I'm having a little trouble understanding the question, but here's what I think you're asking: can some always-running process (like a daemon) automatically recompile a project whenever source is changed? Is that the question?

If that's what you're looking for, I recommend this article, which talks about combining JUnit and ant into a system for continuous rebuilding and testing of projects.

If you want the java compiler to solve this problem by itself, it can't: it just compiles the files you ask it to and then it exits. But the article describes how to use several tools together to set up such a system.