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Chapter 49. Multi-project Builds

The powerful support for multi-project builds is one of Gradle's unique selling points. This topic is also the most intellectually challenging.

49.1. Cross project configuration

Let's start with a very simple multi-project build. After all Gradle is a general purpose build tool at its core, so the projects don't have to be java projects. Our first examples are about marine life.

49.1.1. Defining common behavior

We have the following project tree. This is a multi-project build with a root project water and a subproject bluewhale.

Example 49.1. Multi-project tree - water & bluewhale projects

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/

Note: The code for this example can be found at samples/userguide/multiproject/firstExample/water which is in both the binary and source distributions of Gradle.

settings.gradle

include 'bluewhale'

And where is the build script for the bluewhale project? In Gradle build scripts are optional. Obviously for a single project build, a project without a build script doesn't make much sense. For multiproject builds the situation is different. Let's look at the build script for the water project and execute it:

Example 49.2. Build script of water (parent) project

build.gradle

Closure cl = { task -> println "I'm $task.project.name" }
task hello << cl
project(':bluewhale') {
    task hello << cl
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale

Gradle allows you to access any project of the multi-project build from any build script. The Project API provides a method called project(), which takes a path as an argument and returns the Project object for this path. The capability to configure a project build from any build script we call cross project configuration. Gradle implements this via configuration injection.

We are not that happy with the build script of the water project. It is inconvenient to add the task explicitly for every project. We can do better. Let's first add another project called krill to our multi-project build.

Example 49.3. Multi-project tree - water, bluewhale & krill projects

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/
  krill/

Note: The code for this example can be found at samples/userguide/multiproject/addKrill/water which is in both the binary and source distributions of Gradle.

settings.gradle

include 'bluewhale', 'krill'

Now we rewrite the water build script and boil it down to a single line.

Example 49.4. Water project build script

build.gradle

allprojects {
    task hello << { task -> println "I'm $task.project.name" }
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
I'm krill

Is this cool or is this cool? And how does this work? The Project API provides a property allprojects which returns a list with the current project and all its subprojects underneath it. If you call allprojects with a closure, the statements of the closure are delegated to the projects associated with allprojects. You could also do an iteration via allprojects.each, but that would be more verbose.

Other build systems use inheritance as the primary means for defining common behavior. We also offer inheritance for projects as you will see later. But Gradle uses configuration injection as the usual way of defining common behavior. We think it provides a very powerful and flexible way of configuring multiproject builds.

49.2. Subproject configuration

The Project API also provides a property for accessing the subprojects only.

49.2.1. Defining common behavior

Example 49.5. Defining common behaviour of all projects and subprojects

build.gradle

allprojects {
    task hello << {task -> println "I'm $task.project.name" }
}
subprojects {
    hello << {println "- I depend on water"}
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
I'm krill
- I depend on water

49.2.2. Adding specific behavior

You can add specific behavior on top of the common behavior. Usually we put the project specific behavior in the build script of the project where we want to apply this specific behavior. But as we have already seen, we don't have to do it this way. We could add project specific behavior for the bluewhale project like this:

Example 49.6. Defining specific behaviour for particular project

build.gradle

allprojects {
    task hello << {task -> println "I'm $task.project.name" }
}
subprojects {
    hello << {println "- I depend on water"}
}
project(':bluewhale').hello << {
    println "- I'm the largest animal that has ever lived on this planet."
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
- I'm the largest animal that has ever lived on this planet.
I'm krill
- I depend on water

As we have said, we usually prefer to put project specific behavior into the build script of this project. Let's refactor and also add some project specific behavior to the krill project.

Example 49.7. Defining specific behaviour for project krill

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/
    build.gradle
  krill/
    build.gradle

Note: The code for this example can be found at samples/userguide/multiproject/spreadSpecifics/water which is in both the binary and source distributions of Gradle.

settings.gradle

include 'bluewhale', 'krill'

bluewhale/build.gradle

hello.doLast { println "- I'm the largest animal that has ever lived on this planet." }

krill/build.gradle

hello.doLast {
    println "- The weight of my species in summer is twice as heavy as all human beings."
}

build.gradle

allprojects {
    task hello << {task -> println "I'm $task.project.name" }
}
subprojects {
    hello << {println "- I depend on water"}
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
- I'm the largest animal that has ever lived on this planet.
I'm krill
- I depend on water
- The weight of my species in summer is twice as heavy as all human beings.

49.2.3. Project filtering

To show more of the power of configuration injection, let's add another project called tropicalFish and add more behavior to the build via the build script of the water project.

