The Cobertura tool is a free and easy to use source code test coverage analyser. It helps you to discover where your source-code lacks in test coverage.
For more information regarding Cobertura check out the project homepage.
Cobertura requires compiling with debug on, which is the Maven default. If running maven-cobertura-plugin yields no results, start by checking the setting of maven.compile.debug.
To use the plugin:
<reports> ... <report>maven-cobertura-plugin</report> ... </reports>