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

I think the simplest way is to simply trash the target directory first. Hopefully copying all the .jar files in isn't going to be time-consuming.

Otherwise you're going to have to somehow compare sizable files (whether directly, via computed checksum or similar). Which doesn't sound very nice at all.

Answer 2

You can write a simple script to compare the md5 sum of every jar file to every other jar file and deleting duplicates along the way.

Answer 3

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 4

System.getProperty("java.class.path"), split it, sort it, look at it with the human eye :-).

It will not include the classpath derived from manifests inside other jars thou :-(.

Or use http://www.jboss.org/tattletale as one of the posters suggested.