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javastaticlazy-initialization

Java interface static field lazy initialization

Given example

interface A {
    static int aInit() {
        System.out.println("Interface field");
        return 42;
    }
    int a = aInit();
}

class B implements A {
    static int bInit() {
        System.out.println("Class field");
        return 42;
    }
    static final int b = bInit();
}

A a = new B();

on both JDK8 and JDK10 prints just "Class field". Direct access to A.a spawns its initialization and "Interface field" output.

This shows that interface static field initialization is lazy, which is not true for final static class field.

I can see OpenJDK JEP draft about such laziness for classes, but is it a documented feature for interface? Or just a detail of JVM implementation?

Solution

  • It is a documented behavior. The interface A will not be initialized as per https://docs.oracle.com/javase/specs/jls/se12/html/jls-12.html#jls-12.4.1. It is only initialized when either the field a or the method aInit() are called.