Now we look at how Gradle provides build-by-convention and out of the box functionality. These features are decoupled from the core of Gradle, and are provided via plugins. Although the plugins are decoupled, we would like to point out that the Gradle core plugins are NEVER updated or changed for a particular Gradle distribution. If there is a bug in the compile functionality of Gradle, we will release a new version of Gradle. There is no change of behavior for the lifetime of a given distribution of Gradle.
If you want to use the plugin for building a Java project, simply include
in your script. That's all. From a technological point of view plugins use just the same operations as you can use from your build scripts. That is, they use the Project and Task API. The Gradle plugins generally use this API to:
Add tasks to the project (e.g. compile, test)
Create dependencies between those tasks to let them execute in the appropriate order.
Add dependency configurations to the project.
Add a so called convention object to the project.
Let's check this out:
Example 18.2. Applying a plugin by id
build.gradle
apply plugin: 'java'
task show << {
println relativePath(compileJava.destinationDir)
println relativePath(processResources.destinationDir)
}Output of gradle -q show
> gradle -q show build/classes/main build/resources/main
The Java plugin adds a compileJava task and a processResources task
to the project object which can be accessed by a build script. It has configured the destinationDir
property of both of these tasks.
The apply() method either takes a string or a class as an argument. You can write
Thanks to Gradle's default imports (see Appendix D, Existing IDE Support and how to cope without it) you can also write in this case.
Any class, which implements the Plugin interface, can be used as a
plugin. Just pass the class as an argument. You don't need to configure anything else for this.
If you want to use your own plugins, you must make sure that they are accessible via the build script classpath (see Chapter 45, Organizing Build Logic for more information). To learn more about how to write custom plugins, see Chapter 44, Writing Custom Plugins.
If you use the Java plugin
for example, there are a compileJava and a processResources task for
your production code (the same is true for your test
code). What if you want to change the default configuration? Let's try:
Example 18.5. Configuring a plugin task
build.gradle
apply plugin: 'java' task show << { processResources.destinationDir = new File(buildDir, 'output') println relativePath(processResources.destinationDir) println relativePath(compileJava.destinationDir) }
Output of gradle -q show
> gradle -q show build/output build/classes/main
Setting the destinationDir
of the processResources task had only an effect on the processResources
task. Maybe this was what you wanted. But what if
you want to change the output directory for all tasks? It would be unfortunate if you had to do this for
each task separately.
Gradle's tasks are usually convention aware. A plugin can add a convention object to your project, and map certain values of this convention object to task properties.
Example 18.6. Plugin convention object
build.gradle
apply plugin: 'java' task show << { sourceSets.main.output.classesDir = new File(buildDir, 'output/classes') sourceSets.main.output.resourcesDir = new File(buildDir, 'output/resources') println relativePath(compileJava.destinationDir) println relativePath(processResources.destinationDir) }
Output of gradle -q show
> gradle -q show build/output/classes build/output/resources
The Java plugin has added a convention object with a sourceSets
property, which we use to set the classes directory.
By setting a task attribute explicitly (as we have done in the first example) you overwrite the convention value for this particular task.
Not all of the tasks attributes are mapped to convention object values. It is the decision of the plugin to decide what are the shared properties and then bundle them in a convention object and map them to the tasks.
The properties of a convention object can be accessed as project properties. As shown in the following example, you can also access the convention object explicitly.
Example 18.7. Using the plugin convention object
build.gradle
apply plugin: 'java' task show << { // Access the convention property as a project property println relativePath(sourceSets.main.output.classesDir) println relativePath(project.sourceSets.main.output.classesDir) // Access the convention property via the convention object println relativePath(project.convention.plugins.java.sourceSets.main.output.classesDir) }
Output of gradle -q show
> gradle -q show build/classes/main build/classes/main build/classes/main
Every project object has a Convention object which is a
container for convention objects contributed
by the plugins declared for your project. If you simply access or set a property or access a method in
your build script, the project object first looks if this is a property of itself. If not, it delegates
the request to its convention object. The convention object checks if any of the plugin convention
objects can fulfill the request (first wins and the order is not defined). The plugin convention objects
also introduce a namespace.
A plugin is only called once for a given project, even if you have multiple
apply()
statements. An additional call after the first call has no effect but doesn't hurt either. This can be
important if you use plugins which extend other plugins. For example
the Groovy plugin automatically applies the Java plugin. We say the Groovy plugin extends the Java plugin. But you might as well
write:
Example 18.8. Explicit application of an implied plugin
build.gradle
apply plugin: 'java' apply plugin: 'groovy'
If you use cross-project configuration in multi-project builds this is a useful feature.