一直不明白pipe是如何唤醒selector的，所以又去看了jdk的源码(openjdk下载)，整理了如下：

以Java nio自带demo : OperationServer.java   OperationClient.java(见附件)

其中server端的核心代码：

public voidinitSelector() {try{

selector=SelectorProvider.provider().openSelector();this.serverChannel1 =ServerSocketChannel.open();

serverChannel1.configureBlocking(false);

InetSocketAddress isa= new InetSocketAddress("localhost", this.port1);

serverChannel1.socket().bind(isa);

serverChannel1.register(selector, SelectionKey.OP_ACCEPT);

}catch(IOException e) {//TODO Auto-generated catch block

e.printStackTrace();

}

}

从头开始，

先看看SelectorProvider.provider()做了什么：

public staticSelectorProvider provider() {synchronized(lock) {if (provider != null)returnprovider;returnAccessController.doPrivileged(new PrivilegedAction() {publicSelectorProvider run() {if(loadProviderFromProperty())returnprovider;if(loadProviderAsService())returnprovider;

provider=sun.nio.ch.DefaultSelectorProvider.create();returnprovider;

}

});

}

}

其中provider = sun.nio.ch.DefaultSelectorProvider.create();会根据操作系统来返回不同的实现类，windows平台就返回WindowsSelectorProvider；

而if (provider != null) returnprovider;

保证了整个server程序中只有一个WindowsSelectorProvider对象；

再看看WindowsSelectorProvider. openSelector():

public AbstractSelector openSelector() throwsIOException {return new WindowsSelectorImpl(this);

}newWindowsSelectorImpl(SelectorProvider)代码：

WindowsSelectorImpl(SelectorProvider sp)throwsIOException {super(sp);

pollWrapper= newPollArrayWrapper(INIT_CAP);

wakeupPipe=Pipe.open();

wakeupSourceFd=((SelChImpl)wakeupPipe.source()).getFDVal();//Disable the Nagle algorithm so that the wakeup is more immediate

SinkChannelImpl sink =(SinkChannelImpl)wakeupPipe.sink();

(sink.sc).socket().setTcpNoDelay(true);

wakeupSinkFd=((SelChImpl)sink).getFDVal();

pollWrapper.addWakeupSocket(wakeupSourceFd,0);

}

其中Pipe.open()是关键，这个方法的调用过程是：

Java代码

public static Pipe open() throwsIOException {returnSelectorProvider.provider().openPipe();

}

SelectorProvider 中：public Pipe openPipe() throwsIOException {return new PipeImpl(this);

}

再看看怎么new PipeImpl()的：

Java代码

PipeImpl(SelectorProvider sp) {long pipeFds = IOUtil.makePipe(true);int readFd = (int) (pipeFds >>> 32);int writeFd = (int) pipeFds;

FileDescriptor sourcefd= newFileDescriptor();

IOUtil.setfdVal(sourcefd, readFd);

source= newSourceChannelImpl(sp, sourcefd);

FileDescriptor sinkfd= newFileDescriptor();

IOUtil.setfdVal(sinkfd, writeFd);

sink= newSinkChannelImpl(sp, sinkfd);

}

其中IOUtil.makePipe(true)是个native方法：

/**

* Returns two file descriptors for a pipe encoded in a long.

* The read end of the pipe is returned in the high 32 bits,

* while the write end is returned in the low 32 bits.

*/

staticnativelong makePipe(boolean blocking);

具体实现：

JNIEXPORT jlong JNICALL

Java_sun_nio_ch_IOUtil_makePipe(JNIEnv*env, jobject this, jboolean blocking)

{int fd[2];if (pipe(fd) < 0) {

JNU_ThrowIOExceptionWithLastError(env,"Pipe failed");return 0;

}if (blocking ==JNI_FALSE) {if ((configureBlocking(fd[0], JNI_FALSE) < 0)|| (configureBlocking(fd[1], JNI_FALSE) < 0)) {

JNU_ThrowIOExceptionWithLastError(env,"Configure blocking failed");

close(fd[0]);

close(fd[1]);return 0;

}

}return ((jlong) fd[0] << 32) | (jlong) fd[1];

}static intconfigureBlocking(intfd, jboolean blocking)

{int flags =fcntl(fd, F_GETFL);int newflags = blocking ? (flags & ~O_NONBLOCK) : (flags |O_NONBLOCK);return (flags == newflags) ? 0: fcntl(fd, F_SETFL, newflags);

}

正如这段注释：

/**

* Returns two file descriptors for a pipe encoded in a long.

* The read end of the pipe is returned in the high 32 bits,

* while the write end is returned in the low 32 bits.

*/

High32位存放的是通道read端的文件描述符FD(file descriptor)，low 32 bits存放的是write端的文件描述符。所以取到makepipe()返回值后要做移位处理。

pollWrapper.addWakeupSocket(wakeupSourceFd, 0);

这行代码把返回的pipe的write端的FD放在了pollWrapper中(后面会发现，这么做是为了实现selector的wakeup())

ServerSocketChannel.open()的实现：

public static ServerSocketChannel open() throwsIOException {returnSelectorProvider.provider().openServerSocketChannel();

}

SelectorProvider：public ServerSocketChannel openServerSocketChannel() throwsIOException {return new ServerSocketChannelImpl(this);

}

可见创建的ServerSocketChannelImpl也有WindowsSelectorImpl的引用。

ServerSocketChannelImpl(SelectorProvider sp) throwsIOException {super(sp);this.fd = Net.serverSocket(true); //打开一个socket，返回FD

this.fdVal =IOUtil.fdVal(fd);this.state =ST_INUSE;

