Several mature open-source and commercial tools exist. Below is a comparison of the most widely used.
jar tf myapp.jar | head -20 # Look for readable package names (com/mycompany) vs cryptic (a/b/c) decompile jar
This is the "Swiss Army Knife" of reverse engineering. It incorporates several different decompilers (like Fernflower, Procyon, and CFR) into one interface. Deep analysis and security auditing. Several mature open-source and commercial tools exist