java - 类加载、静态 block

Question

我有这段代码，我使用 -verbose:class 选项运行它来查看已加载的类。令我惊讶的是，它显示它加载了类 A1 和 A2，但静态 block 没有被调用。

有人可以解释一下这种行为

package P1;



import java.lang.reflect.InvocationTargetException;

public class DemoReflection {

    static {
        System.out.println("Loading Demo");
    }

    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, SecurityException,
            InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
        System.out.println("A2 " + A2.class.getClassLoader().getClass());
        System.out.println("Demo " + DemoReflection.class.getClassLoader().getClass());
        System.out.println("A1 " + A1.class.getClassLoader().getClass());
    }

}

class A1 {
    static {
        System.out.println("Loading A1");
    }
}

class A2 extends A1 {

    static {
        System.out.println("Loading A2");
    }

    public A2() {
        System.out.println("m2");
    }

    public void m1() {
        System.out.println("m1");
    }
}

class A3 {

    static int a3Id = 3;

    static {
        System.out.println("Loading A3");
    }

}

输出:

Answer 1

JLS §8.7说:

A static initializer declared in a class is executed when the class is initialized (§12.4.2).

那么初始化是什么意思呢？我们引用JLS §12.4.2 。这描述了详细的初始化过程。不过点JLS §12.4.1放在这里可能更合适。它说:

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.

T is a class and 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).

T is a top level class (§7.6), and an assert statement (§14.10) lexically nested within T (§8.1.3) is executed.

这些选项都不适用于您的情况，因此不会调用静态 block 。