Is it possible to view bytecode of Class file?

Question

In Java,source code is compiled into bytecode which is actually Class file. Is it possible that we can view bytecode of Class file? If it possible, Can we edit it?

Is there any eclipse plugin available?

-Abhishek Jain

Answer 1

The JDK comes with javap which is a tool to disassemble the byte code inside a class file. Editing on byte code level is possible. Have a look at BCEL, a java library designed to read, manipulate and write class files.

A list of tool and libraries to edit byte code can be found on java-net. For example JBE, a Java Byte Code editor that even comes with a GUI.

Answer 2

Yes, you can use the javap command that's included with the JDK to see the bytecode of a class. For example:

javap -c com.mypackage.MyClass

There are several libraries and tools that help you to work with Java bytecode, for example ASM and Jasmin.

Answer 3

To my experience, jclasslib is one of the best bytecode viewers.

As for editors, there are two types: bytecode manipulation libraries, and editors with GUIs. This question has been asked few times on SO, you could check the answers and the links that were provided.

Just be careful that editing bytecode in not as straightforward as you think. The JVMS imposes many restrictions on how class files should be, and there is a great chance that one of your edit will violate one of them.

Check these other questions:

Editing a .class file directly, playing around with opcodes

Is it possible to view bytecode of Class file?

Programming in Java bytecode

Answer 4

To view the bytecodes

Forget javap! The best plugin I have ever used is the "ASM - Bytecode Outline plugin for Eclipse"

http://asm.ow2.org/eclipse/index.html

It is from ASM (a bytecode manipulation framework).

It shows the bytecodes (that you asked for), stack elements (jvm style), and how to generate the same result (to produce the same bytecodes) using the asm framework methods.

Better still is the fact that it does so while you have the source code selected. You don't have to find the .class file in the bin directory to inspect it's bytecode.

To edit them

Using code:

• ASM: Visitors based, very, very fast.
• BCEL: Loads the bytecode as an in memory description of the class file.
• Javassit: the easiest one to use, allows you to do pattern matching and expression replacement.

By hand: JBE