单例模式（懒汉模式）——调用时实例化

特点：调用时实例化，需要线程同步

主方法

public class main {
    public static void main(String[] args) throws InterruptedException {
        long time = System.currentTimeMillis();
        MyThread[] mts = new MyThread[10];
        for (int i = 0; i < 10; i++) {
            mts[i] = new MyThread();
        }
        for (int i = 0; i < 10; i++) {
            mts[i].start();
            mts[i].join();
        }
        System.out.println("耗时：" + (System.currentTimeMillis() - time));
    }
}

实现类

public class Singleton {
    private static Singleton instance = null;

    private Singleton() {
    }

    public synchronized static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

线程类

public class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println(Singleton.getInstance().hashCode());
    }
}