Some tools you can use. However note that none of them has the ability to recompile classes, i.e you cannot decompile a single class to source, modify it, and then recompile back. It may be possible using Reflection API but then you need to do a lot of modification on the decompiled source itself. Other ways may be to decompile the entire bunch of classes and then recompile all when done.
This is a tool to open Java class file binaries, view their internal
structure, modify portions of it if required and save the class file
back. It also generates readable reports similar to the javap utility.
Easy to use Java Swing GUI. The user interface tries to display as
much detail as possible and tries to present a structure as close as
the actual Java class file structure. At the same time ease of use and
class file consistency while doing modifications is also stressed. For
example, when a method is deleted, the associated constant pool entry
will also be deleted if it is no longer referenced. In built verifier
checks changes before saving the file. This tool has been used by
people learning Java class file internals. This tool has also been
used to do quick modifications in class files when the source code is
JBE - Java Bytecode Editor
JBE is a bytecode editor suitable for viewing and modifying java class
files. It is built on top of the open-source jclasslib bytecode viewer
by ej-technologies. For verification and exporting the class files,
JBE uses the the Bytecode Engineering Library by Apache's Jakarta
Class Construction Kit
The Class Construction Kit is a tool for the visual creation or
modification of Java class files. It is implemented using BCEL and
The reJ project aims to allow improved visibility into Java class files, whether they were created compiling from Java
sources, another language or by any other kind of tool. Basically
anything that will run in a Java Virtual Machine.
reJ is a library and a graphical tool for inspection (viewing,
searching and comparing) and manipulation (modification, obfuscation,
refactoring of methods, fields, attributes and code) of classfiles for
the Java platform.
BcelEditor is a tool for the visual modification of Java class files.
It is implemented using BCEL.
Note: You need to register on the site to get access
MethodBodyEditor for Java
A Java class editor
Note: You need to register on the site to
Other than this, some other generic java tools are available on tuts4you