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
Incremental Build
An incremental build is a build that avoids running tasks whose inputs and outputs did not change, making the execution of such tasks unnecessary. An incremental build does not execute one or more tasks if it does not need to. Incremental builds save time, because they avoid doing unnecessary work.
The central element that enables such behavior is the capacity of tasks to analyze their inputs and outputs, detect that nothing that would require it to run changed and decide that they don't need to run.
An even more sophisticated behavior is identifying which of the input elements had changed and only performing the work 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.
Task Inputs and Outputs
Gradle determines whether a task is up to date if the task's inputs and outputs have not changed since the last execution, by comparing their values with those preserved in the build cache. Does the build cache need to be enabled for inputs and outputs to be relevant?
If the input and output snapshots are identical to the previous ones, the task is not executed. 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.
Annotations
@Input
⚠️ Note that in Java, the annotation must be attached to the getter method. Annotations on setters or just the field in Java are ignored.
@OutputFile
Up-to-date Check
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.