java开发中为什么双重效验锁要加volatile

public class Singleton {
    // 1.防止外部直接 new 对象破坏单例模式
    private Singleton() {}
    // 2.通过私有变量保存单例对象【添加了 volatile 修饰】
    private static volatile Singleton instance = null;
    // 3.提供公共获取单例对象的方法
    public static Singleton getInstance() {
        if (instance == null) { // 第 1 次效验
            synchronized (Singleton.class) {
                if (instance == null) { // 第 2 次效验
                    instance = new Singleton(); 
                }
            }
        }
        return instance;
    }
}

private static boolean flag = false;
public static void main(String[] args) {
    Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {
            // 如果 flag 变量为 true 就终止执行
            while (!flag) {
            }
            System.out.println("终止执行");
        }
    });
    t1.start();
    // 1s 之后将 flag 变量的值修改为 true
    Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("设置 flag 变量的值为 true！");
            flag = true;
        }
    });
    t2.start();
}

public class volatileTest {
    private static volatile boolean flag = false;
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                // 如果 flag 变量为 true 就终止执行
                while (!flag) {
                }
                System.out.println("终止执行");
            }
        });
        t1.start();
        // 1s 之后将 flag 变量的值修改为 true
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("设置 flag 变量的值为 true！");
                flag = true;
            }
        });
        t2.start();
    }
}

public class Singleton {
    private Singleton() {}
    // 使用 volatile 禁止指令重排序
    private static volatile Singleton instance = null;
    public static Singleton getInstance() {
        if (instance == null) { // ①
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton(); // ②
                }
            }
        }
        return instance;
    }
}