You can do a lot of cool things with Byte Buddy, although not everything is very clearly laid out in their docs, here’s how you can change the output of a function:

Skipping function execution

The first step in changing the return value is most likely skipping the execution of the function. You can change the return value of a function without skipping execution however we won’t do that in this blogpost.

In Byte Buddy you have a handy skipOn attribute which you can set in your OnMethodEnter annotation, to quote from Byte Buddy docs :

A value defining what return values of the advice method indicate that the instrumented method should be skipped or void if the instrumented method should never be skipped.

TL;DR you set this to the class that represents the return value of your OnMethodEnter function, and if that value is “positive” (meaning true, 1, not null, …) the function execution will be skipped.

@Advice.OnMethodEnter(skipOn = boolean.class)
public static boolean onEnter() {
    if (wantToSkipExecution) {
        return true;
    }
    return false;
}

Skipping function execution and rewriting output

Now that we know how to skip the execution, we also want to start rewriting the output, you can do this using an @Advice.Return. What this does is basically give you the return value of the function, with the ability to overwrite it, example :

@Advice.OnMethodExit
public static void onExit(
    @Advice.Return(readOnly = false) String returnValue
) {
    returnValue = "My new return string";
}

Et voila! As easy as pie.

Skipping function execution and writing from onEnter

If you want to skip or not depending on something in your OnMethodEnter function but you also generate the return value in your OnMethodEnter function you can use a wrapper class like this :

public record SkipOnWrapper(String newReturnValue) {}

And making your OnMethodEnter function return a SkipOnWrapper, if you want to skip the function you return this object with your new return value, if you do not want to skip you return null :

@Advice.OnMethodEnter(skipOn = SkipOnWrapper.class)
public static SkipOnWrapper onEnter() {
    if (wantToSkipExecution) {
        return new SkipOnWrapper("My new return value");
    }
    return null;
}

@Advice.OnMethodExit
public static void onExit(
    @Advice.Enter SkipOnWrapper enterResult,
    @Advice.Return(readOnly = false) String returnValue
) {
    // Overwrite return value based on code in onEnter : 
    if (enterResult != null) {
        returnValue = enterResult.newReturnValue();
    }
}

And that’s it, thanks for reading this blogpost. See you soon with more Java madness :0