springboot线程池结合CountDownLatch使用

使用场景

等待多个子线程并行执行完毕，通知主线程继续执行任务

1 配置线程池

配置spring boot 线程池

2 多线程调用

2.1 调用代码

使用 CountDownLatch 统计线程数，实现等待多个子线程执行完毕

@RestController
public class TestController {

    private static Logger logger = LoggerFactory.getLogger(TestController.class);

    @Autowired
    private TestService testService;
    @Autowired
    private ThreadPoolTaskExecutor ThreadPoolA;


    @RequestMapping("/test")
    public void test()throws Exception{
        logger.info("main->start");
        //初始化子线程个数 2
        final CountDownLatch latch =new CountDownLatch(2);
        ThreadPoolA.execute(() -> {
            try {
                logger.info("thread1->doWork");
                Thread.sleep(3000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                logger.info("thread1->end");
                //线程数-1
                latch.countDown();
            }
        });
        ThreadPoolA.execute(()->{
            try {
                logger.info("thread2->doWork");
                Thread.sleep(1000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                logger.info("thread2->end");
                //线程数-1
                latch.countDown();
            }
        });
        logger.info("main->doWork");
        //等待线程执行完毕
        latch.await();
        logger.info("main->end");
    }
}
           

需获取线程返回值时 ThreadPoolA.execute 改为 ThreadPoolA.submit，例如：

//省略
        Future<Long> future = ThreadPoolA.submit(() -> {
            //省略
            return 1L;
        });
        //省略
        latch.await();
        //调用future.get()将会在该线程执行完毕后才会往下执行
        logger.info(future.get());
           

为避免线程数统计不正确导致死锁，建议使用latch.await(long timeout, TimeUnit unit)

//十分钟内线程计数未归零，将自动释放
latch.await(10, TimeUnit.MINUTES);
           

2.2 执行结果

2 CountDownLatch 源码展示

public class CountDownLatch {
    
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    private final Sync sync;

    /**
     * 初始化线程个数
     */
    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

    /**
     * 当前线程等待直到线程数为0
     */
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * 当前线程等待直到线程数为0或超过等待时间
     */
    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    /**
     * 统计线程个数，如果计数达到零，则释放所有等待线程
     */
    public void countDown() {
        sync.releaseShared(1);
    }
    public long getCount() {
        return sync.getCount();
    }
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }
}
           

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