Android Studio Gradle - JavaExec classpath configuration - Java Reflection - class access

Im currently working on an android project where i have to process .java-files to possibly generate another .java-files which should then be compiled and packed into the .apk-file.

Lets assume i have 2 files which will be processed by my library, FILE_A.java and FILE_B.java.

Now i need to access these files within my library via reflection, e.g. with:

Class.forName("com.test.entities.FILE_A"); 
Class.forName("com.test.entities.FILE_B");

The problem is that i'm not able to access the class files, i think because of the missing classpath configuration. Currently i use this task to call my .jar-file:

task (mytask, type: org.gradle.api.tasks.JavaExec) {
    classpath(files('libs/myjar.jar'))
    main('com.test.TestMain')
}

preBuild.dependsOn mytask

I found some ressources on the web, but they all don't work.

I tried to add the following to the classpath:

sourceSets.main.runtimeClasspath (main is unknown) android.sourceSets.main.runtimeClasspath (runtimeClasspath is unkown).

So how can i access the class files in my library?