Java NIO，Selector机制源码分析

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();
        }
}
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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() { public SelectorProvider run() { if (loadProviderFromProperty()) return provider; if (loadProviderAsService()) return provider;
                            provider = sun.nio.ch.DefaultSelectorProvider.create(); return provider;
                        }
                    });
        }
    }
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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);
    }
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其中Pipe.open()是关键，这个方法的调用过程是：

public static Pipe open() throws IOException { return SelectorProvider.provider().openPipe();
}
SelectorProvider 中： public Pipe openPipe() throws IOException { return new PipeImpl(this);
}
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再看看怎么new PipeImpl()的：

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);
 }
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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.

     */
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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);
}
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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.

     */
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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);
}
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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;
}
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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;
    }
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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
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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);
}
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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。

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();
}
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