Failed to find Premain-Class manifest attribute ... spock-mockable

I am trying to stub the final class java.net.UR in my spock testL. Based on the answer here: https://stackoverflow.com/questions/75691233/how-to-stub-a-java-final-static-method-or-class-in-spock I'm attempting to use spock-mockable.

I have modified my build.gradle as follows:

testImplementation (
        'org.junit.jupiter:junit-jupiter-api:5.8.1',
        'org.spockframework:spock-core:2.4-M1-groovy-4.0',
        'io.github.joke:spock-mockable:3d2e39a',
        'cglib:cglib-nodep:3.3.0'
)

...

// to load the agent even earlier add it as a JVM argument
tasks.withType(Test) {
    jvmArgs += ["-javaagent:${classpath.find { it.name.contains('spock-mockable') }.absolutePath}"]
}

From reading the readme file, it looks like I can now just run my test, but when I try it, I get the following error:

Error occurred during initialization of VM
Failed to find Premain-Class manifest attribute in /home/thomas/.gradle/caches/modules-2/files-2.1/io.github.joke/spock-mockable/3d2e39a/d3a086b4c82c8290a2013ebd5a6b697f959918bf/spock-mockable-3d2e39a.jar
agent library failed to init: instrument

I'm not exactly sure what a javaagent is and tried to find some reading on it, but found several examples with no explanation. I can kind of guess.

At any rate, the spock-mockable shows up in my external libraries. (From IntelliJ where I'm running this test.)

Looking for what to do next.

Spock Mockable version 3d2e39a is old (May 2022), not a recent version as the leading "3" implies. According to both the GitHub site and MvnRepository.com, the latest release is 2.3.1.

If you inspect the JAR of version 3d2e39a, you will find an empty META-INF/MANIFEST.MF file, which is the root cause for your error message. There really is no Premain-Class entry there, which would be necessary to run the JAR as a Java agent. But 2.3.1 has the correct manifest entries.