多线程单例--双重校验锁

package caolihua.general;
//双重检验锁
public class ObjSingleton {
    private static ObjSingleton obj;
    public static ObjSingleton getObj() {
        //首次判断是否为空
        if (obj == null) {
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            //保证只有一个线程进来创建对象
            synchronized (ObjSingleton.class) {
                if (obj == null) {
                    obj = new ObjSingleton();
                }
            }

        }
        return obj;
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable(){
            @Override
            public void run() {
                System.out.println(getObj().hashCode());
            }
            
        },"t1");
        
        Thread t2 = new Thread(new Runnable(){
            @Override
            public void run() {
                System.out.println(getObj().hashCode());
            }
            
        },"t2");
        
        Thread t3 = new Thread(new Runnable(){
            @Override
            public void run() {
                System.out.println(getObj().hashCode());
            }
            
        },"t3");
        t1.start();
        t2.start();
        t3.start();
    }

}

可以看到，三个线程创建的都是同一个对象。