49.2.3.1. Filtering by name

Example 49.8. Adding custom behaviour to some projects (filtered by project name)

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/
    build.gradle
  krill/
    build.gradle
  tropicalFish/

Note: The code for this example can be found at samples/userguide/multiproject/addTropical/water which is in both the binary and source distributions of Gradle.

settings.gradle

include 'bluewhale', 'krill', 'tropicalFish'

build.gradle

allprojects {
    task hello << {task -> println "I'm $task.project.name" }
}
subprojects {
    hello << {println "- I depend on water"}
}
configure(subprojects.findAll {it.name != 'tropicalFish'}) {
    hello << {println '- I love to spend time in the arctic waters.'}
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
- I love to spend time in the arctic waters.
- I'm the largest animal that has ever lived on this planet.
I'm krill
- I depend on water
- I love to spend time in the arctic waters.
- The weight of my species in summer is twice as heavy as all human beings.
I'm tropicalFish
- I depend on water

The configure() method takes a list as an argument and applies the configuration to the projects in this list.

49.2.3.2. Filtering by properties

Using the project name for filtering is one option. Using extra project properties is another. (See Section 16.4.2, “Extra properties” for more information on extra properties.)

Example 49.9. Adding custom behaviour to some projects (filtered by project properties)

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/
    build.gradle
  krill/
    build.gradle
  tropicalFish/
    build.gradle

Note: The code for this example can be found at samples/userguide/multiproject/tropicalWithProperties/water which is in both the binary and source distributions of Gradle.

settings.gradle

include 'bluewhale', 'krill', 'tropicalFish'

bluewhale/build.gradle

ext.arctic = true
hello.doLast { println "- I'm the largest animal that has ever lived on this planet." }

krill/build.gradle

ext.arctic = true
hello.doLast {
    println "- The weight of my species in summer is twice as heavy as all human beings."
}

tropicalFish/build.gradle

ext.arctic = false

build.gradle

allprojects {
    task hello << {task -> println "I'm $task.project.name" }
}
subprojects {
    hello {
        doLast {println "- I depend on water"}
        afterEvaluate { Project project ->
            if (project.arctic) { doLast {
                println '- I love to spend time in the arctic waters.' }
            }
        }
    }
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
- I'm the largest animal that has ever lived on this planet.
- I love to spend time in the arctic waters.
I'm krill
- I depend on water
- The weight of my species in summer is twice as heavy as all human beings.
- I love to spend time in the arctic waters.
I'm tropicalFish
- I depend on water

In the build file of the water project we use an afterEvaluate notification. This means that the closure we are passing gets evaluated after the build scripts of the subproject are evaluated. As the property arctic is set in those build scripts, we have to do it this way. You will find more on this topic in Section 49.6, “Dependencies - Which dependencies?”

49.3. Execution rules for multi-project builds

When we have executed the hello task from the root project dir things behaved in an intuitive way. All the hello tasks of the different projects were executed. Let's switch to the bluewhale dir and see what happens if we execute Gradle from there.

Example 49.10. Running build from subproject

Output of gradle -q hello

> gradle -q hello
I'm bluewhale
- I depend on water
- I'm the largest animal that has ever lived on this planet.
- I love to spend time in the arctic waters.

The basic rule behind Gradle's behavior is simple. Gradle looks down the hierarchy, starting with the current dir, for tasks with the name hello an executes them. One thing is very important to note. Gradle always evaluates every project of the multi-project build and creates all existing task objects. Then, according to the task name arguments and the current dir, Gradle filters the tasks which should be executed. Because of Gradle's cross project configuration every project has to be evaluated before any task gets executed. We will have a closer look at this in the next section. Let's now have our last marine example. Let's add a task to bluewhale and krill.

Example 49.11. Evaluation and execution of projects

bluewhale/build.gradle

ext.arctic = true
hello << { println "- I'm the largest animal that has ever lived on this planet." }

task distanceToIceberg << {
    println '20 nautical miles'
}

krill/build.gradle

ext.arctic = true
hello << { println "- The weight of my species in summer is twice as heavy as all human beings." }

task distanceToIceberg << {
    println '5 nautical miles'
}

Output of gradle -q distanceToIceberg

> gradle -q distanceToIceberg
20 nautical miles
5 nautical miles

Here the output without the -q option:

Example 49.12. Evaluation and execution of projects

Output of gradle distanceToIceberg

> gradle distanceToIceberg
:bluewhale:distanceToIceberg
20 nautical miles
:krill:distanceToIceberg
5 nautical miles

BUILD SUCCESSFUL

Total time: 1 secs

The build is executed from the water project. Neither water nor tropicalFish have a task with the name distanceToIceberg. Gradle does not care. The simple rule mentioned already above is: Execute all tasks down the hierarchy which have this name. Only complain if there is no such task!

