ time java -Xshare:on -cp . HelloWorldApp
Hello World!

real    0m0.051s
user    0m0.038s
sys 0m0.012s

$ time java -Xshare:off -cp . HelloWorldApp
Hello World!

real    0m0.076s
user    0m0.061s
sys 0m0.012s

In Windows and MacOS systems, the cache is generated automatically after installation. I have filed a bug for Fedora to do the same: