Gradle Incremental Builds

External

• https://docs.gradle.org/current/userguide/more_about_tasks.html#sec:up_to_date_checks
• https://docs.gradle.org/current/userguide/custom_tasks.html#incremental_tasks
• https://docs.gradle.org/current/userguide/build_cache.html

Internal

• Gradle Tasks

Incremental Build

An incremental build is a build that avoids running tasks whose inputs and outputs did not change since the previous build, making the execution of such tasks unnecessary. An incremental build does not execute a task if it does not need to. The build determines whether is needed or not to run the task by performing up-to-date checks. Incremental builds save time, because they avoid doing unnecessary and some times lengthy work.

The central element that enables such behavior is the capacity of Gradle to analyze a task's inputs and outputs relative to the last build, detect that nothing that would require the task to run changed and decide that it does not need to run. A task must have at least one output to be considered as part of an incremental build. If it does not, it will be executed every time the build runs.

An even more sophisticated behavior consists in identifying which of the input elements had changed and perform work only for those elements. An incremental build is not required to support this behavior, it only has to avoid to run tasks unnecessarily. However, Gradle supports the behavior described above with incremental tasks.

All builds in Gradle are incremental builds by default, Build caching does not have to be enabled to allow them.

Task Inputs and Outputs

In most cases, a task takes some inputs and generates some outputs. For Java compilation, the inputs are the source files, target JDK version, whether we want debugging information or not, and the outputs are the generated class files. The amount of memory allocated to the compiler is not an input, but an internal task property, as its change does not influence the tasks' outputs.

Up-to-Date Check

When it is about to execute a task, during the execution phase, Gradle performs an "up-to-date" check: it tests externally to the task whether any of the following changed from the last build:

• any of the task inputs changed.
• any of the task outputs changed or it was removed.
• the task code changed.

If none of the above elements changed, Gradle considers the task to be UP-TO_DATE and skips executing its actions. If gradle is executed with the -i command line option, the task outcome will be displayed. To be considered as part of an incremental build, a task must have at least one output.

The cleanest way to designate task inputs and outputs is with annotation when implementing the task as a custom enhanced task: the corresponding properties are marked with annotations. Alternatively, a Runtime API can be used to achieve the same results. The Runtime API can be used with simple tasks.

The remaining of this section shows how to declare inputs and outputs by annotating the getter method of the corresponding property with appropriate annotations.

⚠️ If the task is written in Java, the annotation must be attached to the getter method. Annotations on setters or on the field will be ignored. Groovy allows annotating properties as well.

Inputs

The essential characteristic of an input is whether it affects one or more outputs in any way. In the above example, source files, target JDK version, enabling debugging are all task inputs. The amount of memory allocated to the compiler is not an input, but an internal task property.

If a task property affects the output of the task, it must be registered as input, otherwise the task will be considered UP-TO-DATE when the corresponding property changes, and the task is not actually up-to-date. Conversely, properties that do not affect the output must not be registered as inputs, otherwise the task will potentially execute when does not need to.

Simple Values

@Input

Strings or numbers, and in general, any type that implements Serializable, can be handled as a simple value. enums implement Serializable automatically, so they can be used here. Java example:

private String customVersion;

@Input
public String getCustomVersion() {
  return customVersion;
}

public void setCustomVersion(String s) {
  this.customVersion = s;
}

Note that the result of the @Input evaluation is displayed when running with -i command line option:

> Task :example
Task ':example' is not up-to-date because:
  Value of input property 'customVersion' has changed for task ':example'
...

File Types

@InputFile

@InputDirectory

@InputFiles

@InputFile can be used to annotate a single input file (not directory). @InputDirectory can be used to annotate a single input directory (not file). @InputFiles can be used to annotate an iterable of input files and directories. The properties that can be annotated with these annotations are standard Java File instances, but also derivatives of Gradle's org.gradle.api.file.FileCollection type and anything else that can be passed to either the Project.file(Object) method for single/file directory properties or the Project.files(Object...) method.

Nested Values

org.gradle.api.tasks.@Nested

@Nested can be used to annotate custom types that do not conform the other two categories but have their own properties that are inputs or outputs - the task inputs or outputs are nested inside these custom types.

Internal Task Properties

Anything that is not a task input, meaning that it does not influence one or more outputs, it is an internal task property.

Outputs

@OutputFile

@OutputDirectory

@OutputFiles

@OutputDirectories

To be considered as part of an incremental build, a task must have at least one output and zero or more inputs.

If a task declares outputs but those are not configured in the configuration phase, the build will fail with a message similar to "No value has been specified for property '...'".

Outputs used as Inputs for another Tasks

Expand on this. The palantir docker task accepts tasks.jar.outputs as "files".

Declaring Inputs and Outputs with Runtime API

Annotations in custom task types are the cleanest way to declare inputs and outputs. However, simple tasks can be configured to participate in incremental builds by using the Runtime API.

Declaring Inputs and Outputs with Runtime API

Examples

• Gradle documentation example for @Input, @InputFiles, @Nested and @OutputDirectory.
• Playground incremental build for a task with one output and zero inputs.
• Playground simple value input.

Non-Deterministic Tasks

If a task generates different output for exactly the same inputs, these tasks should not be configured for an incremental build, by declaring inputs and outputs, as up-to-date checks won't work and the task will not be executed when it should be.

Incremental Task

Incremental Task

Build Cache

Gradle allows caching task output in build caches, if the tasks producing those outputs are cacheable tasks, hence enabling sharing of outputs between builds. Build caching does not have to be enabled to allow incremental builds.

Builds Cache

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TO PROCESS

An input can be a property, a directory or one or more files. An output is a directory or one or more files. Inputs and outputs are fields of the DefaultTask class. Task inputs and outputs are obtained with Task.getInputs() and Task.getOutputs(), which return TaskInputs and TaskOutputs instances, respectively.

Task inputs and outputs are evaluated during the configuration phase to wire up the task dependencies. That is why they need to be defined in a configuration block. To avoid unexpected behavior, make sure that the value you assign to inputs and outputs is accessible at configuration time. If you need to implement programmatic output evaluation, the method upToDateWhen(Closure) on TaskOutputs comes in handy. In contrast to regular inputs/outputs evaluation, this method is evaluated at execution time. If the closure returns true, the task is considered up to date.

Debug a Java compilation task to see how input/output works.