Imagine you are working on a project based upon a traditional 3 layered architecture, in which the layers are named presentation, business and integration. Of course, you want each layer to be independant of all the other layers and so you want to produce one artifact for each one. Following the Maven norm, you will need to have one project for each layer. Hence, your setup should look like this :
So to compile your presentation project, you would first need to compile the business project. But wait, the business project depends upon the integration project so it should be compiled first. Ok, let's recapitulate. So to compile your presentation project, you would need to type the following commands (we assume the projects have been put in a common root directory):
/root_directory$/ cd integration
Few! 8 lines just to compile! I thought Maven was simple. Of course, Maven offers a solution to this problem in the form of a multi-modules project.
A multi-modules project is a very particular type of project - it doesn't produce any artifact and is composed of several other projects known as modules. When you run a command on the project, it will execute it on all of its children projects. Even better, Maven is able, through its reactor component (don't worry about it for now), to discover the correct execution order and to detect circular dependencies. So let's apply this solution to the last example. Your project should have the following structure at the end of this lesson:
First, create a new directory named after your project, something like myproject (original isn't it?). Now let's write the pom file for your multi-modules project:
As you can see, there is nothing very complex in this pom file. It is a normal parent project's pom file, which was introduced in the lesson [todo : link]. Only, this time there is something new: the modules section. The modules section allows you to declare the child modules of a parent project, hence the name multi-modules project. Modules are regular Maven projects which must be located under a subdirectory so the parent project can retrieve them easily.
Maven doesn't force you to place your modules at a specific location. You can put them wherever you want as long as they are under the parent project's directory and their paths are declared correctly in the modules section (for instance, modules/presentation). However, there is no point in not placing them directly under your parent project unless you really need to because of some external constraints.
Next, create the following subdirectories and the pom.xml files that go with them [todo : explains the directory name restriction]:
Now let's fill the presentation project's pom file: