Update
This article is a little old now, so I've started a new series (as of April, 2010) on Java decompilers and obfuscators. Please follow that link for much more recent information. The content below is kept here only for legacy reasons.
Introduction
In our previous article, we demonstrated a simple method that can be used to easily download the class files of Java applets. In this article we'll show you how to decompile the Java class, turning the .class file back into a .java source code file.
Why do this?
If you're just joining us in Part 2 of our series, I feel the need to explain why we're doing this series in the first place:
- First, we're a Java educational center. Telling you the bad about Java as well as the good is also educational. Whether it's bad or good, it is truth. The fact is that other people can decompile your Java classes, and you need to know that, and it's helpful to know how they can do it.
- Second, and even more importantly, if you know how to decompile Java classes, you can also learn how to try stop it (assuming you're interested in stopping it).
Obtaining software to decompile Java class files
To decompile a Java class file, you're going to need some software specially made for decompiling Java class files (also called reverse-engineering). In this article we're going to show you how to decompile Java class files with a software package that goes by the name of Mocha.
Mocha is probably not the best decompiling software available today, but it is one of the first packages available - if not the first - and it's also free. Mocha is written in Java, so you'll also need a copy of the Java JDK, or the JRE.
Step 1: Download Mocha
Assuming that you already have a copy of the Java JDK, the first step in the decompilation process is to download Mocha. Click here to download Mocha.
Step 2: Install Mocha
After you've downloaded Mocha, install it in a directory according to the installation instructions that came with the Mocha software. At the time of this writing, the installation process simply consists of unpacking a ZIP file into a desired directory.
Decompiling a Java class file
In this article we're going to decompile the AnimatedAd.class Java class file we downloaded in Part 1 of this article. If you're at all familiar with our web site, you might know that the original source code for this class file is also available. This is good, because it gives us a chance to compare the original source code to the decompiled version of the source code.
If you're ready, let's begin the process of decompiling the class file. If you've installed Mocha properly, there's really very little to the process other than running Mocha.
Step 1: Open a DOS or Unix window
Running Mocha is very simple. Assuming you're using a Windows system, just open a DOS window, and move to the directory where you installed Mocha.
If you're using a version of Unix, open a Unix command-line window and move to the Mocha installation directory.
Step 2: Modify your CLASSPATH variable
In this step, modify your CLASSPATH environment variable to include the Mocha.zip file you just downloaded and installed.
For instance, using Visual Cafe 2.5 on a Windows95 computer system, I modified my CLASSPATH to look like this:
set CLASSPATH=C:\Apps\Win95\Symantec\VisualCafe2.5\BIN\COMPONENTS\SYMBEANS.JAR; C:\Apps\Win95\Symantec\VisualCafe2.5\JAVA\LIB\CLASSES.ZIP; C:\Apps\Win95\Symantec\VisualCafe2.5\JAVA\LIB; C:\Temp\DevDaily\Java\Decompilers\Mocha.zip
This should all be one continuous line. (Actually, because my CLASSPATH is so long, I cheated and combined this step and the command from Step 4 into a DOS batch file named runmocha.bat.)
Step 3: Copy the class file to the Mocha installation directory
If you haven't done so already, copy the AnimatedAd.class file into the Mocha installation directory.
Step 4: Run the Mocha program
Once the CLASSPATH is set, you can run Mocha like this to decompile the AnimatedAd.class file:
java -classpath %CLASSPATH% mocha.Decompiler -v AnimatedAd.class
Notice that I included the -v option (verbose) to see a little more output than normal. It's not required, but it is helpful.
When Mocha runs, you'll see some output on your screen similar to Figure 1.
Decompiling AnimatedAd.class -> AnimatedAd.mocha Method init.................................................................... ................................................................................ ................................................................................ ................................................................................ ................................................................................ ........................ Method start................ Method stop.............. Method run..................................................................... ......................................................................... Method mouseDown............... Method pause........ Method drawMyString.............. Method paint................ Method numStringsUsed......................... Method getRandomInt................................................. Method <init>.............................
Figure 1 (above): This is the output Mocha generated and printed to my DOS window.
The decompiled source code
When Mocha runs, it also creates an output file named AnimatedAd.mocha. This file contains the decompiled Java source code - the source code it created by reading the AnimatedAd.class binary file.
The AnimatedAd.mocha file is too large to list here, but you can click here to view the reverse-engineered source code.
As a point of comparison, you can click here to view the original AnimatedAd.java source code.
Summary
I hope you enjoyed this two-part series on downloading and decompiling Java applets. If you have any questions or comments, please leave them in the comments section below.
(Editor's Note: The process of decompiling Java class files belonging to other businesses or individuals may be illegal in your city, state, or country. Frankly, I don't know for sure, because, to coin a phrase from Star Trek's (TM) Doctor McCoy, "I'm an editor, not a lawyer." In any case, this series of articles is not written to encourage that practice. These articles are presented only so you can learn to protect your own Java class files from reverse engineering.)
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