Need help on how to debug java code without main in Eclipse IDE

We have .jar file that is runnable. We don't have the source of this jar file. We have our java code that does not have main and our jar acts like a library where the runnable jar calls our jar file. We only have he source of our jar file but not the runnable jar. How do we debug our code in Eclipse using the runnable jar which we don't have eh source. Our jar file has function called start() which the runnable jar calls to our jar file to execute the start function. We like to know if we can debug our code that has the start function? This is where we need help. No coding needed. We only want to know if possible to debug our code that another jar calls into our code?