探索NIO wakeup内部原理

最近在重新梳理对Kafka的知识，看一遍源码加深印象。源码实现中，Kafka的客户端与服务端都是基于Java NIO机制提高网络IO的效率（当然，这不是唯一因素）。

这里就不再重复说明阻塞/非阻塞，同步/异步IO模型之间的区别了，也不赘述非阻塞IO在不同平台上的实现，例如select，epoll，kqueue等，这些都可以在网络上找到很丰富的文档以及示例。

wakeup 接口

Java NIO里面，我觉得蛮有意思的是这么一个接口，蛮trick的：

 /**
 * 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
 */
public abstract Selector wakeup();

从上面的注释可以看出，这个接口可以立刻中断正在执行的select()调用，或者接下来第一次被调用的select()调用。

这么神奇？我印象中的非阻塞实现好像不支持“唤醒”功能，Java NIO是怎么做到的？

本着好学求知的精神，我转向Google寻求帮助，终于在StackOverflow上找到了一个相似的问题：

如何在没有IO事件时中断epoll_wait

里面提到了一个技巧：self-pipe，通过管道文件IO的方式唤醒确实是可以做到的，JDK源码也证实了这一点。

public void interrupt() {
    interrupt(this.interruptFD);
}

private native int poll0(long var1, int var3, long var4);

private static native void interrupt(int var0);

从图中可以看出，最终的wakeup调用了类sun.nio.ch.PollArrayWrapper的interrupt本地方法。

进一步，我们找到了这个本地方法的具体C语言实现，具体定义在src/linux/native/sun/nio/ch/PollArrayWrapper.c文件中：

JNIEXPORT void JNICALL
Java_sun_nio_ch_PollArrayWrapper_interrupt(JNIEnv *env, jobject this, jint fd)
{
    int fakebuf[1];
    fakebuf[0] = 1;
    if (write(fd, fakebuf, 1) < 0) {
         JNU_ThrowIOExceptionWithLastError(env,
                                          "Write to interrupt fd failed");
    }
}

通过往管道FD中写入一个整数1的方式，触发了管道FD的可读事件，进而中断了NIO的select()调用。

self-pipe 还是一个蛮有意思的trick。

本文链接 http://yexingzhe54.github.io/passages/java-nio-wakeup-trick/