Bytecode Viewer

Bytecode Viewer is an Advanced Lightweight Java Bytecode Viewer, GUI Java Decompiler, GUI Bytecode Editor, GUI Smali, GUI Baksmali, GUI APK Editor, GUI Dex Editor, GUI APK Decompiler, GUI DEX Decompiler, GUI Procyon Java Decompiler, GUI Krakatau, GUI CFR Java Decompiler, GUI FernFlower Java Decompiler, GUI DEX2Jar, GUI Jar2DEX, GUI Jar-Jar, Hex Viewer, Code Searcher, Debugger and more. It's written completely in Java, and it's open sourced. It's currently being maintained and developed by Konloch.

There is also a plugin system that will allow you to interact with the loaded classfiles, for example you can write a String deobfuscator, a malicious code searcher, or something else you can think of. You can either use one of the pre-written plugins, or write your own. It supports groovy scripting. Once a plugin is activated, it will execute the plugin with a ClassNode ArrayList of every single class loaded in BCV, this allows the user to handle it completely using ASM.