EventBus 源码关于 BackgroundPoster 的疑惑

源码如下：

 final class BackgroundPoster implements Runnable, Poster {

    private final PendingPostQueue queue;
    private final EventBus eventBus;

    private volatile boolean executorRunning;

    BackgroundPoster(EventBus eventBus) {
        this.eventBus = eventBus;
        queue = new PendingPostQueue();
    }

    public void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            queue.enqueue(pendingPost);
            if (!executorRunning) {
                executorRunning = true;
                eventBus.getExecutorService().execute(this);
            }
        }
    }

    @Override
    public void run() {
        try {
            try {
                while (true) {
                    PendingPost pendingPost = queue.poll(1000);
                    if (pendingPost == null) {
                        synchronized (this) {
                            // Check again, this time in synchronized
                            pendingPost = queue.poll();
                            if (pendingPost == null) {
                                executorRunning = false;
                                return;
                            }
                        }
                    }
                    eventBus.invokeSubscriber(pendingPost);
                }
            } catch (InterruptedException e) {
                eventBus.getLogger().log(Level.WARNING, Thread.currentThread().getName() + " was interruppted", e);
            }
        } finally {
            executorRunning = false;
        }
    }

从源码我得知BackgroundPoster从线程池中抽取了一个单独线程，循环读取队列中的PendingPost执行。那么但是既然是单线程，run()方法中的加锁的目的在哪里呢？我感觉完全可以去除加锁代码，简写成如下的代码：

public void run() {
        try {
            try {
                while (true) {
                    PendingPost pendingPost = queue.poll(1000);
                    if (pendingPost == null) {
                                executorRunning = false;
                                return;
                        }
                    }
                    eventBus.invokeSubscriber(pendingPost);
                }
            } catch (InterruptedException e) {
                eventBus.getLogger().log(Level.WARNING, Thread.currentThread().getName() + " was interruppted", e);
            }
        } finally {
            executorRunning = false;
        }
    }

我是在哪方面没有考虑到呢？

ChenFanlin

2019-08-05 10:49:21 +08:00

EventBusBuilder#DEFAULT_EXECUTOR_SERVICE 是 Executors.newCachedThreadPool();
然后这个 BackgroundPoster 是加了个变量`executorRunning`来使他同时只在一个线程中运行,两个锁是确保`executorRunning`的值不出错,两个加锁的代码块同时只能执行一个
假如去掉了下面一个, 假设一种情况, 如果队列里是空的, 先执行`pendingPost = queue.poll();` 取到 null, **executorRunning 还未赋值为 false 的时候** , 这时候`queue.enqueue()`进去了,
```java
if (!executorRunning) {
executorRunning = true;
eventBus.getExecutorService().execute(this);
}
```
这段代码就不会执行, 就得被推迟到下一次 background 的分发了
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