[java] How to find unused/dead code in java projects

What tools do you use to find unused/dead code in large java projects? Our product has been in development for some years, and it is getting very hard to manually detect code that is no longer in use. We do however try to delete as much unused code as possible.

Suggestions for general strategies/techniques (other than specific tools) are also appreciated.

Edit: Note that we already use code coverage tools (Clover, IntelliJ), but these are of little help. Dead code still has unit tests, and shows up as covered. I guess an ideal tool would identify clusters of code which have very little other code depending on it, allowing for docues manual inspection.

I'm suprised ProGuard hasn't been mentioned here. It's one of the most mature products around.

ProGuard is a free Java class file shrinker, optimizer, obfuscator, and preverifier. It detects and removes unused classes, fields, methods, and attributes. It optimizes bytecode and removes unused instructions. It renames the remaining classes, fields, and methods using short meaningless names. Finally, it preverifies the processed code for Java 6 or for Java Micro Edition.

Some uses of ProGuard are:

• Creating more compact code, for smaller code archives, faster transfer across networks, faster loading, and smaller memory footprints.
• Making programs and libraries harder to reverse-engineer.
• Listing dead code, so it can be removed from the source code.
• Retargeting and preverifying existing class files for Java 6 or higher, to take full advantage of their faster class loading.

Here example for list dead code: https://www.guardsquare.com/en/products/proguard/manual/examples#deadcode

An Eclipse plugin that works reasonably well is Unused Code Detector.

It processes an entire project, or a specific file and shows various unused/dead code methods, as well as suggesting visibility changes (i.e. a public method that could be protected or private).

• FindBugs is excellent for this sort of thing.
• PMD (Project Mess Detector) is another tool that can be used.

However, neither can find public static methods that are unused in a workspace. If anyone knows of such a tool then please let me know.

CodePro was recently released by Google with the Eclipse project. It is free and highly effective. The plugin has a 'Find Dead Code' feature with one/many entry point(s). Works pretty well.

Use a test coverage tool to instrument your codebase, then run the application itself, not the tests.

Emma and Eclemma will give you nice reports of what percentage of what classes are run for any given run of the code.

DCD is not a plugin for some IDE but can be run from ant or standalone. It looks like a static tool and it can do what PMD and FindBugs can't. I will try it.

P.S. As mentioned in a comment below, the Project lives now in GitHub.

There is a Java project - Dead Code Detector (DCD). For source code it doesn't seem to work well, but for .jar file - it's really good. Plus you can filter by class and by method.

I use Doxygen to develop a method call map to locate methods that are never called. On the graph you will find islands of method clusters without callers. This doesn't work for libraries since you need always start from some main entry point.

In Eclipse Goto Windows > Preferences > Java > Compiler > Errors/Warnings
and change all of them to errors. Fix all the errors. This is the simplest way. The beauty is that this will allow you to clean up the code as you write.

Screenshot Eclipse Code :

Eclipse can show/highlight code that can't be reached. JUnit can show you code coverage, but you'd need some tests and have to decide if the relevant test is missing or the code is really unused.

Code coverage tools, such as Emma, Cobertura, and Clover, will instrument your code and record which parts of it gets invoked by running a suite of tests. This is very useful, and should be an integral part of your development process. It will help you identify how well your test suite covers your code.

However, this is not the same as identifying real dead code. It only identifies code that is covered (or not covered) by tests. This can give you false positives (if your tests do not cover all scenarios) as well as false negatives (if your tests access code that is actually never used in a real world scenario).

I imagine the best way to really identify dead code would be to instrument your code with a coverage tool in a live running environment and to analyse code coverage over an extended period of time.

If you are runnning in a load balanced redundant environment (and if not, why not?) then I suppose it would make sense to only instrument one instance of your application and to configure your load balancer such that a random, but small, portion of your users run on your instrumented instance. If you do this over an extended period of time (to make sure that you have covered all real world usage scenarios - such seasonal variations), you should be able to see exactly which areas of your code are accessed under real world usage and which parts are really never accessed and hence dead code.

