How to detect duplicate JARs in the classpath?

Question

Does anyone have code to detect duplicate JARs in the classpath?

Background: When there are two versions of the same JAR in the classpath, really strange things can happen. This can even happen when using tools like Maven: Change a dependency and build the WAR without cleaning first. Since target/webapp/WEB-INF/lib wasn't cleaned, the dependency will be in there twice.

Is there a safety-net for this?

Answer 1

JBoss Tattletale might help you with this.

It's a free tool which scans the JAR files used by your project and gives you a report about them.

Amongst its feature are:

• Spot if a class is located in multiple JAR files
• Spot if the same JAR file is located in multiple locations
• Find similar JAR files that have different version numbers

Answer 2

There is a Maven plugin to do just that: maven-duplicate-finder-plugin

[EDIT] If you want to do it in unit tests yourself, use

getClass().getClassLoader().getResources( "com/pany/package/Java.class" ) 

If this returns more than one URL, you have duplicates on the classpath.

The drawback is that this only works for cases where you had conflicts in the past. On the positive side, it's just a few lines of code (or one line when you write a helper method) and it works on your build server.