使用Reflection时，Java中的奇怪静态初始化

Question

Given the following code-snippet I get the output "B" and "error":

public class Test {

    private static class A {
        static final B c = new B();
    }

    private static class B extends A {
        static final B c = A.c;
    }

    public static void main(String[] args) throws Exception {
        for(Class<?> cls : Test.class.getDeclaredClasses()) {
            if(cls.getDeclaredFields()[0].get(null) == null) {
                System.out.println(cls.getSimpleName());
            }
        }
        if(B.class.getDeclaredField("c").get(null) == null) {
            System.out.println("error");
        }
    }

}

But it gets even more bizarre. If I comment-out the for-loop I get no output - so no error. Also if I access the field Bc directly without reflection before doing the reflection stuff the problem does not occur. Why is that so and how can I fix it?

Answer 1

The problem is not related with reflection, is related with the order in which classes are initialized.

If class A is initialized first, it has a dependency in class B, class B tries to initialize Bc to a static field of A, but this field is still null.

If the class B is initialized first, it has a dependency in class A, the class A is initialized and Ac is set to a new B instance and Bc is initialized correctly.

To make an example more simple, this code shows the problem simplified:

public static void main(String[] args) throws Exception {
    System.out.println(A.c);
    System.out.println(B.c);
}

Output:

Test$B@f72617
null

And this other example:

public static void main(String[] args) throws Exception {
    System.out.println(B.c);
    System.out.println(A.c);
}

Output:

Test$B@f72617
Test$B@f72617

To solve the problem you could change your design to remove the circular dependency or make sure the classes are initialized in the right order.