Java NIO——Selector机制源码分析---转

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：

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：

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：

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：

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();
}