Java NIO

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距离上次看nio相关知识想来也快一年了，感觉对于nio的理解总是停留在IO复用的io模型，知其然但不知其所以然，故而今天来解开Java NIO的神秘面纱。

首先来回顾下NIO基本概念，Java NIO主要由Buffer、Channel、Selector三大组件组成。其他组件比如Pipe、FileLock只不过是这三个组件的公共工具类。
Buffer是与NIO Channel交互的载体，提供了一系列便于操作内存块的方法。读数据是从Channel读取到Buffer中，写数据是从Buffer写入到Channel。
使用Buffer进行读写数据通常需要4步：
将数据写入到Buffer

  • 调用buffer.flip()
  • 从Buffer中读取数据
  • 调用buffer.clear()或者buffer.compact()

TCP/IP 三次握手

Java NIO API基本原理
Selector.open -> Pipe.open 创建一个Selector实例 初始化Selector(windows为WindowsSelectorImpl Linux为PollSelectorImpl) Selector同时会初始化并持有PollArrayWapper类
(功能近似于一个数组，保存注册的Socket句柄、感兴趣的事件掩码以及调用系统调用poll后的返回的就绪事件掩码)
ServerSocketChannel.open 初始化channel，构造ServerSocketChannelImpl 对象
ServerSocketChannel.register 注册需要监听的通道 将channel(socket)以及感兴趣的事件注册到pollArray中

Selector.select 获取已经就绪的通道

以Linux为例：

Selector.open

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public static Selector open() throws IOException {
    return SelectorProvider.provider().openSelector();
}

从上可知Selecotr是由SelectorProvider.openSelector提供。则要初始化Selector必须先拿到SelectorProvier。

获取SelectorProvider

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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;
                    }
                });
    }
}
  1. 通过获取系统变量java.nio.channels.spi.SelectorProvider的方式获取SelectorProvider
  2. 通过SPI扩展加载获取SelectorProvider
  3. 通过sun.nio.ch.DefaultSelectorProvider.create()获取

不一样的DefaultSelectorProvider

通过阅读JDK源码 我们可以找到linux、macosx、windows三个版本的实现

  • linux版本：

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    public class DefaultSelectorProvider {
        /**
         * Prevent instantiation.
         */
        private DefaultSelectorProvider() { }

        @SuppressWarnings("unchecked")
        private static SelectorProvider createProvider(String cn) {
            Class<SelectorProvider> c;
            try {
                c = (Class<SelectorProvider>)Class.forName(cn);
            } catch (ClassNotFoundException x) {
                throw new AssertionError(x);
            }
            try {
                return c.newInstance();
            } catch (IllegalAccessException | InstantiationException x) {
                throw new AssertionError(x);
            }

        }
        /**
         * Returns the default SelectorProvider.
         */
        public static SelectorProvider create() {
            String osname = AccessController
                .doPrivileged(new GetPropertyAction("os.name"));
            if (osname.equals("SunOS"))
                return createProvider("sun.nio.ch.DevPollSelectorProvider");
            if (osname.equals("Linux"))
                return createProvider("sun.nio.ch.EPollSelectorProvider");
            return new sun.nio.ch.PollSelectorProvider();
        }
    }

  • macosx版本：

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    public class DefaultSelectorProvider {
        /**
         * Prevent instantiation.
         */
        private DefaultSelectorProvider() { }

        /**
         * Returns the default SelectorProvider.
         */
        public static SelectorProvider create() {
            return new sun.nio.ch.KQueueSelectorProvider();
        }
    }

  • windows版本：

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    public class DefaultSelectorProvider {
        /**
         * Prevent instantiation.
         */
        private DefaultSelectorProvider() { }

        /**
         * Returns the default SelectorProvider.
         */
        public static SelectorProvider create() {
            return new sun.nio.ch.WindowsSelectorProvider();
        }
    }

