Java中静态变量和初始化顺序的文本顺序

Question

I only spend five minutes to find a duplicate in SO.

My question is simple. Does the following code always work?

public class LexicalOrderStatic {
    private static Integer a1 = initA1();

    private static Integer a2 = initA2();


    private static Integer initA2(){
        return new Integer(5) / a1;
    }

    private static Integer initA1(){
        return new Integer(5);
    }

    public Integer getA1(){
        return new Integer(a2);
    }

    public static void main(String[] args) {
        LexicalOrderStatic lexLuthor = new LexicalOrderStatic();
        System.out.println(lexLuthor.getA1());

    }
}

In java can I be sure that a1 is always initialized before a2 ?

Thanks. Dw is ok if it is asked or if it is very simple.

Answer 1

In java can I be sure that a1 is always initialized before a2 ?

Yes, because the specification (section 12.4.2) guarantees it (emphasis mine):

Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order , as though they were a single block.

Note that constants are initialized earlier than non-constants (step 6 compared with step 9 quoted above).

Answer 2

Yes. The chain of elements group ( name of the class, static attributes, instance attributes, static method, inner static block of code, etc) (1) is defined in compile time, not in run time: it has a deterministic behaviour.

When you run main , you have loaded at the first time LexicalOrderStatic and the static elements (attributes, methods) are loaded very quickly if this is the first loading.

If you load a second obejct of LexicalOrderStatic , the attribute are shared between the two instance.

Yu can see this statement in running this modified main

public static void main(String[] args) {
    LexicalOrderStatic lexLuthor = new LexicalOrderStatic();
    System.out.println(lexLuthor.getA1());
    LexicalOrderStatic lexLuthor2 = new LexicalOrderStatic();
    System.out.println(lexLuthor2.getA1());
}

(1) before I referred to inheritance, but it'snt the situation, as pointed by the first comment