16.4. Declaring variables
There are two kinds of variables that can be declared in a build script: local variables and extra properties.
16.4.1. Local variables
Local variables are declared with the def keyword. They are only visible in the scope where they have been declared. Local variables are a feature of the underlying Groovy language.
Example 16.2. Using local variables
build.gradle
def dest = "dest"
task copy(type: Copy) { from "source"
into dest
}
16.4.2. Extra properties
All enhanced objects in Gradle's domain model can hold extra user-defined properties. Thi includes, but is not limited to, projects, tasks, and source sets. Extra properties can be added, read and set via the owning object'sext property.
Example 16.3. Using extra properties
build.gradle
apply plugin: "java"
sourceSets.all { ext.purpose = null }
sourceSets { main {
purpose = "production"
}
test {
purpose = "test"
}
plugin {
ext.purpose = "production"
}
}
task printProductionSourceDirs << {
sourceSets.matching { it.purpose == "production" }.each { println it.java.
}
In this example, a property named purpose is added to all source sets by setting ext.purpose to null (null is a permissible value). Once the property has been added, it can be read and set
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like a predefined property. Alternatively, ext. can be used as well.
By requiring special syntax for adding a property, Gradle can fail fast when an attempt is made to set a (predefined or extra) property but the property is misspelled or does not exist. [6] Extra properties can be accessed from anywhere their owning object can be accessed, giving them a wider scope than local variables. Extra properties on a parent project are visible from subprojects.
For further details on extra properties and their API, see ExtraPropertiesExtension.
16.5. Some Groovy basics
Groovy provides plenty of features for creating DSLs, and the Gradle build language takes advantage of these. Understanding how the build language works will help you when you write your build script, and in particular, when you start to write customs plugins and tasks.
16.5.1. Groovy JDK
Groovy adds lots of useful methods to JVM classes. For example, Iterable gets an each method, which iterates over the elements of the Iterable:
Example 16.4. Groovy JDK methods
build.gradle
// Iterable gets an each() method configurations.runtime.each { File f -> println f }
Have a look at http://groovy.codehaus.org/groovy-jdk/ for more details.
16.5.2. Property accessors
Groovy automatically converts a property reference into a call to the appropriate getter or setter method.
Example 16.5. Property accessors
build.gradle
//Using a getter method println project.buildDir
println getProject().getBuildDir()
//Using a setter method project.buildDir = 'target' getProject().setBuildDir('target')
16.5.3. Optional parentheses on method calls
Parentheses are optional for method calls.
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Example 16.6. Method call without parentheses
build.gradle
test.systemProperty 'some.prop', 'value' test.systemProperty('some.prop', 'value')
16.5.4. List and map literals
Groovy provides some shortcuts for defining List and Map instances.
Example 16.7. List and map literals
build.gradle
// List literal
test.includes = ['org/gradle/api/**', 'org/gradle/internal/**']
List<String> list = new ArrayList<String>() list.add('org/gradle/api/**') list.add('org/gradle/internal/**') test.includes = list
// Map literal
apply plugin: 'java'
Map<String, String> map = new HashMap<String, String>() map.put('plugin', 'java')
apply(map)
16.5.5. Closures as the last parameter in a method
The Gradle DSL uses closures in many places. You can find out more about closures here. When the last parameter of a method is a closure, you can place the closure after the method call:
Example 16.8. Closure as method parameter
build.gradle
repositories {
println "in a closure"
}
repositories() { println "in a closure" } repositories({ println "in a closure" })
16.5.6. Closure delegate
Each closure has a delegate object, which Groovy uses to look up variable and method references which are not local variables or parameters of the closure. Gradle uses this for configuration closures, where the delegate object is set to the object to be configured.
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Example 16.9. Closure delegates
build.gradle
dependencies {
assert delegate == project.dependencies compile('junit:junit:4.8.2') delegate.compile('junit:junit:4.8.2')
}
[6] As of Gradle 1.0 milestone 9, using ext to add extra properties is strongly encouraged but not yet enforced. Therefore, Gradle will not fail when an unknown property is set. However, it will print a warning.
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