49.4. Running tasks by their absolute path

As we have seen, you can run a multi-project build by entering any subproject dir and execute the build from there. All matching task names of the project hierarchy starting with the current dir are executed. But Gradle also offers to execute tasks by their absolute path (see also Section 49.5, “Project and task paths”):

Example 49.13. Running tasks by their absolute path

Output of gradle -q :hello :krill:hello hello

> gradle -q :hello :krill:hello hello
I'm water
I'm krill
- I depend on water
- The weight of my species in summer is twice as heavy as all human beings.
- I love to spend time in the arctic waters.
I'm tropicalFish
- I depend on water

The build is executed from the tropicalFish project. We execute the hello tasks of the water, the krill and the tropicalFish project. The first two tasks are specified by there absolute path, the last task is executed on the name matching mechanism described above.

49.5. Project and task paths

A project path has the following pattern: It starts always with a colon, which denotes the root project. The root project is the only project in a path that is not specified by its name. The path :bluewhale corresponds to the file system path water/bluewhale in the case of the example above.

The path of a task is simply its project path plus the task name. For example :bluewhale:hello. Within a project you can address a task of the same project just by its name. This is interpreted as a relative path.

Originally Gradle has used the '/' character as a natural path separator. With the introduction of directory tasks (see Section 14.1, “Directory creation”) this was no longer possible, as the name of the directory task contains the '/' character.

49.6. Dependencies - Which dependencies?

The examples from the last section were special, as the projects had no Execution Dependencies. They had only Configuration Dependencies. Here is an example where this is different:

49.6.1. Execution dependencies

49.6.1.1. Dependencies and execution order

Example 49.14. Dependencies and execution order

Build layout

messages/
  settings.gradle
  consumer/
    build.gradle
  producer/
    build.gradle

Note: The code for this example can be found at samples/userguide/multiproject/dependencies/firstMessages/messages which is in both the binary and source distributions of Gradle.

settings.gradle

include 'consumer', 'producer'

consumer/build.gradle

task action << {
    println("Consuming message: " +
            (rootProject.hasProperty('producerMessage') ? rootProject.producerMessage : 'null'))
}

producer/build.gradle

task action << {
    println "Producing message:"
    rootProject.producerMessage = 'Watch the order of execution.'
}

Output of gradle -q action

> gradle -q action
Consuming message: null
Producing message:

This did not work out. If nothing else is defined, Gradle executes the task in alphanumeric order. Therefore :consumer:action is executed before :producer:action. Let's try to solve this with a hack and rename the producer project to aProducer.

Example 49.15. Dependencies and execution order

Build layout

messages/
  settings.gradle
  aProducer/
    build.gradle
  consumer/
    build.gradle

settings.gradle

include 'consumer', 'aProducer'

aProducer/build.gradle

task action << {
    println "Producing message:"
    rootProject.producerMessage = 'Watch the order of execution.'
}

consumer/build.gradle

task action << {
    println("Consuming message: " +
            (rootProject.hasProperty('producerMessage') ? rootProject.producerMessage : 'null'))
}

Output of gradle -q action

> gradle -q action
Producing message:
Consuming message: Watch the order of execution.

Now we take the air out of this hack. We simply switch to the consumer dir and execute the build.

Example 49.16. Dependencies and execution order

Output of gradle -q action

> gradle -q action
Consuming message: null

For Gradle the two action tasks are just not related. If you execute the build from the messages project Gradle executes them both because they have the same name and they are down the hierarchy. In the last example only one action was down the hierarchy and therefore it was the only task that got executed. We need something better than this hack.

49.6.1.2. Declaring dependencies

Example 49.17. Declaring dependencies

Build layout

messages/
  settings.gradle
  consumer/
    build.gradle
  producer/
    build.gradle

Note: The code for this example can be found at samples/userguide/multiproject/dependencies/messagesWithDependencies/messages which is in both the binary and source distributions of Gradle.

settings.gradle

include 'consumer', 'producer'

consumer/build.gradle

task action(dependsOn: ":producer:action") << {
    println("Consuming message: " +
            (rootProject.hasProperty('producerMessage') ? rootProject.producerMessage : 'null'))
}

producer/build.gradle

task action << {
    println "Producing message:"
    rootProject.producerMessage = 'Watch the order of execution.'
}

Output of gradle -q action

> gradle -q action
Producing message:
Consuming message: Watch the order of execution.

Running this from the consumer directory gives:

Example 49.18. Declaring dependencies

Output of gradle -q action

> gradle -q action
Producing message:
Consuming message: Watch the order of execution.

We have now declared that the action task in the consumer project has an execution dependency on the action task on the producer project.

49.6.1.3. The nature of cross project task dependencies

Of course, task dependencies across different projects are not limited to tasks with the same name. Let's change the naming of our tasks and execute the build.

