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

Question

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.

I'm a huge fan of organized sources, so I always create the following directory structure:

/src - for your packages & classes
/test - for unit tests
/docs - for documentation, generated and manually edited
/lib - 3rd party libraries
/etc - unrelated stuff
/bin (or /classes) - compiled classes, output of your compile
/dist - for distribution packages, hopefully auto generated by a build system

In /src I'm using the default Java patterns: Package names starting with your domain (org.yourdomain.yourprojectname) and class names reflecting the OOP aspect you're creating with the class (see the other commenters). Common package names like util, model, view, events are useful, too.

I tend to put constants for a specific topic in an own class, like SessionConstants or ServiceConstants in the same package of the domain classes.

Where I'm working, we're using Maven 2 and we have a pretty nice archetype for our projects. The goal was to obtain a good separation of concerns, thus we defined a project structure using multiple modules (one for each application 'layer'): - common: common code used by the other layers (e.g., i18n) - entities: the domain entities - repositories: this module contains the daos interfaces and implementations - services-intf: interfaces for the services (e.g, UserService, ...) - services-impl: implementations of the services (e.g, UserServiceImpl) - web: everything regarding the web content (e.g., css, jsps, jsf pages, ...) - ws: web services

Each module has its own dependencies (e.g., repositories could have jpa) and some are project wide (thus they belong in the common module). Dependencies between the different project modules clearly separate things (e.g., the web layer depends on the service layer but doesn't know about the repository layer).

Each module has its own base package, for example if the application package is "com.foo.bar", then we have:

com.foo.bar.common
com.foo.bar.entities
com.foo.bar.repositories
com.foo.bar.services
com.foo.bar.services.impl
...

Each module respects the standard maven project structure:

   src\
   ..main\java
     ...\resources
   ..test\java
     ...\resources

Unit tests for a given layer easily find their place under \src\test... Everything that is domain specific has it's place in the entities module. Now something like a FileStorageStrategy should go into the repositories module, since we don't need to know exactly what the implementation is. In the services layer, we only know the repository interface, we do not care what the specific implementation is (separation of concerns).

There are multiple advantages to this approach:

• clear separation of concerns
• each module is packageable as a jar (or a war in the case of the web module) and thus allows for easier code reuse (e.g., we could install the module in the maven repository and reuse it in another project)
• maximum independence of each part of the project

I know this doesn't answer all your questions, but I think this could put you on the right path and could prove useful to others.

Class names should always be descriptive and self-explanatory. If you have multiple domains of responsibility for your classes then they should probably be refactored.

Likewise for you packages. They should be grouped by domain of responsibility. Every domain has it's own exceptions.

Generally don't sweat it until you get to a point where it is becoming overwhelming and bloated. Then sit down and don't code, just refactor the classes out, compiling regularly to make sure everything works. Then continue as you did before.

Use packages to group related functionality together.

Usually the top of your package tree is your domain name reversed (com.domain.subdomain) to guarantee uniqueness, and then usually there will be a package for your application. Then subdivide that by related area, so your FileStorageStrategy might go in, say, com.domain.subdomain.myapp.storage, and then there might be specific implementations/subclasses/whatever in com.domain.subdomain.myapp.storage.file and com.domain.subdomain.myapp.storage.database. These names can get pretty long, but import keeps them all at the top of files and IDEs can help to manage that as well.

Exceptions usually go in the same package as the classes that throw them, so if you had, say, FileStorageException it would go in the same package as FileStorageStrategy. Likewise an interface defining constants would be in the same package.

There's not really any standard as such, just use common sense, and if it all gets too messy, refactor!

One thing that I found very helpful for unit tests was to have a myApp/src/ and also myApp/test_src/ directories. This way, I can place unit tests in the same packages as the classes they test, and yet I can easily exclude the test cases when I prepare my production installation.