Chapter 41. Organizing Build Logic

Any method or property defined in a project build script is also visible to all the sub-projects. You can use this to define common configurations, and to extract build logic into methods which can be reused by the sub-projects.

// Define an extra property
ext.srcDirName = 'src/java'

// Define a method
def getSrcDir(project) {
    return project.file(srcDirName)
}

task show << {
    // Use inherited property
    println 'srcDirName: ' + srcDirName

    // Use inherited method
    File srcDir = getSrcDir(project)
    println 'srcDir: ' + rootProject.relativePath(srcDir)
}

> gradle -q show
srcDirName: src/java
srcDir: child/src/java

You can use the configuration injection technique discussed in Section 24.1, “Cross project configuration” and Section 24.2, “Subproject configuration” to inject properties and methods into various projects. This is generally a better option than inheritance, for a number of reasons: The injection is explicit in the build script, You can inject different logic into different projects, And you can inject any kind of configuration such as repositories, plug-ins, tasks, and so on. The following sample shows how this works.

subprojects {
    // Define a new property
    ext.srcDirName = 'src/java'

    // Define a method using a closure as the method body
    ext.srcDir = { file(srcDirName) }

    // Define a task
    task show << {
        println 'project: ' + project.path
        println 'srcDirName: ' + srcDirName
        File srcDir = srcDir()
        println 'srcDir: ' + rootProject.relativePath(srcDir)
    }
}

// Inject special case configuration into a particular project
project(':child2') {
    ext.srcDirName = "$srcDirName/legacy"
}

// Use injected property and method. Here, we override the injected value
srcDirName = 'java'
def dir = srcDir()

> gradle -q show
project: :child1
srcDirName: java
srcDir: child1/java
project: :child2
srcDirName: src/java/legacy
srcDir: child2/src/java/legacy

You can configure the current project using an external build script. All of the Gradle build language is available in the external script. You can even apply other scripts from the external script.

apply from: 'other.gradle'

println "configuring $project"
task hello << {
    println 'hello from other script'
}

> gradle -q hello
configuring root project 'configureProjectUsingScript'
hello from other script

When you run Gradle, it checks for the existence of a directory called buildSrc. Gradle then automatically compiles and tests this code and puts it in the classpath of your build script. You don't need to provide any further instruction. This can be a good place to add your custom tasks and plugins.

For multi-project builds there can be only one buildSrc directory, which has to be in the root project directory.

Listed below is the default build script that Gradle applies to the buildSrc project:

apply plugin: 'groovy'
dependencies {
    compile gradleApi()
    compile localGroovy()
}

This means that you can just put your build source code in this directory and stick to the layout convention for a Java/Groovy project (see Table 45.4, “Java plugin - default project layout”).

If you need more flexibility, you can provide your own build.gradle. Gradle applies the default build script regardless of whether there is one specified. This means you only need to declare the extra things you need. Below is an example. Notice that this example does not need to declare a dependency on the Gradle API, as this is done by the default build script:

repositories {
    mavenCentral()
}

dependencies {
    testCompile 'junit:junit:4.12'
}

The buildSrc project can be a multi-project build, just like any other regular multi-project build. However, all of the projects that should be on the classpath of the actual build must be runtime dependencies of the root project in buildSrc. You can do this by adding this to the configuration of each project you wish to export:

rootProject.dependencies {
  runtime project(path)
}

You can use the GradleBuild task. You can use either of the dir or buildFile properties to specify which build to execute, and the tasks property to specify which tasks to execute.

task build(type: GradleBuild) {
    buildFile = 'other.gradle'
    tasks = ['hello']
}

task hello << {
    println "hello from the other build."
}

> gradle -q build
hello from the other build.

If your build script needs to use external libraries, you can add them to the script's classpath in the build script itself. You do this using the buildscript() method, passing in a closure which declares the build script classpath.

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath group: 'commons-codec', name: 'commons-codec', version: '1.2'
    }
}

The closure passed to the buildscript() method configures a ScriptHandler instance. You declare the build script classpath by adding dependencies to the classpath configuration. This is the same way you declare, for example, the Java compilation classpath. You can use any of the dependency types described in Section 23.4, “How to declare your dependencies”, except project dependencies.

Having declared the build script classpath, you can use the classes in your build script as you would any other classes on the classpath. The following example adds to the previous example, and uses classes from the build script classpath.

import org.apache.commons.codec.binary.Base64

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath group: 'commons-codec', name: 'commons-codec', version: '1.2'
    }
}

task encode << {
    def byte[] encodedString = new Base64().encode('hello world\n'.getBytes())
    println new String(encodedString)
}

> gradle -q encode
aGVsbG8gd29ybGQK

For multi-project builds, the dependencies declared with a project's buildscript() method are available to the build scripts of all its sub-projects.

Build script dependencies may be Gradle plugins. Please consult Chapter 25, Gradle Plugins for more information on Gradle plugins.

Every project automatically has a buildEnvironment task of type BuildEnvironmentReportTask that can be invoked to report on the resolution of the build script dependencies.

For reasons we don't fully understand yet, external dependencies are not picked up by Ant's optional tasks. But you can easily do it in another way. ^[21]

configurations {
    ftpAntTask
}

dependencies {
    ftpAntTask("org.apache.ant:ant-commons-net:1.9.6") {
        module("commons-net:commons-net:1.4.1") {
            dependencies "oro:oro:2.0.8:jar"
        }
    }
}

task ftp << {
    ant {
        taskdef(name: 'ftp',
                classname: 'org.apache.tools.ant.taskdefs.optional.net.FTP',
                classpath: configurations.ftpAntTask.asPath)
        ftp(server: "ftp.apache.org", userid: "anonymous", password: "me@myorg.com") {
            fileset(dir: "htdocs/manual")
        }
    }
}

This is also a good example for the usage of client modules. The POM file in Maven Central for the ant-commons-net task does not provide the right information for this use case.

Gradle offers you a variety of ways of organizing your build logic. You can choose what is right for your domain and find the right balance between unnecessary indirections, and avoiding redundancy and a hard to maintain code base. It is our experience that even very complex custom build logic is rarely shared between different builds. Other build tools enforce a separation of this build logic into a separate project. Gradle spares you this unnecessary overhead and indirection.