When we use Rule based model configuration in our Gradle project we can give Gradle rules on how to manage objects from the model space. These rules are defined in a class that extends RuleSource. When we want to set some default values for properties of a model object (in Gradle terms this is a subject) we can use the @Defaults annotation. Rules annotated with @Defaults are invoked right after the object is created and before any other methods that can mutate the state of the object.

The method, to set the default values, must have the type of the object as first parameter. Other parameters are considered input parameters and can be used to set a default value based on other model objects.

In a previous post we worked with a managed object VersionFile. Let's add a default value for the version property. We want the version default value to be the unspecified:

// File: buildSrc/src/main/groovy/mrhaki/gradle/VersionFileTaskRules.groovy
package mrhaki.gradle

import org.gradle.api.Task
import org.gradle.model.Defaults
import org.gradle.model.Model
import org.gradle.model.ModelMap
import org.gradle.model.Mutate
import org.gradle.model.Path
import org.gradle.model.RuleSource

class VersionFileTaskRules extends RuleSource {

    @Model
    void versionFile(final VersionFile versionFile) { }

    /**
     * Method to set default values for {@link VersionFile} object
     * created by the {@link #versionFile} method.
     *
     * @param versionFile First argument is the type we want to set default values for
     */
    @Defaults
    void defaultsVersionFile(
            final VersionFile versionFile) {

        // Set default value for version property to.
        versionFile.version = 'unspecified'
    }

    @Mutate
    void createVersionFileTask(final ModelMap tasks, final VersionFile versionFile) {
        tasks.create('generateVersionFile', VersionFileTask) { task ->
            task.version = versionFile.version
            task.outputFile = versionFile.outputFile
        }
    }

} 

When we run the model task we can see Gradle knows the defaultsVersionFile methods was used to change the state of the VersionFile instance:

$ gradle -q model
...
+ versionFile
      | Type:           mrhaki.gradle.VersionFile
      | Creator:        VersionFileTaskRules#versionFile(VersionFile)
      | Rules:
         ⤷ VersionFileTaskRules#defaultsVersionFile(VersionFile)
    + outputFile
          | Type:       java.io.File
          | Value:      /Users/mrhaki/Projects/sample/build/version.txt
          | Creator:    VersionFileTaskRules#versionFile(VersionFile)
    + version
          | Type:       java.lang.String
          | Value:      null
          | Creator:    VersionFileTaskRules#versionFile(VersionFile)
...
$

Written with Gradle 3.2.

Original blog post