I had to patch a class file today and I spent several hours to find a working tool. reJ threw a NullPointerException when I tried to insert the instruction “ifne”, it seems to be unfinished, it’s a shame as it could be a very nice editor, it already has numerous nice features, an hex viewer, … The famous Java Decompiler works but it’s not an editor. I didn’t succeed in editing with ClassEditor.
After that, I tried JByteMod and it worked. It’s a very complete editor, it uses several decompilers (3 including Procyon the last time I gave it a try). It allows to edit everything in class files, the colored syntax drives the code more pleasant to read, it supports Java 1.8, it shows a control flow graph which helps to understand what you’re doing especially when you’re not a bytecode expert.