In Maven, Why Run 'mvn clean'?

Question

I am wondering what the major difference between running mvn compile and mvn clean compile are, in practicality.

I understand what the actual difference is, that mvn clean compile deletes all the generated files and starts again from scratch, but why would we want to do this? I can assume mvn compile will regenerate files if it's necessary, right?

One thing I noticed in my project was that if you had deleted a source file, without running clean, the compiled file remains, which usually wouldn't be a problem, but could be I suppose.

Answer 1

For example: If you rename a class, the previous compiled version will remain in target/classes until you run clean. This maybe completely harmless, but it could cause issues if it is autodetected by classpath scanning and the like.

Answer 2

Certain plugins require a clean in order to work properly. For instance (at least in Maven 2), the maven-war-plugin explodes each dependent WAR into an existing directory tree. It requires a clean to get rid of files that have been removed from the dependent WARs.

Another problem is that when you rename a class, the old compiled version can hang around in the build tree, and will get included in JAR files, etcetera ... until you run mvn clean.

I can assume "mvn compile" will regenerate files if it's necessary, right?

For mainstream plugins, that is a fair assumption. However, if you are using a plugin to generate source code components, I'd look carefully at the documentation, and at where you put the generated source code. For instance, there are a couple of unsupported plugins whose purpose is to drive the Eclipse EMF code generator.