When building out your architecture, I would encourage you to break out your APIs as first-class modules. (Sorry, a ‘module’ is Maven speak for the parts of a project that create their own artifacts such as JAR or WAR files.)
For an example of a simple suggested structure, look at the following image. (Arrows denote compile-time dependencies.):
In some of my recent projects I received some push-back on breaking out the APIs. The concern was that breaking them out as modules would add overhead to the build process. I do understand that concern, but I think it is a minimized threat if you are using a fairly sophisticated build system.
I acquiesced at the time, and now we have a structure that looks similar to the next image. (Can you see the problem already?):
The black arrow indicates just one of the weaknesses of this model. Here, you can see that a UI component can (at the least offensive) access the DAO API as well as (at the most offensive) access the concrete DAO implementations.
When you have a structure like this, the question is not if, but when will one of your fellow engineers take advantage of this and circumvent the nice functional striations that you have taken so much time to isolate?