With

compile-testing

when generating Java you can write tests like this:

Compilation compilation =
    javac()
        .withProcessors(new MyAnnotationProcessor())
        .compile(JavaFileObjects.forResource("HelloWorld.java"));
assertThat(compilation).succeeded();
assertThat(compilation)
    .generatedSourceFile("GeneratedHelloWorld")
    .hasSourceEquivalentTo(JavaFileObjects.forResource("GeneratedHelloWorld.java"));

Or for errors:

JavaFileObject helloWorld = JavaFileObjects.forResource("HelloWorld.java");
Compilation compilation =
    javac()
        .withProcessors(new NoHelloWorld())
        .compile(helloWorld);
assertThat(compilation).failed();
assertThat(compilation)
    .hadErrorContaining("No types named HelloWorld!")
    .inFile(helloWorld)
    .onLine(23)
    .atColumn(5);

to be more specific, currently I actually use

compile-testing

to compile the code in my tests (writing the sources I compile in my tests in Java), but the processor generate Kotlin code, so I can use some features like error cases, but I can't use any java-specific things like

generatedSourceFile

or

hasSourceEquivalentTo

(both only handle Java stuff), instead I redirect the output and test it myself (with a few regexp)