Example 49.19. Cross project task dependencies

consumer/build.gradle

task consume(dependsOn: ':producer:produce') << {
    println("Consuming message: " +
            (rootProject.hasProperty('producerMessage') ? rootProject.producerMessage : 'null'))
}

producer/build.gradle

task produce << {
    println "Producing message:"
    rootProject.producerMessage = 'Watch the order of execution.'
}

Output of gradle -q consume

> gradle -q consume
Producing message:
Consuming message: Watch the order of execution.

49.6.2. Configuration time dependencies

Let's have one more example with our producer-consumer build before we enter Java land. We add a property to the producer project and create now a configuration time dependency from consumer on producer.

Example 49.20. Configuration time dependencies

consumer/build.gradle

message = rootProject.hasProperty('producerMessage') ? rootProject.producerMessage : 'null'

task consume << {
    println("Consuming message: " + message)
}

producer/build.gradle

rootProject.producerMessage = 'Watch the order of evaluation.'

Output of gradle -q consume

> gradle -q consume
Consuming message: null

The default evaluation order of the projects is alphanumeric (for the same nesting level). Therefore the consumer project is evaluated before the producer project and the key value of the producer is set after it is read by the consumer project. Gradle offers a solution for this.

Example 49.21. Configuration time dependencies - evaluationDependsOn

consumer/build.gradle

evaluationDependsOn(':producer')

message = rootProject.hasProperty('producerMessage') ? rootProject.producerMessage : 'null'

task consume << {
    println("Consuming message: " + message)
}

Output of gradle -q consume

> gradle -q consume
Consuming message: Watch the order of evaluation.

The command evaluationDependsOn triggers the evaluation of producer before consumer is evaluated. The example is a bit contrived for the sake of showing the mechanism. In this case there would be an easier solution by reading the key property at execution time.

Example 49.22. Configuration time dependencies

consumer/build.gradle

task consume << {
    println("Consuming message: " +
            (rootProject.hasProperty('producerMessage') ? rootProject.producerMessage : 'null'))
}

Output of gradle -q consume

> gradle -q consume
Consuming message: Watch the order of evaluation.

Configuration dependencies are very different to execution dependencies. Configuration dependencies are between projects whereas execution dependencies are always resolved to task dependencies. Another difference is that always all projects are configured, even when you start the build from a subproject. The default configuration order is top down, which is usually what is needed.

To change the the default configuration order to be bottom up, That means that a project configuration depends on the configuration of its child projects, the evaluationDependsOnChildren() method can be used.

On the same nesting level the configuration order depends on the alphanumeric position. The most common use case is to have multi-project builds that share a common lifecycle (e.g. all projects use the Java plugin). If you declare with dependsOn a execution dependency between different projects, the default behavior of this method is to create also a configuration dependency between the two projects. Therefore it is likely that you don't have to define configuration dependencies explicitly.

49.6.3. Real life examples

Gradle's multi-project features are driven by real life use cases. The first example for describing such a use case, consists of two webapplication projects and a parent project that creates a distribution out of them. [19] For the example we use only one build script and do cross project configuration.

Example 49.23. Dependencies - real life example - crossproject configuration

Build layout

webDist/
  settings.gradle
  build.gradle
  date/
    src/main/java/
      org/gradle/sample/
        DateServlet.java
  hello/
    src/main/java/
      org/gradle/sample/
        HelloServlet.java

Note: The code for this example can be found at samples/userguide/multiproject/dependencies/webDist which is in both the binary and source distributions of Gradle.

settings.gradle

include 'date', 'hello'

build.gradle

allprojects {
    apply plugin: 'java'
    group = 'org.gradle.sample'
    version = '1.0'
}

subprojects {
    apply plugin: 'war'
    repositories {
        mavenCentral()
    }
    dependencies {
        compile "javax.servlet:servlet-api:2.5"
    }
}

task explodedDist(dependsOn: assemble) << {
    File explodedDist = mkdir("$buildDir/explodedDist")
    subprojects.each {project ->
        project.tasks.withType(Jar).each {archiveTask ->
            copy {
                from archiveTask.archivePath
                into explodedDist
            }
        }
    }
}

We have an interesting set of dependencies. Obviously the date and hello projects have a configuration dependency on webDist, as all the build logic for the webapp projects is injected by webDist. The execution dependency is in the other direction, as webDist depends on the build artifacts of date and hello. There is even a third dependency. webDist has a configuration dependency on date and hello because it needs to know the archivePath. But it asks for this information at execution time. Therefore we have no circular dependency.

Such and other dependency patterns are daily bread in the problem space of multi-project builds. If a build system does not support such patterns, you either can't solve your problem or you need to do ugly hacks which are hard to maintain and massively afflict your productivity as a build master.

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