通过反射 API 使 Java 类成为非最终类

Question

I have a Java class similar to the following one:

public final class Node {
    public Node() {}
}

I have learned already how to change change the accessibility of 'final' fields via the reflection API, but is this also possible for classes? Can I turn a final class into a non-final class at runtime?

Answer 1

You can re-write a class file using a library like ASM.

There may be no point changing the final status of a class at runtime as it needs to be non-final at compile time to compile a sub-class.

Answer 2

I don't see how this can work, because the compiler will check to see if the class you're trying to make non-final at compile time. You'll get an error if you try to inherit from it.

I think it's fine to ask the question if you're curious, but the larger question to ask yourself is why do you want to do this? Reflection allows you to do a lot of things to get around final, private, etc., but that doesn't mean it's a good idea. If the designer of a 3rd party library thought that final was a good idea, you might be well advised to honor that.

Answer 3

Edit: Thanks to the comment of @kriegaex. 'accessFlags' field only exists in Class class implementation in Android SDK. Therefore, unless you are developing for Android, this solution will not work for you since such a field does not exist in the first place.

Here is how you can make the class extendable by removing the 'final' modifier:

Class classClass = Class.class;

Field accessFlagsField = classClass.getDeclaredField("accessFlags");

accessFlagsField.setAccessible(true);

Class nodeClass = Node.class;

accessFlagsField.setInt(nodeClass, nodeClass.getModifiers() & ~Modifier.FINAL);

I do not agree with the answers above. One might want to remove the 'final' modifier to extend the class during runtime by using 'ASM' or libraries like 'byte-buddy'. I agree that if it is a final class, it is probably for a good reason but custom business logic may require to do so.