Capture task classpath fingerprints in a configuration cache–compatible way

captureFingerprints.gradle

def fingerprinter = services.get(org.gradle.internal.fingerprint.classpath.ClasspathFingerprinter)
tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach { task ->
    def objectFactory = task.project.objects
    def buildScan = rootProject.buildScan
    doFirst {
        ClassLoader classLoader = task.getClass().classLoader
        while (classLoader instanceof URLClassLoader) {
            def fingerprints = [] as Set
            def allFiles = [] as Set
            classLoader.getURLs().each {
                def fingerprint = fingerprinter.fingerprint(objectFactory.fileCollection().from(it.file)).hash
                fingerprints.add(["${task.path}:${new File(it.file).name}", "${fingerprint}"])
                allFiles.add(it.file)
            }
            fingerprints.each { buildScan.value it[0], it[1] }
            buildScan.value "${task.path}:classpath", allFiles.join("\n")
            classLoader = classLoader.parent
        }
    }
}

For each Task of the given type, this script registers a <task>:classpath=<classpath> custom value, as well as a <task>:<fileName>=<fileHash> custom value for each of that task's classpath's files.

One possible use is investigating cache misses caused by "Task implementation changed". This script can then be used to register the contents of that Task's classpath in the Build Scan® itself, and quickly compare it across builds in Develocity.

Change KotlinCompile to whatever task type you need, e.g. com.google.devtools.ksp.gradle.KspTaskJvm.

Usage

This can be applied to a single project or to multiple projects. Applying to all projects is also possible, but you're more likely to hit the custom value limits.

To apply it to projects, the preferable way is to

1. save this file somewhere
2. apply it from your projects' build.gradle or build.gradle.kts:

// my-project/build.gradle.kts
appy(from = "$rootDir/captureFingerprints.gradle")