How to go about RE Java Plugins for Intellij?

[CryptXor]

Originally Posted by chessgod101

If these plugins are coded in java, tools such as ByteCode Viewer, jd-gui, and DJ Java Decompiler are great for decompiling the code. If you need to modify it, tools such as Java Bytecode Editor(JBE or CJBE), DirtyJoe, or ReCaf can help you change byte code instructions.
It is fair to note that I have never used Intellij or any of its plugins, as I generally use Netbeans and Eclipse for the rare instance that I need to code in java, so I am commenting somewhat generically/hypothetically.

ByteCode Viewer is sort of an AIO Java RE tool in that regard, and it infact includes the JD-GUI core as one of its decompilers. Its my go-to for anything Java related.

Something chessgod101 missed is a decent deobfuscator, since a lot of java is obfuscated: https://github.com/java-deobfuscator/deobfuscator

As far as reversing intellij plugins go, there are a few ways you can go about it, but generally just trace from the entry point (Main()) and you will very quickly find the licensing code you are wanting to modify.