Selector的wakeup()

最新推荐文章于 2021-09-06 15:26:05 发布

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原文链接：https://blog.youkuaiyun.com/weixin_44601714/article/details/100081131

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本文深入解析Java NIO中Selector的wakeup()方法，揭示其如何唤醒阻塞的select()操作，使线程立即返回并确保返回值不为零。探讨了在无进行中的选择操作时，该方法如何在下次select调用时生效。

版权声明：本文为优快云博主「木易九日111」的原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.youkuaiyun.com/weixin_44601714/article/details/100081131

/**
 * Causes the first selection operation that has not yet returned to return
 * immediately.
 *
 * <p> If another thread is currently blocked in an invocation of the
 * {@link #select()} or {@link #select(long)} methods then that invocation
 * will return immediately.  If no selection operation is currently in
 * progress then the next invocation of one of these methods will return
 * immediately unless the {@link #selectNow()} method is invoked in the
 * meantime.  In any case the value returned by that invocation may be
 * non-zero.  Subsequent invocations of the {@link #select()} or {@link
 * #select(long)} methods will block as usual unless this method is invoked
 * again in the meantime.
 *
 * <p> Invoking this method more than once between two successive selection
 * operations has the same effect as invoking it just once.  </p>
 *
 * @return  This selector
 */
 翻译一下，就是说

其他线程如果因为调用了selector.select()或者selector.select(long)这两个方法而阻塞，
调用了selector.wakeup()之后，就会立即返回结果，并且返回的值!=0，
如果当前Selector没有阻塞在select方法上，
那么本次 wakeup调用会在下一次select阻塞的时候生效。
public abstract Selector wakeup();

也就是说，这个方法是用来唤醒阻塞的select()方法的

我们知道select()方法会阻塞，是因为用户态将socket的文件描述符和感兴趣的事件传递给操作系统底层的pipe，底层函数执行完成，触发事件之后底层就会向用户态返回数据，这样才会打破阻塞，
在这里插入图片描述

wakeup是如何唤醒阻塞的select()呢，我们从源码角度来看

public Selector wakeup() {
    synchronized(this.interruptLock) {
        if (!this.interruptTriggered) {
            this.setWakeupSocket();
            this.interruptTriggered = true;
        }

        return this;
    }
}
//这里追到了naive方法
private native void setWakeupSocket0(int var1);

setWakeupSocket()方法会直接向pipe中添加wakeupSinkFD，这个FD会立即执行完成并且返回数据，这样底层就会给用户态数据，然后结束阻塞