/** * @author JHX * @date 2018/12/1 13:58 */public class SingleClass {    private static  SingleClass singleClass = null;    public static SingleClass getSingleClass() {        if (singleClass == null) {            synchronized (SingleClass.class) {                if (singleClass == null) {                    singleClass = new SingleClass();                }            }        }        return singleClass;    }}复制代码

这是双重锁单例的模式，然后写一个测试类

/** * @author JHX * @date 2018/12/1 14:00 */public class CoountDownLatchTest {    public static void main(String[] args) {        final CountDownLatch countDownLatch = new CountDownLatch(1);        ExecutorService exce = new ThreadPoolExecutor(15, 15, 20, TimeUnit.SECONDS, new ArrayBlockingQueue
    
     (20));        for (int i = 0; i < 15; i++) {            final int num = i;            Thread thread = new Thread(new Runnable() {                @Override                public void run() {                    System.out.println("第" + num + "开始");                    try {                        countDownLatch.await();                        System.out.println(SingleClass.getSingleClass().toString());                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                }            });            exce.execute(thread);        }        try {            Thread.sleep(10000);        } catch (InterruptedException e) {            e.printStackTrace();        }        countDownLatch.countDown();        System.out.println("main end");        exce.shutdown();    }}复制代码
    

上述测试结果发现，很小概率的打印出来的地址不一样

singleClass = new SingleClass();复制代码

singleClass 类对象的 引用存在栈上

new SingleClass()在堆上开辟内存

指针指向堆上的内存

上述就是出现问题的原因