synchronized

Question

了解一下，一个线程中是否可以调用多个synchronized方法(包含方法，静态方法，代码块)

```java
public class SyncTest {
    //获取CPU个数
    private static int cpuCount = Runtime.getRuntime().availableProcessors();
    private static ThreadPoolExecutor threadPool = new ThreadPoolExecutor(cpuCount, cpuCount * 2, 20,
            TimeUnit.SECONDS, new LinkedBlockingQueue<>(20000));

    static {
        System.out.println("我的cpu颗数: " + cpuCount);
    }

    //成员变量，可被多个线程 共享
    private static long a = 0L;
    private static long b = 0L;
    private static long c = 0L;

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(10000);
        SyncTest lockobj = new SyncTest();
        for (int i = 0; i < 10000; i++) {
            threadPool.execute(() -> {
                lockobj.sync(countDownLatch);
                sync2(countDownLatch);
                lockobj.sync3(countDownLatch);
            });
        }
        // 等待所有的线程执行完再打印
        countDownLatch.await();
        System.out.println("a=" + a);
        System.out.println("b=" + b);
        System.out.println("c=" + c);
    }

    /**
     * 修饰生源方法，其使用的锁对应时当前类所在的实例对象(或者说当前方法的调用对象),也就是this
     *
     * @param countDownLatch
     */
    private synchronized void sync(CountDownLatch countDownLatch) {
        try {
            a++;
        } finally {
            countDownLatch.countDown();
        }
    }

    /**
     * 修饰静态方法，使用的锁对象是当前类
     *
     * @param countDownLatch
     */
    private synchronized static void sync2(CountDownLatch countDownLatch) {
        try {
            b++;
        }  finally {
            countDownLatch.countDown();
        }
    }

    private void sync3(CountDownLatch countDownLatch) {
        try {
            synchronized (countDownLatch) {
                c++;
            }
        }  finally {
            countDownLatch.countDown();
        }
    }

}

结果:
a=3339
b=3334
c=3332

比如上面这段代码，如果三个代码在同一个线程，甚至两个代码在同一个线程，得出的结果集都不正确。

Answer 1

你的结果不对，是因为你的线程还没结束，你就输出结果了，不要用countDownLatch.await();
用threadPool.getActiveCount()判断它是不是执行完了，只要不等于0，就是还没结束
这一段改一下

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(10000);
        SyncTest lockobj = new SyncTest();
        for (int i = 0; i < 10000; i++) {
            threadPool.execute(() -> {
                lockobj.sync(countDownLatch);
                sync2(countDownLatch);
                lockobj.sync3(countDownLatch);
            });
        }

        while (threadPool.getActiveCount()!=0){
            Thread.sleep(100);
        }

        threadPool.shutdown();

        System.out.println("a=" + a);
        System.out.println("b=" + b);
        System.out.println("c=" + c);
    }