Static Circular Dependency in Java

问题

for the following code:

class A
{
    public static int X;
    static { X = B.Y + 1;}
}
public class B
{
    public static int Y = A.X + 1;
    static {}
    public static void main(String[] args) {
        System.out.println("X = "+A.X+", Y = "+B.Y);
    }
}

the output is: X = 1, Y = 2

Why? and How???

-Ivar

P.S: Code snippet taken from JavaCamp.org

回答1:

Here is what happens in chronological order:

1. Class B contains the main-method so it is loaded by the class loader.

2. Initialization of B references A, so class A is loaded.

3. A has a static variable X initialized to B.Y + 1.

   The initialization of B.Y hasn't been executed yet, so B.Y evaluates to 0, and thus 1 is assigned to A.X

4. Now A has finished loading, and the initialization of B.Y can take place.

   The value of A.X + 1 (1 + 1) is assigned to B.Y.

5. The values of A.X and B.Y are printed as 1 and 2 respectively.
   
   

Further reading:

Java Language Specification, §12.4.1 When Initialization Occurs

回答2:

This is only my guess:

1. Class B is loaded because it contains main which you have requested to be executed.
2. Class loader discovers that B requires A to operate (it uses A by accessing its static field)
3. Class B is loaded.
4. Class A requires class B which happens to be already loaded, but not yet initialized
5. A carelessly loads B.Y (initialized to 0 by that time by default), since the class looks like loaded (language design flaw?)
6. A.X = 0 + 1
7. A is now loaded, class loader can continue to load and initialize B
8. B.Y = 1 + 1.

回答3:

B being public starts to load itself.

Sees A.X = B.Y +1 == which is 0 to start with (default int object value) hence 1;

initializes B = 1 + 1 = 2;

hence the answer.

来源：https://stackoverflow.com/questions/6416408/static-circular-dependency-in-java