How can I open Java .class files in a human-readable way?

I'm trying to figure out what a Java applet's class file is doing under the hood. Opening it up with Notepad or Textpad just shows a bunch of gobbledy-gook.

Is there any way to wrangle it back into a somewhat-readable format so I can try to figure out what it's doing?

• Environment == Windows w/ VS 2008 installed.

Solution 1:

jd-gui is the best decompiler at the moment. it can handle newer features in Java, as compared to the getting-dusty JAD.

Solution 2:

If you don't mind reading bytecode, javap should work fine. It's part of the standard JDK installation.

Usage: javap <options> <classes>...

where options include:
   -c                        Disassemble the code
   -classpath <pathlist>     Specify where to find user class files
   -extdirs <dirs>           Override location of installed extensions
   -help                     Print this usage message
   -J<flag>                  Pass <flag> directly to the runtime system
   -l                        Print line number and local variable tables
   -public                   Show only public classes and members
   -protected                Show protected/public classes and members
   -package                  Show package/protected/public classes
                             and members (default)
   -private                  Show all classes and members
   -s                        Print internal type signatures
   -bootclasspath <pathlist> Override location of class files loaded
                             by the bootstrap class loader
   -verbose                  Print stack size, number of locals and args for methods
                             If verifying, print reasons for failure

Solution 3:

As pointed out by @MichaelMyers, use

javap -c <name of java class file> 

to get the JVM assembly code. You may also redirect the output to a text file for better visibility.

javap -c <name of java class file> > decompiled.txt

Solution 4:

You want a java decompiler, you can use the command line tool javap to do this. Also, Java Decompiler HOW-TO describes how you can decompile a class file.

Solution 5:

what you are looking for is a java de-compiler. I recommend JAD http://www.kpdus.com/jad.html It's free for non commercial use and gets the job done.

Note: this isn't going to make the code exactly the same as what was written. i.e. you're going to lose comments and possibly variable names, so it's going to be a little bit harder than just reading normal source code. If the developer is really secretive they will have obfuscated their code as well, making it even harder to read.