Unfortunately checking Java code for compliance with a given software architecture is not done on every project, as far as I know only a few experts do it. This is not good because "if it's not checked, it's not there". Free tool support for enforcing architectural rules has been lacking since long but recently things have changed. Well, maybe not recently but recently I thought about it :-)
There have always been a few basic tools to check the references of classes. One of these tools is Macker. It's quite old and not actively maintained any more, but I've been using it for years and it worked great for me. It's small and simple and yet immensely powerful. I love it and use it whenever possible. Everybody should use it. And don't turn it off!
A Shameless Plug
Macker and some other tools that can be used to enforce architecture rules are described in the fourth part of my 'Code Cop' series, published in the German magazine iX last summer: Automatisierte Architektur-Reviews (Automated Architecture Reviews) (iX 6/2010).
Architecture Reviews with Ant
The article describes several free tools (PMD, Macker and Classycle) and how to use them with Ant to verify different aspects of an architecture. Each of them has its advantages and disadvantages and when used together they are useful. The examples of the Ant integration and the PMD/Macker/Classycle architecture rules as given in the article should help you getting started with your own checks.
Macker and Maven
As I said above, Macker comes with proper Ant support, but Maven integration has been lacking. Last year I wanted to use Macker on an Maven project but was disappointed by the immaturity of the MackerMavenPlugin available on Codehaus. So I had to enhance it a bit. I submitted some patches which got accepted but still the MOJO wouldn't get promoted out of its sandbox state. Fortunately I keep an unofficial release (0.9) in my own Maven repository. So finally proper Maven integration of Macker is available.
I guess there are other tools available to be used with Maven. For example there is Architecture Rules with its maven-architecture-rules-plugin which uses JDepend under the hood. It looks promising but I haven't used it in production yet. And I hear that Sonar 2.4 is able to check architecture rules as well but I didn't try it till now.
(List of all my publications with abstracts.)