optimization - How to determine which classes are used by a Java program?

Question

Is there any tool that lists which and when some classes are effectively used by an app or, even-better, automatically trims JAR libraries to only provide classes that are both referenced and used?

Answer 1

Bear in mind that, as proven by the halting problem, you can't definitely say that a particular class is or isn't used. At least on any moderately complex application. That's because classes aren't just bound at compile-time but can be loaded:

• based on XML config (eg Spring);
• loaded from properties files (eg JDBC driver name);
• added dynamically with annotations;
• loaded as a result of external input (eg user input, data from a database or remote procedure call);
• etc.

So just looking at source code isn't enough. That being said, any reasonable IDE will provide you with dependency analysis tools. IntelliJ certainly does.

What you really need is runtime instrumentation on what your application is doing but even that isn't guaranteed. After all, a particular code path might come up one in 10 million runs due to a weird combination of inputs so you can't be guaranteed that you're covered.

Tools like this do have some value though. You might want to look at something like Emma. Profilers like Yourkit can give you a code dump that you can do an analysis on too (although that won't pick up transient objects terribly well).

Personally I find little value beyond what the IDE will tell you: removing unused JARs. Going more granular than that is just asking for trouble for little to no gain.