When a static variable is in RIWO state, can it be accessed directly ?

Question

If a static variable is in RIWO (Read Indirectly Write Only) state. the static variable can not be accessed directly.

here is the code

class Test {
    static{
        System.out.println(x);
    }

    static int x = 10;

    public static void main(String[] args) {

    }
}

in this case illegal forward reference compile time error is coming.

but if you are using the class name to access the static variable, it can be accessed.

here is the code example

class Test {
    static{
        System.out.println(Test.x);
    }

    static int x = 10;

    public static void main(String[] args) {

    }
}

answer is : 0

how is this possible ? isn't this a direct access ?

Answer 1

As per JLS 12.4.1. When Initialization Occurs and this answer:

a class's static initialization normally happens immediately before the first time one of the following events occur:

• an instance of the class is created
• a static method of the class is invoked
• a static field of the class is assigned
• a non-constant static field is used

In your case reading Test.x falls under point 4, a non-constant static field. Your code reads and output 0 which is the default int value, however you change it by marking the field final as

static {
  System.out.println(Test.x);
}

final static int x = 10;

The code will now output 10. You can see it by comparing output of javap -c Test.class for both non-final and final field cases, the order of bytecode will be reversed around:

6: invokevirtual #4                  // Method java/io/PrintStream.println:(I)V
9: bipush        10

To me it looks like the Java compiler's forward reference error is a workaround for gotchas in static initialization in JVM.