How should I structure a Java application, where do I put my classes?

First of all, I know how to build a Java application. But I have always been puzzled about where to put my classes. There are proponents for organizing the packages in a strictly domain oriented fashion, others separate by tier.

I myself have always had problems with

• naming,
• placing

So,

1. Where do you put your domain specific constants (and what is the best name for such a class)?
2. Where do you put classes for stuff which is both infrastructural and domain specific (for instance I have a FileStorageStrategy class, which stores the files either in the database, or alternatively in database)?
3. Where to put Exceptions?
4. Are there any standards to which I can refer?

I've really come to like Maven's Standard Directory Layout.

One of the key ideas for me is to have two source roots - one for production code and one for test code like so:

MyProject/src/main/java/com/acme/Widget.java
MyProject/src/test/java/com/acme/WidgetTest.java

(here, both src/main/java and src/test/java are source roots).

Advantages:

• Your tests have package (or "default") level access to your classes under test.
• You can easily package only your production sources into a JAR by dropping src/test/java as a source root.

One rule of thumb about class placement and packages:

Generally speaking, well structured projects will be free of circular dependencies. Learn when they are bad (and when they are not), and consider a tool like JDepend or SonarJ that will help you eliminate them.