Introduction to multi-project builds

1 Structure of a multi-project build

• A settings.gradle file in the root or master directory of the project
• A build.gradle file in the root or master directory
• Child directories that have their own *.gradle build files (some multi-project builds may omit child project build scripts)

The settings.gradle file tells Gradle how the project and subprojects are structured. Fortunately, you don’t have to read this file simply to learn what the project structure is as you can run the command gradle projects.

gradle -q projects

By default, Gradle uses the name of the directory it finds the settings.gradle as the name of the root project. This usually doesn't cause problems since all developers check out the same directory name when working on a project. On Continuous Integration servers, like Jenkins, the directory name may be auto-generated and not match the name in your VCS. For that reason, it's recommended that you always set the root project name to something predictable, even in single project builds. You can configure the root project name by setting rootProject.name.

Each project will usually have its own build file, but that's not necessarily the case. In the above example, the services project is just a container or grouping of other subprojects. There is no build file in the corresponding directory. However, multiproject does have one for the root project.

The root build.gradle is often used to share common configuration between the child projects, for example by applying the same sets of plugins and dependencies to all the child projects. It can also be used to configure individual subprojects when it is preferable to have all the configuration in one place. This means you should always check the root build file when discovering how a particular subproject is being configured.

Another thing to bear in mind is that the build files might not be called build.gradle. Many projects will name the build files after the subproject names, such as api.gradle and services.gradle from the previous example. Such an approach helps a lot in IDEs because it’s tough to work out which build.gradle file out of twenty possibilities is the one you want to open. This little piece of magic is handled by the settings.gradle file, but as a build user you don’t need to know the details of how it’s done. Just have a look through the child project directories to find the files with the .gradle suffix.

Once you know what subprojects are available, the key question for a build user is how to execute the tasks within the project.

2 Executing a multi-project build

Use a qualified task name from any directory, although this is usually done from the root. For example: gradle :services:webservice:build will build the webservice subproject and any subprojects it depends on.

if you want to know what tasks are in a particular subproject, just use the tasks task, e.g. gradle :services:webservice:tasks