Decompiling Java code

Occasionally, there comes a need to “look under the hood” of the code in a JAR file. While java is a compiled language, it isn’t quite machine code, but rather exists in a psuedo-code form to be used by the Java Virtual Machine’s JIT (Just in Time) compiler.

A lot can be learned from looking at other source code, unfortunately when using decompiled code you don’t get the original variable names or javadoc, but you can often better understand the API’s and performance issues in particular code.

I’m personally fond of DJ Decompiler, but I list several here for your use:


Java Code Coverage

In my “day job” I do lot’s of code reviews. I’m a big fan of Agile Programming and JUnits, recently I was introduced to the world of code coverage tools available (for free!) to Java developers.

IMHO, here’s the three front-runners.

Personally I prefer the Eclipse integration provided by ECLEMMA, but I agree that no one tool is ever ‘best’ for all scenarios.

Some background on this topic if you are interested in learning more:

Happy coding.

Automated Java code review tools

I recently found out about ‘static analysis’ of Java code. I’ve found two of these tools that are both free and easy to use. Both provide review of java bytecode and look for common development errors and inefficiencies…
FindBugs is based on the concept of bug patterns. A bug pattern is a code idiom that is often an error. Bug patterns arise for a variety of reasons:

  • Difficult language features
  • Misunderstood API methods
  • Misunderstood invariants when code is modified during maintenance
  • Garden variety mistakes: typos, use of the wrong boolean operator

PMD scans Java source code and looks for potential problems like:

  • Possible bugs – empty try/catch/finally/switch statements
  • Dead code – unused local variables, parameters and private methods
  • Suboptimal code – wasteful String/StringBuffer usage
  • Overcomplicated expressions – unnecessary if statements, for loops that could be while loops
  • Duplicate code – copied/pasted code means copied/pasted bugs

Both integrate easily within Eclipse based (and other IDE’s) is typically done with the use of a simple plugin.

FindBugs can also run as a Java WebStart (JNLP) application, however a different UI is shown for JRE 1.4 vs. 1.5 and above (look out!).
More information:

While no tool can identify all problems, these will help you find some troublesome problems and give you areas to take a deeper look at.

Happy reviewing and fixing.