java - Can multiple JVM processes share memory for common classes?

Question

I'd like to run multiple Java processes on my web server, one for each web app. I'm using a web framework (Play) that has a lot of supporting classes and jar files, and the Java processes use a lot of memory. One Play process shows about 225MB of "resident private" memory. (I'm testing this on Mac OS X, with Java 1.7.0_05.) The app-specific code might only be a few MB. I know that typical Java web apps are jars added to one server process (Tomcat, etc), but it appears the standard way to run Play is as a standalone app/process. If these were C programs, most of that 200MB would be shared library and not duplicated in each app. Is there a way to make this happen in Java? I see some pages about class data sharing, but that appears to apply only to the core runtime classes.

Answer 1

At this time and with the Oracle VM, this isn't possible.

But I agree, it would be a nice feature, especially since Java has all the information it needs to do that automatically.

Of the top of my hat, I think that the JIT is the only reason why this can't work: The JIT takes runtime behavior into account. So if app A uses some code in a different pattern than app B, that would result in different assembler code generated at runtime.

But then, the usual "pattern" is "how often is this code used." So if app A called some method very often and B didn't, they could still share the code because A has already paid the price for optimizing/compiling it.

What you can try is deploy several applications as WAR files into a single VM. But from my experience, that often causes problems with code that doesn't correctly clean up thread locals or shutdown hooks.