How is "mvn clean install" different from "mvn install"?

clean is its own build lifecycle phase (which can be thought of as an action or task) in Maven. mvn clean install tells Maven to do the clean phase in each module before running the install phase for each module.

What this does is clear any compiled files you have, making sure that you're really compiling each module from scratch.

Maven lets you specify either goals or lifecycle phases on the command line (or both).

clean and install are two different phases of two different lifecycles, to which different plugin goals are bound (either per default or explicitly in your pom.xml)

The clean phase, per convention, is meant to make a build reproducible, i.e. it cleans up anything that was created by previous builds. In most cases it does that by calling clean:clean, which deletes the directory bound to ${project.build.directory} (usually called "target")

You can call more than one target goal with maven. mvn clean install calls clean first, then install. You have to clean manually, because clean is not a standard target goal and not executed automatically on every install.

clean removes the target folder - it deletes all class files, the java docs, the jars, reports and so on. If you don't clean, then maven will only "do what has to be done", like it won't compile classes when the corresponding source files haven't changed (in brief).

^{we call it target in ant and goal in maven}

To stick with the Maven terms:

• "clean" is a phase of the clean lifecycle
• "install" is a phase of the default lifecycle

http://maven.apache.org/guides/introduction/introduction-to-the-lifecycle.html#Lifecycle_Reference