When upgrading from Gradle 3 to Gradle 4, I got a deprecation warning in my build. The message looked like this: 

Gradle now uses separate output directories for each JVM language, but this build assumes a single directory for all classes from a source set. This behaviour has been deprecated and is scheduled to be removed in Gradle 5.0
        at build_6ku42cnecj8o3fcbd4eugmgr$_run_closure3.doCall(/Users/tsu/projects/tsu/thinkcode.se/build.gradle:37)
The setTestClassesDir(File) method has been deprecated and is scheduled to be removed in Gradle 5.0. Please use the setTestClassesDirs(FileCollection) method instead.
        at build_6ku42cnecj8o3fcbd4eugmgr$_run_closure3.doCall(/Users/tsu/projects/tsu/thinkcode.se/build.gradle:37)

My question was "What? How do I upgrade my build to get rid of the warning?". I searched but neither Google nor Gradle had a migration plan ready for me.

Trouble shooting

The code Gradle was complaining about looked like this: 

task acceptanceTest(type: Test) {
    description = 'Runs the acceptance tests'
    group = 'verification'
    testClassesDir = sourceSets.acceptanceTest.output.classesDir
    classpath = sourceSets.acceptanceTest.runtimeClasspath
}

The line Gradle complains about is this: testClassesDir = sourceSets.acceptanceTest.output.classesDir where I define where the compiled tests should be found.

This test task runs my acceptance tests that are defined in a separate source set. This allows me to define a separate set of acceptance tests. My acceptance tests are slow so I don't want them mixed with my unit tests.

The source set I use for the separation looks like this: 

sourceSets {
    acceptanceTest {
        java.srcDir file('src/acceptanceTest/java')
        resources.srcDir file('src/acceptanceTest/resources')
        compileClasspath += main.output + test.output + configurations.testRuntime
        runtimeClasspath += output + compileClasspath
    }
}

The deprecation warning wants me to refer to all of the output directories for the test classes. In this case I only have one.

The solution

My solution is to update how I refer to the output directory. I updated the test task like this: 

task acceptanceTest(type: Test) {
    description = 'Runs the acceptance tests'
    group = 'verification'
    testClassesDirs = sourceSets.acceptanceTest.output
    classpath = sourceSets.acceptanceTest.runtimeClasspath
}

The offending line has been updated to testClassesDirs = sourceSets.acceptanceTest.output and the deprecation warning is gone.

Acknowledgements

I would like to thank Malin Ekholm for finding my typos.

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