Is it possible to replace field accesses?

[Azn9]

I have a class that looks like this:

public class Foo {

    private String foo;
    
    public String getFoo() {
        return this.foo;
    }
    
    public void doSomething() {
        String temp = this.foo;

        // ...
    }
}

Is there any way to modify this class so that the resulting "fake" class would act like the following?

//...
    public void doSomething() {
        String temp = this.getFoo(); // <--- replace "this.foo" by its getter
//...
}

I'm already replacing the getter, but sometimes in the source code we use "direct" access to the fields, that would not give the same value than the getter.

[raphw]

Yes, you are looking for MemberSubstitution. You'd replace a field read with a method invocation.

[Azn9]

I tried with something like the following, but I got the error "class redefinition failed: attempted to change the schema (add/remove fields)", do you see any mistake that I made? I don't find it :'(
It's my first time using bytebuddy and I'm a bit lost with all the possibilities this library offer 😅

DynamicType.Builder<T> builder = LiveObjectUtils.BYTE_BUDDY
        .redefine(clazz)
        .method(this::isBlockingGetter).intercept(MethodDelegation.to(BlockingGetterInterceptor.class))
        .method(this::isReactiveGetter).intercept(MethodDelegation.to(ReactiveGetterInterceptor.class))
        .method(this::isReactiveSetter).intercept(MethodDelegation.to(ReactiveSetterInterceptor.class))
        .method(this::isBlockingSetter).intercept(MethodDelegation.to(BlockingSetterInterceptor.class));

for (Field declaredField : clazz.getDeclaredFields()) {
    builder = builder.visit(MemberSubstitution.relaxed()
                    .field(ElementMatchers.named(declaredField.getName()))
                    .onRead()
                    .replaceWithMethod(new GetterMatcher(declaredField.getName(), declaredField.getType()))
                    .on(ElementMatchers.definedMethod(ElementMatchers.any()))
            );
}

try (DynamicType.Unloaded<T> unloadedT = builder.make()) {
    Class<? extends T> dynamicType = unloadedT
            .load(this.getClass().getClassLoader(), ClassReloadingStrategy.fromInstalledAgent())//ClassLoadingStrategy.Default.INJECTION)
            .getLoaded();

// ...

[Azn9]

Okay, if I remove all my .method().intercept(), it works (but I get a stackoverflow as it also replaces the one in the getter, making it call itself)
Does the interception adds methods to the class?

[raphw]

Interception adds metgods containing the original code. You'd need to use Advice then and use visit to decorate methods.

[Azn9]

I finally managed to get it work as I expected, thank you very much for your help!

I just have one last question, I used the following code to skip the method execution and replace the return value, but I was wondering if this is the correct way to do it?

@Advice.OnMethodEnter(skipOn = Boolean.class)
static Object enter() {
    return Boolean.TRUE;
}

@Advice.OnMethodExit(onThrowable = Throwable.class)
static void exit(@Advice.Return(readOnly = false, typing = Assigner.Typing.DYNAMIC) Object returnValue) {
    returnValue = "Returned from advice :o";
}

[raphw]

That's one way. You can also set Advice.OnNonDefaultValue.class and return a primitive, but I doubt it makes a performance difference.

[Azn9]

Thank you very much for all the help!