java static code

static{}代码块在一个进程中只会执行一次。

public class XX {

    public XX() {
        System.out.println("XX()");
    }
}

public class Test {

    static {
        System.out.println("static code thread ID:"
                + Thread.currentThread().getId());
    }

    private static final XX x = new XX();

    public static void main(String[] args) {
        System.out.println("main thread ID:" + Thread.currentThread().getId());

        new Thread() {
            @Override
            public void run() {
                new Test().print();
            }
        }.start();

        new Thread() {
            @Override
            public void run() {
                new Test().print();
            }
        }.start();

    }

    public void print() {
        System.out.println("thread ID:" + Thread.currentThread().getId());
    }
}

-----------------------------------
static code thread ID:1
XX()
main thread ID:1
thread ID:8
thread ID:9

 ---------------------------------------------------------------------------------------

有2种情况下静态代码会执行：

1. new 一个Extend类
2. 调用一个Extend类中实现的(而非继承下来的)静态方法

public class Base {
    
    public static String who = "Mr. Base";
     
    Base(){
    }
    static {
        System. out.println("static block in Base" );
    }
    public static void functionInBase(){
        System. out.println("static function in Base! -- " + who);
    }
    public void showWho(){
        System. out.print(who);
    }
}

public class Extend extends Base {

    public static String who = "Mr. Extend";

    static {
        System.out.println("static block in Extend");
    }

    public static void functionInExtend() {
        System.out.println("static function in Extend!");
    }

    Extend() {
    }
}

public class TT {

    public static void main(String[] args) {
        Extend.functionInBase();
        System.out.println();
        
        Extend ex = new Extend();
        System.out.println();
        
        ex.showWho();
        System.out.println(Extend.who);
    }
}

static block in Base
static function in Base! -- Mr. Base

static block in Extend

Mr. BaseMr. Extend