Chapter 13. Writing Build Scripts

This chapter looks at some of the details of writing a build script.

13.1. Project API

In the tutorial in Chapter 6, Java Quickstart we used, for example, the apply() method. Where does this method come from? We said earlier that the build script defines a project in Gradle. For each project in the build creates an instance of type Project and associates this Project object with the build script. As the build script executes, it configures this Project object:

Getting help writing build scripts

Don't forget that your build script is simply Groovy code that drives the Gradle API. And the Project interface is your starting point for accessing everything in the Gradle API. So, if you're wondering what 'tags' are available in your build script, you can start with the documentation for the Project interface.

Any method you call in your build script, which is not defined in the build script, is delegated to the Project object.
Any property you access in your build script, which is not defined in the build script, is delegated to the Project object.

Let's try this out and try to access the name property of the Project object.

Example 13.1. Accessing property of the Project object

build.gradle

println name
println project.name

task check << {
}

Output of gradle -q check

> gradle -q check
projectApi
projectApi

Both println statements print out the same property. The first uses auto-delegation to the Project object, for properties not defined in the build script. The other statement uses the project property available to any build script, which returns the associated Project object. Only if you define a property or a method which has the same name as a member of the Project object, you need to use the project property.

13.1.1. Standard project properties

The Project object provides some standard properties, which are available in your build script. The following table lists a few of the commonly used ones.

Table 13.1. Project Properties

Name	Type	Default Value
`project`	`Project`	The `Project` instance
`name`	`String`	The name of the directory containing the build script.
`path`	`String`	The absolute path of the project.
`description`	`String`	A description for the project.
`buildFile`	`File`	The build script.
`projectDir`	`File`	The directory containing the build script.
`buildDirName`	`String`	`build`
`buildDir`	`File`	`projectDir/build`
`group`	`Object`	`unspecified`
`version`	`Object`	`unspecified`
`ant`	`AntBuilder`	An `AntBuilder` instance

Below is a sample build which demonstrates some of these properties.

Example 13.2. Project properties

Build layout

projectCoreProperties/
  build.gradle
  subProject/
    build.gradle

build.gradle

task check << {
    allprojects {
        println "project path $path"
        println "  project name = $name"
        println "  project dir = '${rootProject.relativePath(projectDir)}'"
        println "  build file = '${rootProject.relativePath(buildFile)}'"
        println "  build dir = '${rootProject.relativePath(buildDir)}'"
    }
}

Output of gradle -q check

> gradle -q check
project path :
  project name = projectCoreProperties
  project dir = '.'
  build file = 'build.gradle'
  build dir = 'build'
project path :subProject
  project name = subProject
  project dir = 'subProject'
  build file = 'subProject/build.gradle'
  build dir = 'subProject/build'

13.2. Script API

When Gradle executes a script, it compiles the script into a class which implements Script.