How to ask gradle if anything is not up-to-date

Question

Is there a way to ask gradle if anything is not up-to-date and would need to be built (without actually building it then)?

Answer 1

This is not possible. The up-to-date check happens in the execution phase, just before the doFirst is run. It is calculated and this value is not stored. Moreover, the up-to-date check might depend on the previous task(s) outputs. Therefore to resolve a task's up-to-date status you need to execute all it's dependencies (dependsOn tasks). Therefore what you are asking would theoretically work only for the first task that is not up-to-date.

To see how the lifecycle works, here's a simple example:

task hello {
    println "CONFIG1"
    outputs.upToDateWhen {
        println "UPTODATE"
        return false
    }
    println "CONFIG2"
    doFirst {
        println "DOFIRST"
    }
    doLast {
        println "DOLAST"
    }
}

If you execute this with the --debug flag, you will see this first:

08:05:17.294 [DEBUG] [org.gradle.model.internal.registry.DefaultModelRegistry] Project : - Registering model element 'tasks.hello' (hidden = false)
08:05:17.302 [QUIET] [system.out] CONFIG1
08:05:17.329 [QUIET] [system.out] CONFIG2
08:05:17.333 [DEBUG] [org.gradle.configuration.project.BuildScriptProcessor] Timing: Running the build script took 0.764 secs

The config phase sets up the up-to-date check. Then Gradle calculates the graph of tasks:

08:16:26.212 [DEBUG] [org.gradle.execution.taskgraph.DefaultTaskGraphExecuter] Timing: Creating the DAG took 0.007 secs

And only then it executes the tasks:

08:05:17.430 [INFO] [org.gradle.execution.taskgraph.AbstractTaskPlanExecutor] :hello (Thread[main,5,main]) started.
08:05:17.431 [LIFECYCLE] [class org.gradle.internal.buildevents.TaskExecutionLogger] :hello
08:05:17.432 [DEBUG] [org.gradle.api.internal.tasks.execution.ExecuteAtMostOnceTaskExecuter] Starting to execute task ':hello'
08:05:17.461 [QUIET] [system.out] UPTODATE
08:05:17.465 [INFO] [org.gradle.api.internal.tasks.execution.ResolveTaskArtifactStateTaskExecuter] Putting task artifact state for task ':hello' into context took 0.032 secs.
08:05:17.465 [DEBUG] [org.gradle.api.internal.tasks.execution.SkipUpToDateTaskExecuter] Determining if task ':hello' is up-to-date
08:05:17.465 [INFO] [org.gradle.api.internal.tasks.execution.SkipUpToDateTaskExecuter] Executing task ':hello' (up-to-date check took 0.0 secs) due to:
  Task.upToDateWhen is false.
08:05:17.466 [DEBUG] [org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter] Executing actions for task ':hello'.
08:05:17.467 [QUIET] [system.out] DOFIRST
08:05:17.467 [QUIET] [system.out] DOLAST

Up-to-date check happens only when the tree of tasks is built and tasks are being executed in order.

Since no value is stored from this check, it is not possible to determine the up-to-date status of a task without executing all tasks it depends on.