Gradle Task Dependencies and Ordering

External

• https://docs.gradle.org/current/dsl/org.gradle.api.Task.html#N18D13
• https://docs.gradle.org/current/javadoc/org/gradle/api/Task.html#dependencies
• https://docs.gradle.org/current/userguide/more_about_tasks.html#sec:adding_dependencies_to_tasks

Internal

• Gradle Task

Overview

May times, a task requires another task to run first, especially if the task depends on the produced output of its dependency task. A task may declared its dependencies explicitly. A task may depend on other tasks implicitly, as described in the Implicit Dependencies section.

In Gradle, task execution order is automatically determined taking into account explicit dependencies and implicit dependencies, and a specific execution order for the tasks that declared dependencies among themselves is not guaranteed. The only thing that is guaranteed is that the dependencies will be honored. This architectural decision has several benefits: you don't need to know the whole chain of task dependencies to make a change, and because the tasks don't have to be executed strictly sequentially, they can be parallelized.

However, Gradle also offers a way to request an execution order between tasks, in absence of dependency.

If tasks belonging to different sub-projects that are part of the same multi-project define dependencies among themselves, that automatically introduces a dependency between the owning projects:

Multi-project builds | Inter-project Dependencies

Explicit Dependencies

A task's explicit dependencies are maintained and can be configured with the task's "dependsOn" property. The Task API used to declare explicit task dependencies is Task.dependsOn(Object... paths), which surfaces in the DSL as:

task red {
  dependsOn 'blue', 'green' // dependency expressed using task names, comma is required for more than one dependencies
  ...
}

// dependency expressed using task objects
task red(dependsOn: [blue, green] {
  ...
}

task red {
  ...
}
red.dependsOn(blue, green) // dependency expressed using task objects

task red {
  ...
}
red.dependsOn blue // dependency expressed using task objects
red.dependsOn green // dependency expressed using task objects

Note that a task dependency may be another task name, the task instance itself or another objects.

If multiple tasks are provides as argument of dependsOn(...), the order in which they are declared does not influence the order in which they are executed. The only thing that is guaranteed is that all will be executed before the task that declares the dependency.

Adding Explicit Dependencies using a Lazy Block

Task dependencies can be defined using a lazy block. When evaluated, the block is passed the task whose dependencies are being calculated. The block should return one task or a collection of tasks, which are then treated as dependencies of the task:

task red {
  ...
}

red.dependsOn {
  tasks.findAll { task -> task.name.startsWith('b') }
}

Other Methods to Set Explicit Dependencies

Other methods that can be used to set explicit dependencies are:

• Task.setDependsOn(Iterable)

Explicit Dependencies between Tasks belonging to Different Projects

In a multi-project situation, tasks belonging to two different projects can express the dependency on one another by using task paths:

red.dependsOn(':project-a:yellow')

task red(..., dependsOn: [':project-a:yellow', ':project-b:brown']) {
  ...
}

task red {
  dependsOn ':project-a:yellow', ':project-b:brown' // comma is required
}

This explicit dependency also introduces a dependency between projects, causing dependency project ("project-a") to be configured before the current project in the configuration phase.

Finalizer Tasks

A finalizer task is a task that will be scheduled to run after the task that requires finalization, regardless of whether the task succeeds or fails.

task a { ... }
task b { ... }
a.finalizedBy b

Ordering Tasks

https://docs.gradle.org/current/userguide/more_about_tasks.html#sec:ordering_tasks

It is possible to request a specific order in which two tasks execute, without introducing a dependency relationship between tasks: if both tasks are executed, they will be executed in the requested order; if only one of the tasks is executed, but not the other, the order between tasks becomes irrelevant. Dependency is mandatory execution of both task, in the required order, so dependency implies order. Order does not imply mandatory execution, just ordered execution if both tasks are executed, so order does not imply dependency. If there is an order relationship between two tasks A and B, it is still possible to execute task A without task B and vice-versa.

Task ordering can be useful for example in the situation when both unit and integration tests are executed and it is useful to run the unit tests first and get feedback faster. There is no dependency relationship between tasks, one can be executed without the other. However, it is useful if they execute in a specific order, if they both execute.

There are two ordering rules: mustRunAfter and shouldRunAfter. In both cases, the arguments are task names, task instances or any argument that is accepted by Task.dependsOn(Object ...).

mustRunAfter

"must run after" ordering rule specifies that a task must always run after other task, whenever both tasks are run.

• Task.mustRunAfter(Object...)
• Task.setMustRunAfter(Iterable)

task a { ... }
task b { ... }
b.mustRunAfter a

shouldRunAfter

"should run after" ordering rule specifies that a task should run after other task, whenever both tasks are run, but it is less strict than mustRunAfter and it will be ignored in two situations:

• If using the rule introduces an ordering cycle.
• When using parallel execution and all dependencies of a task have been satisfied apart from "should run after", then this task will be run regardless of whether its "should run after" dependencies have been run or not. "should run after" should be used where the ordering is helpful but not strictly required.

The rule can be invoked with the following API:

• Task.shouldRunAfter(Object ...)
• Task.setShouldRunAfter(Iterable)

Implicit Dependencies

Implicit dependencies are determined base on task inputs and outputs.