如何使用私有构造函数从final类运行该方法？

Question

How to run the method foo() in class A without any changes on this class

public final class A{

    private A(){
        System.exit(0);
    }
    public void foo(){

        System.out.println("from foo");
    }

}

Answer 1

Without doing something like using reflection or bytecode manipulation to mess with accessibility...

The "right" way to do this is to get an instance of the class some other way. For example, if there's a static factory method or pre-made instances that you can access. The reason for having a private constructor like this is to control construction of the class. (For example, enum implementations have a private constructor so that you don't create additional instances beyond the static ones supplied.)

If you side-step this then someone (either you or the original class author) is doing something wrong.

Answer 2

You can create an instance of a class without calling the constructor. See this question: Is it possible to create an instance of an object in Java without calling the constructor?

You can use objenesis to do this for you. Once you've got an instance of A without calling the constructor, calling foo is easy.

Answer 3

You can alter the byte code before it is loaded by the jvm. Here's a place to start: http://www.ibm.com/developerworks/java/library/j-dyn0916/index.html