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 void initSelector() { 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 static SelectorProvider provider() { synchronized (lock) { if (provider != null) return provider; return AccessController.doPrivileged( new PrivilegedAction<SelectorProvider>() { public SelectorProvider run() { if (loadProviderFromProperty()) return provider; if (loadProviderAsService()) return provider; provider = sun.nio.ch.DefaultSelectorProvider.create(); return provider; } }); } }
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) return provider;
保证了整个server程序中只有一个WindowsSelectorProvider对象；
再看看WindowsSelectorProvider. openSelector():
public AbstractSelector openSelector() throws IOException { return new WindowsSelectorImpl(this); } new WindowsSelectorImpl(SelectorProvider)代码： WindowsSelectorImpl(SelectorProvider sp) throws IOException { super(sp); pollWrapper = new PollArrayWrapper(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() throws IOException { return SelectorProvider.provider().openPipe(); } SelectorProvider 中： public Pipe openPipe() throws IOException { 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 = new FileDescriptor(); IOUtil.setfdVal(sourcefd, readFd); source = new SourceChannelImpl(sp, sourcefd); FileDescriptor sinkfd = new FileDescriptor(); IOUtil.setfdVal(sinkfd, writeFd); sink = new SinkChannelImpl(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 int configureBlocking(int fd, 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() throws IOException { return SelectorProvider.provider().openServerSocketChannel(); } SelectorProvider： public ServerSocketChannel openServerSocketChannel() throws IOException { return new ServerSocketChannelImpl(this); }
可见创建的ServerSocketChannelImpl也有WindowsSelectorImpl的引用。
ServerSocketChannelImpl(SelectorProvider sp) throws IOException { 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) throws IOException { if (channelArray == null) throw new ClosedSelectorException(); 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(); return updated; }
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 return poll0(pollWrapper.pollArrayAddress, Math.min(totalChannels, MAX_SELECTABLE_FDS), readFds, writeFds, exceptFds, timeout); } private native int poll0(long pollAddress, int numfds, int[] readFds, int[] writeFds, int[] exceptFds, long timeout); // 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():
public Selector wakeup() { synchronized (interruptLock) { if (!interruptTriggered) { setWakeupSocket(); interruptTriggered = true; } } return this; } // Sets Windows wakeup socket to a signaled state. private void setWakeupSocket() { setWakeupSocket0(wakeupSinkFd); } private native void setWakeupSocket0(int wakeupSinkFd); JNIEXPORT void JNICALL 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 static SelectorProvider create() { PrivilegedAction pa = new GetPropertyAction("os.name"); String osname = (String) AccessController.doPrivileged(pa); if ("SunOS".equals(osname)) { return new sun.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 new sun.nio.ch.EPollSelectorProvider(); } } catch (NumberFormatException x) { // format not recognized } } } return new sun.nio.ch.PollSelectorProvider(); }