}

然后通过serverChannel1.register(selector, SelectionKey.OP_ACCEPT);把selector和channel绑定在一起，也就是把new ServerSocketChannel时创建的FD与selector绑定在了一起。

到此，server端已启动完成了，主要创建了以下对象：

WindowsSelectorProvider：单例

WindowsSelectorImpl中包含：

pollWrapper：保存selector上注册的FD，包括pipe的write端FD和ServerSocketChannel所用的FD

wakeupPipe：通道(其实就是两个FD，一个read，一个write)

再到Server 中的run():

selector.select();主要调用了WindowsSelectorImpl中的这个方法：

protected int doSelect(long timeout) throwsIOException {if (channelArray == null)throw newClosedSelectorException();this.timeout = timeout; //set selector timeout

processDeregisterQueue();if(interruptTriggered) {

resetWakeupSocket();return 0;

}//Calculate number of helper threads needed for poll. If necessary//threads are created here and start waiting on startLock

adjustThreadsCount();

finishLock.reset();//reset finishLock//Wakeup helper threads, waiting on startLock, so they start polling.//Redundant threads will exit here after wakeup.

startLock.startThreads();//do polling in the main thread. Main thread is responsible for//first MAX_SELECTABLE_FDS entries in pollArray.

try{

begin();try{

subSelector.poll();

}catch(IOException e) {

finishLock.setException(e);//Save this exception

}//Main thread is out of poll(). Wakeup others and wait for them

if (threads.size() > 0)

finishLock.waitForHelperThreads();

}finally{

end();

}//Done with poll(). Set wakeupSocket to nonsignaled for the next run.

finishLock.checkForException();

processDeregisterQueue();int updated =updateSelectedKeys();//Done with poll(). Set wakeupSocket to nonsignaled for the next run.

resetWakeupSocket();returnupdated;

}

其中subSelector.poll()是核心，也就是轮训pollWrapper中保存的FD；具体实现是调用native方法poll0：

private int poll() throws IOException{ //poll for the main thread

returnpoll0(pollWrapper.pollArrayAddress,

Math.min(totalChannels, MAX_SELECTABLE_FDS),

readFds, writeFds, exceptFds, timeout);

}private native int poll0(long pollAddress, intnumfds,int[] readFds, int[] writeFds, int[] exceptFds, longtimeout);//These arrays will hold result of native select().//The first element of each array is the number of selected sockets.//Other elements are file descriptors of selected sockets.

private final int[] readFds = new int [MAX_SELECTABLE_FDS + 1];//保存发生read的FD

private final int[] writeFds = new int [MAX_SELECTABLE_FDS + 1]; //保存发生write的FD

private final int[] exceptFds = new int [MAX_SELECTABLE_FDS + 1]; //保存发生except的FD

这个poll0()会监听pollWrapper中的FD有没有数据进出，这会造成IO阻塞，直到有数据读写事件发生。比如，由于pollWrapper中保存的也有ServerSocketChannel的FD，所以只要ClientSocket发一份数据到ServerSocket,那么poll0()就会返回；又由于pollWrapper中保存的也有pipe的write端的FD，所以只要pipe的write端向FD发一份数据，也会造成poll0()返回；如果这两种情况都没有发生，那么poll0()就一直阻塞，也就是selector.select()会一直阻塞；如果有任何一种情况发生，那么selector.select()就会返回，所有在OperationServer的run()里要用while (true) {，这样就可以保证在selector接收到数据并处理完后继续监听poll();

这时再来看看WindowsSelectorImpl. Wakeup():

publicSelector wakeup() {synchronized(interruptLock) {if (!interruptTriggered) {

setWakeupSocket();

interruptTriggered= true;

}

}return this;

}//Sets Windows wakeup socket to a signaled state.

private voidsetWakeupSocket() {

setWakeupSocket0(wakeupSinkFd);

}private native void setWakeupSocket0(intwakeupSinkFd);

JNIEXPORTvoidJNICALL

Java_sun_nio_ch_WindowsSelectorImpl_setWakeupSocket0(JNIEnv*env, jclass this,

jint scoutFd)

{/*Write one byte into the pipe*/

const char byte = 1;

send(scoutFd,&byte, 1, 0);

}

可见wakeup()是通过pipe的write 端send(scoutFd, &byte, 1, 0)，发生一个字节1，来唤醒poll()。所以在需要的时候就可以调用selector.wakeup()来唤醒selector。

原文：http://goon.iteye.com/blog/1775421

补充linux操作系统下的DefaultSelectorProvider的实现，可以看到，如果内核版本>=2.6则，具体的SelectorProvider为EPollSelectorProvider，否则为默认的PollSelectorProvider

//sun.nio.ch.DefaultSelectorProvider

public staticSelectorProvider create() {

PrivilegedAction pa= new GetPropertyAction("os.name");

String osname=(String) AccessController.doPrivileged(pa);if ("SunOS".equals(osname)) {return newsun.nio.ch.DevPollSelectorProvider();

}//use EPollSelectorProvider for Linux kernels >= 2.6

if ("Linux".equals(osname)) {

pa= new GetPropertyAction("os.version");

String osversion=(String) AccessController.doPrivileged(pa);

String[] vers= osversion.split("\\.", 0);if (vers.length >= 2) {try{int major = Integer.parseInt(vers[0]);int minor = Integer.parseInt(vers[1]);if (major > 2 || (major == 2 && minor >= 6)) {return newsun.nio.ch.EPollSelectorProvider();

}

}catch(NumberFormatException x) {//format not recognized

}

}

}return newsun.nio.ch.PollSelectorProvider();

}