Byte Buddy - How to delegate a private method?

Question

I have the following unit test :

@Test
public void TestPrivateMethodDelegation() throws InstantiationException, IllegalAccessException, IllegalArgumentException, 
    InvocationTargetException, NoSuchMethodException, SecurityException
{
    Foo foo = new ByteBuddy()
        .subclass(Foo.class)
        .method(named("getHello")
            .and(isDeclaredBy(Foo.class)
            .and(returns(String.class))))
        .intercept(MethodDelegation.to(new Bar()))
        .make()
        .load(Foo.class.getClassLoader(), ClassReloadingStrategy.fromInstalledAgent())
        .getLoaded()
        .getDeclaredConstructor().newInstance();

    Method privateMethod = Foo.class.getDeclaredMethod("getHello");
    privateMethod.setAccessible(true);

    assertEquals(privateMethod.invoke(foo), new Bar().getHello());
}

Here are the classes it uses :

@NoArgsConstructor
public class Foo 
{
    @SuppressWarnings("unused")
    private String getHello()
    {
        return "Hello Byte Buddy!";
    }
}

@NoArgsConstructor
public class Bar 
{
    public String getHello()
    {
        return "Hello Hacked Byte Buddy!";
    }
}

When i make the getHello() method public in the Foo class, this test passes. When i leave it private, the test fails since i can only assume the private method is not properly delegated.

Is it even possible to delegate a private method to another class?

Thanks!

Answer 1

No, it is not. Byte Buddy generates byte code just as javac would do and this byte code must be valid to function. You cannot call a private method from another class, therefore, Byte Buddy is throwing an exception.