AQS的例子分析

最新推荐文章于 2022-06-04 16:19:09 发布

原创最新推荐文章于 2022-06-04 16:19:09 发布 · 813 阅读

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本文通过分析阿里Druid源码中SimpleCanalConnector的connect方法，详细介绍了AQS(Abstract Queued Synchronizer)在并发控制中的作用及其实现原理。

https://blog.youkuaiyun.com/AAA821/article/details/79712522这篇博客介绍了一下AQS

然后看了下阿里Druid的源码,举其中一个方法来解释AQS的作用,通过例子可以更好的理解AQS的作用

这个是SimpleCanalConnector的connect方法,

public void connect() throws CanalClientException {
        if (connected) {
            return;
        }

        if (runningMonitor != null) {
            if (!runningMonitor.isStart()) {
                runningMonitor.start();
            }
        } else {
            waitClientRunning();//这里调用了Sync
            doConnect();
            if (filter != null) { 
                subscribe(filter);
            }
            if (rollbackOnConnect) {
                rollback();
            }
        }

        connected = true;
    }

private void waitClientRunning() {
        try {
            if (zkClientx != null) {
                if (!connected) {// 未调用connect
                    throw new CanalClientException("should connect first");
                }

                mutex.get();// 阻塞等待  mutex = BooleanMutex
            }
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            throw new CanalClientException(e);
        }
    }

下面是BooleanMutex的get()方法

/**
     * 阻塞等待Boolean为true
     * 
     * @throws InterruptedException
     */
    public void get() throws InterruptedException {
        sync.innerGet(); //sync = Sync
    }

下面是Sync的innerGet()方法

 void innerGet() throws InterruptedException {
            acquireSharedInterruptibly(0);
        }

下面是AbstractQueuedSynchronizer的acquireSharedInterruptibly

public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

这个方法链的调用,就是在连接前,调用AbstractQueuedSynchronizer的acquireSharedInterruptibly方法,先判断能不用获取到锁(tryAcquireShared 尝试获取共享锁)

１.如果能够获取到锁,则我这个线程就不用阻塞

２.如果获取不到锁,则调用doAcquireSharedInterruptibly方法将线程封装到Node存放到Queue里去,等待获取锁的线程释放锁,才会唤醒你,结束你的阻塞

为什么连接方法就要用到AQS,无非就是想控制并发