Static Variable Initialization that reference each other?

Question

This is somewhat related to static variable initialization order, but this question doesn't really answer my question.

When I run the following code, I expect undefined behavior or something with infinite recursion to happen:

class A {
    static File test = B.test;
    public static void main(String[] args) {
         System.out.println(A.test);
         System.out.println(B.test);
    }
}
class B {
     static File test = A.test;
}

However, instead I see:

null
null

Why does this happen?

Before it's set, the variable is null, and you never set either to anything other than null. — Roddy of the Frozen Peas, Apr 24 '20 at 23:29

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

The answer comes from JLS section 12.4. We start with 12.4.1:

12.4.1. When Initialization Occurs

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

T is a class and an instance of T is created.

A static method declared by T is invoked.

A static field declared by T is assigned.

A static field declared by T is used and the field is not a constant variable (§4.12.4).

When A.main is invoked, the third clause applies, and thus class A (and by extension A.test) must be initialized. The initialization of A.test triggers the fourth clause above, meaning that B must now be initialized. B must now access fields of A, but A has not yet initialized.

We need to now dig into section 12.4.2:

12.4.2. Detailed Initialization Procedure ... For each class or interface C, there is a unique initialization lock LC. The mapping from C to LC is left to the discretion of the Java Virtual Machine implementation. The procedure for initializing C is then as follows:

Synchronize on the initialization lock, LC, for C. This involves waiting until the current thread can acquire LC.

If the Class object for C indicates that initialization is in progress for C by some other thread, then release LC and block the current thread until informed that the in-progress initialization has completed, at which time repeat this step.

If the Class object for C indicates that initialization is in progress for C by the current thread, then this must be a recursive request for initialization. Release LC and complete normally.

Step 3 is the one we take here. Class A is already undergoing initialization by the current thread, so we are in a recursive initialization situation. We thus just give up and complete normally right away, so A.test remains null and B.test gets the value of A.test which is null.

Static Variable Initialization that reference each other?

1 Answers1