单例模式（双重锁定模式）

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));
    }
}

单例模式实现类

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                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());
    }
}