I have never personally seen this done, and do not know how the aforementioned tools can be used to instrument and analyse code that is not being invoked through a test suite - but I am sure they can be.

IntelliJ has code analysis tools for detecting code which is unused. You should try making as many fields/methods/classes as non-public as possible and that will show up more unused methods/fields/classes

I would also try to locate duplicate code as a way of reducing code volume.

My last suggestion is try to find open source code which if used would make your code simpler.

Netbeans here is a plugin for Netbeans dead code detector.

It would be better if it could link to and highlight the unused code. You can vote and comment here: Bug 181458 - Find unused public classes, methods, fields

User coverage tools, such as EMMA. But it's not static tool (i.e. it requires to actually run the application through regression testing, and through all possible error cases, which is, well, impossible :) )

Still, EMMA is very useful.

In theory, you can't deterministically find unused code. Theres a mathematical proof of this (well, this is a special case of a more general theorem). If you're curious, look up the Halting Problem.

This can manifest itself in Java code in many ways:

• Loading classes based on user input, config files, database entries, etc;
• Loading external code;
• Passing object trees to third party libraries;
• etc.

That being said, I use IDEA IntelliJ as my IDE of choice and it has extensive analysis tools for findign dependencies between modules, unused methods, unused members, unused classes, etc. Its quite intelligent too like a private method that isn't called is tagged unused but a public method requires more extensive analysis.

There are tools which profile code and provide code coverage data. This lets you see (as code is run) how much of it is being called. You can get any of these tools to find out how much orphan code you have.

I found Clover coverage tool which instruments code and highlights the code that is used and that is unused. Unlike Google CodePro Analytics, it also works for WebApplications (as per my experience and I may be incorrect about Google CodePro).

The only drawback that I noticed is that it does not takes Java interfaces into account.

The Structure101 slice perspective will give a list (and dependency graph) of any "orphans" or "orphan groups" of classes or packages that have no dependencies to or from the "main" cluster.

One thing I've been known to do in Eclipse, on a single class, is change all of its methods to private and then see what complaints I get. For methods that are used, this will provoke errors, and I return them to the lowest access level I can. For methods that are unused, this will provoke warnings about unused methods, and those can then be deleted. And as a bonus, you often find some public methods that can and should be made private.

But it's very manual.

We've started to use Find Bugs to help identify some of the funk in our codebase's target-rich environment for refactorings. I would also consider Structure 101 to identify spots in your codebase's architecture that are too complicated, so you know where the real swamps are.

Code coverage tools, such as Emma, Cobertura, and Clover, will instrument your code and record which parts of it gets invoked by running a suite of tests. This is very useful, and should be an integral part of your development process. It will help you identify how well your test suite covers your code.

However, this is not the same as identifying real dead code. It only identifies code that is covered (or not covered) by tests. This can give you false positives (if your tests do not cover all scenarios) as well as false negatives (if your tests access code that is actually never used in a real world scenario).

I imagine the best way to really identify dead code would be to instrument your code with a coverage tool in a live running environment and to analyse code coverage over an extended period of time.

If you are runnning in a load balanced redundant environment (and if not, why not?) then I suppose it would make sense to only instrument one instance of your application and to configure your load balancer such that a random, but small, portion of your users run on your instrumented instance. If you do this over an extended period of time (to make sure that you have covered all real world usage scenarios - such seasonal variations), you should be able to see exactly which areas of your code are accessed under real world usage and which parts are really never accessed and hence dead code.

I have never personally seen this done, and do not know how the aforementioned tools can be used to instrument and analyse code that is not being invoked through a test suite - but I am sure they can be.

• FindBugs is excellent for this sort of thing.
• PMD (Project Mess Detector) is another tool that can be used.

However, neither can find public static methods that are unused in a workspace. If anyone knows of such a tool then please let me know.