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 type
usePlugin('java')
at the beginning of 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 (see Chapter 11, The Project and Task API). The Gradle plugins use this API for:
Adding tasks to your build (e.g. compile, test)
Creating dependencies between those tasks to let them execute in the appropriate order.
Adding a so called convention object to your project configuration.
Let's check this out:
Example 15.1. Using plugin
build.gradle
usePlugin('java') task check << { println(compile.destinationDir.name) }
Output of gradle -q check
> gradle -q check classes
The Java Plugin adds a compile
task to the project object which can be accessed by a build script.
The usePlugin method either takes a string or a class as an argument. You can write [14]
usePlugin(org.gradle.api.plugins.JavaPlugin)
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 access a custom plugin via a
string identifier, you must inform Gradle about the mapping. You can do this in the file
plugin.properties
in the top level directory of Gradle. It looks like this for the current release:
Figure 15.1. plugin.properties
java=org.gradle.api.plugins.JavaPlugin eclipse=org.gradle.api.plugins.EclipsePlugin groovy=org.gradle.api.plugins.GroovyPlugin war=org.gradle.api.plugins.WarPlugin osgi=org.gradle.api.plugins.osgi.OsgiPlugin jetty=org.gradle.api.plugins.jetty.JettyPlugin maven=org.gradle.api.plugins.MavenPlugin project-reports=org.gradle.api.plugins.ProjectReportsPlugin
If you want to use your own plugins, you must make sure that they are accessible via the build script classpath (see Chapter 29, Organizing Build Logic for more information). To learn more about how to write custom plugins, see Chapter 24, How to write Custom Plugins.
If you use the Java Plugin
for example, there are a compile and a processResources task for your production code (the same is true for your test
code). The default location for the output of those tasks is the directory build/classes
.
What if you want to change this? Let's try:
Example 15.2. Configuring a plugin
build.gradle
usePlugin('java') task check << { processResources.destinationDir = new File(buildDir, 'output') println(processResources.destinationDir.name) println(compile.destinationDir.name) }
Output of gradle -q check
> gradle -q check output classes
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.
Gradles tasks are usually convention aware. A plugin can add a convention object to your build. It can also map certain values of this convention object to task properties.
Example 15.3. Plugin convention object
build.gradle
usePlugin('java') task check << { classesDirName = 'output' println(processResources.destinationDir.name) println(compile.destinationDir.name) println(convention.classesDirName) }
Output of gradle -q check
> gradle -q check output output output
The Java Plugin has added a convention object with a classesDirName
property. The properties of a convention object can be accessed like project properties. As shown in the
example, you can also access the convention object explicitly.
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.
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.
usePlugin('java') println classesDir println convention.classesDir println convention.plugins.java.classesDir
All three statements print out the same property. The more specific statements are useful if there are ambiguities.
A plugin is only called once for a given project, even if you have multiple
usePlugin()
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
usePlugin('groovy')
calls also the Java Plugin. We say the Groovy plugin extends the Java plugin. But you might as well
write:
usePlugin('java') usePlugin('groovy')
If you use cross-project configuration in multi-project builds this is a useful feature.
Plugins provide tasks, which are glued together via dependsOn relations and a convention object.
[14] Thanks to Gradle's default imports (see Appendix C, Existing IDE Support and how to cope without it) you can also write
usePlugin(JavaPlugin)
in this case.