多线程16--单例--懒汉模式和静态内部类形

1. 懒汉双重检验

public class DubbleSingleton {

    private static DubbleSingleton ds;
    
    public static DubbleSingleton getDs(){
        if(ds == null){
            try {
                //模拟初始化对象的准备时间...
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (DubbleSingleton.class) {
                if(ds == null){
                    ds = new DubbleSingleton();
                }
            }
        }
        return ds;
    }
    
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode()); // 1555073959
            }
        },"t1");
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode()); // 1555073959
            }
        },"t2");
        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode()); // 1555073959
            }
        },"t3");
        
        t1.start();
        t2.start();
        t3.start();
    }
}

2. 静态内部类

public class InnerSingleton {
    
    private static class Singletion {
        private static Singletion single = new Singletion();
    }
    
    public static Singletion getInstance(){
        return Singletion.single;
    }
}