Sonar is an open source web-based application to manage code quality which covers seven axes of code quality as: Architecture and design, comments, duplications, unit tests, complexity, potential bugs and coding rules. Developed in Java and can cover projects in Java, Flex, PHP, PL/SQL, Cobol and Visual Basic 6.

It's very efficient to navigate, offering visual reporting and you can follow metrics evolution of your project and combine them.

There is an online project called Nemo dedicated to open source projects, as you can see projects like Jetty, Apache Lucene and Apache Tomcat.

So, let's config Sonar to work together with Maven. First of all set up Sonar server and other configurations  in Maven's settings.xml file:

<profile>   <id>sonar</id>   <activation>      <activeByDefault>true</activeByDefault>   </activation>   <properties>      <sonar.jdbc.url>jdbc:postgresql://localhost/sonar</sonar.jdbc.url>      <sonar.jdbc.driver>org.postgresql.Driver</sonar.jdbc.driver>      <sonar.jdbc.username>user</sonar.jdbc.username>      <sonar.jdbc.password>password</sonar.jdbc.password>      <!-- SERVER ON A REMOTE HOST -->      <sonar.host.url>http://localhost:9000</sonar.host.url>   </properties></profile>

You must to have a Sonar server running and define it in sonar.host.url parameter. In this example I'm using the default Sonar URL, http://localhost:9000 , and you must set user name and password for your database. To install local Sonar Server you can see this link.

After that, to execute the code analysers and save results in Sonar database just execute mvn sonar:sonar. You can config Sonar to execute in your CI (Continuous Integration) application.  Hudson is a  perfect match, because there is a Sonar plugin for it. See the final result below:

I prefer Teamcity CI, but there isn't a stable plugin as you can see in compatibility matrix. To resolve this plugin's problem, just add sonar:sonar in the maven command executed by Teamcity.

Sonar is an essential tool for your software project to help you evaluating how cohesive your classes are, warning for future problems as code complexity or duplication problems arise, and will help you to have a cleaner code. 

If you have any question or some difficulties, contact me.