How can a Java library detect annotated methods?

Question

There are many ways to find all methods with a specific annotation if you know the package you're searching in (for example by using the Reflections library). But if I'm writing a library, then I don't know the name of the package that's using my library. How would I go about finding all methods with a specific annotation in a case like this?

Answer 1

If you know the package name, you can use a classpath scanner like ClassGraph to find classes within that package, and then you can use reflection to list the methods for each class and find which ones have the correct annotation. Since you do not know the package name, you'll have to think creatively to find it.

I would personally try something like determining the current the call stack, getting the StackTraceElement at the very bottom of the stack, using StackTraceElement#getClassName() and Class#forName(String) to get the implementing program's main class (or the class of the thread which is calling your library), getting that class's package (Class#getPackage()), and then using Package#getName() to get the package's name.

Altogether, it would look something like this:

public static String getImplementingPackageName() throws ClassNotFoundException {
    StackTraceElement[] stackTraceElements = Thread.currentThread().getStackTrace();
    StackTraceElement bottom = stackTraceElements[stackTraceElements.length - 1];
    
    Class<?> caller = Class.forName(bottom.getClassName());
    Package callerPackage = caller.getPackage();
    String packageName = callerPackage.getName();
    
    return packageName;
}