java - 未为派生类调用静态初始值设定项

Question

以下 Java 代码不调用类 B 的静态初始值设定项。为什么？

代码:

class A 
{
    static 
    {
        System.out.println("A static init");
    }

    public static void f() 
    {
        System.out.println("f() called");
    }
}

class B extends A 
{
    static 
    {
        System.out.println("B static init");
    }
}

public class App
{
    public static void main( String[] args)
    {
        B.f(); //invokestatic  #16                 // Method com/db/test/B.f:()V
    }
}

程序输出:

A static init
f() called

在 JDK 1.8.0_25 上测试

Answer 1

没有“静态构造函数”这样的东西。它是一个静态初始化 block ，仅在类初始化时执行。由于您调用的是 A 类的静态方法(即使您是通过 B 类引用它)，因此无需初始化 B 类。调用 B.f(); 与调用 相同code>A.f();.

如果您创建 B 类的实例或访问 B 类的静态成员/方法，将执行 B 类的静态初始化 block 。

以下是触发类初始化的唯一条件(JLS 12.4.1):

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.