java - 为什么主线程在这里没有被抢占？

Question

public class SingletonClass {
private static SingletonClass singletonClass;
public void executeMethod(String string) {
    System.out.print("["+string);
    try {
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    System.out.println("]");
}
private SingletonClass() {

}
public static SingletonClass getInstance() {
    if(singletonClass==null)
        singletonClass=new SingletonClass();
    return singletonClass;
}

public static void main(String args[]) {
    SingletonClass.getInstance().executeMethod("MainThread");
    new SingleTonThread(SingletonClass.getInstance());
}

}

class SingleTonThread implements Runnable{
private SingletonClass sc;
SingleTonThread(SingletonClass singleton){
    this.sc=singleton;
    Thread t = new Thread(this);
    t.start();
}
@Override
public void run() {
    sc.executeMethod("SingleTonThread");
}

}

我期望以下输出(因为主线程应该被 SingleTonThread 抢占): 预期输出:[MainThread[SingleTonThread] ] 实际输出: [主线] [单吨线程]

Answer 1

调用时:

SingletonClass.getInstance().executeMethod("MainThread");

您已通过调用阻塞了主线程

sleep(1000)

下面启动线程类的行将不会被执行，直到

executeMethod("MainThread")

即将执行完毕

顺便说一句，在构造函数中启动线程是一种非常糟糕的做法 as described in this question