    以Linux为例 我们来接着一探究竟，通过源码可知Linux使用的Provider为：EPollSelectorProvider

    EPollSelectorProvider

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    public class EPollSelectorProvider
        extends SelectorProviderImpl
    {
        public AbstractSelector openSelector() throws IOException {
            return new EPollSelectorImpl(this);
        }
        public Channel inheritedChannel() throws IOException {
            return InheritedChannel.getChannel();
        }
    }

    显然Linux下 Selector selector = Selector.open()最终初始化了一个EPollSelectorImpl对象

Selector的真身EPollSelectorImpl

EPollSelectorImpl初始化过程，会建立一个管道pipe 并初始化一个EPollArrayWrapper数组保存pollfd， 并初始化一个维持文件描述符与SelectorKeyImpl映射关系的map

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EPollSelectorImpl(SelectorProvider sp) throws IOException {
    super(sp);
    //1.建立一个管道Pipe 读端fd在高32位 写端fd在低32位
    long pipeFds = IOUtil.makePipe(false);
    fd0 = (int) (pipeFds >>> 32);
    fd1 = (int) pipeFds;
    //2.初始化一个EPollArrayWrapper 本质上是pollfd数组
    pollWrapper = new EPollArrayWrapper();
    //3.将读端fd添加到PollArrayWrapper的fd数组中
    pollWrapper.initInterrupt(fd0, fd1);
    //4.初始化文件描述符到SelectorKey映射的map
    //private Map<Integer,SelectionKeyImpl> fdToKey;
    fdToKey = new HashMap<>();
}

关键的EPollArrayWrapper

EPollArrayWrapper实际通过JNI操作本地epoll_event的数据结构，其定义如下

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typedef union epoll_data {
 void *ptr;
 int fd;
 __uint32_t u32;
 __uint64_t u64;
} epoll_data_t;

struct epoll_event {
 __uint32_t events;
 epoll_data_t data;
};
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EPollArrayWrapper初始化构造器，首先创建epoll的数据结构，并进行内存申请分配

EPollArrayWrapper() throws IOException {
    // creates the epoll file descriptor
    epfd = epollCreate();

    // the epoll_event array passed to epoll_wait
    int allocationSize = NUM_EPOLLEVENTS * SIZE_EPOLLEVENT;
    pollArray = new AllocatedNativeObject(allocationSize, true);
    pollArrayAddress = pollArray.address();

    // eventHigh needed when using file descriptors > 64k
    if (OPEN_MAX > MAX_UPDATE_ARRAY_SIZE)
        eventsHigh = new HashMap<>();
}

epollCreate()

epollCreate通过JNI调用系统方法epoll_create 并返回句柄 完成epoll的初始化工作

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JNIEXPORT jint JNICALL
Java_sun_nio_ch_EPollArrayWrapper_epollCreate(JNIEnv *env, jobject this)
{
    /*
     * epoll_create expects a size as a hint to the kernel about how to
     * dimension internal structures. We can't predict the size in advance.
     */
    int epfd = epoll_create(256);
    if (epfd < 0) {
       JNU_ThrowIOExceptionWithLastError(env, "epoll_create failed");
    }
    return epfd;
}

在上面EpollSelectorImpl初始化过程中，我们看到pollWrapper.initInterrupt(fd0, fd1);的调用

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void initInterrupt(int fd0, int fd1) {
    outgoingInterruptFD = fd1;
    incomingInterruptFD = fd0;
    epollCtl(epfd, EPOLL_CTL_ADD, fd0, EPOLLIN);
}

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JNIEXPORT void JNICALL
Java_sun_nio_ch_EPollArrayWrapper_epollCtl(JNIEnv *env, jobject this, jint epfd,
                                           jint opcode, jint fd, jint events)
{
    struct epoll_event event;
    int res;

    event.events = events;
    event.data.fd = fd;

    RESTARTABLE(epoll_ctl(epfd, (int)opcode, (int)fd, &event), res);

    /*
     * A channel may be registered with several Selectors. When each Selector
     * is polled a EPOLL_CTL_DEL op will be inserted into its pending update
     * list to remove the file descriptor from epoll. The "last" Selector will
     * close the file descriptor which automatically unregisters it from each
     * epoll descriptor. To avoid costly synchronization between Selectors we
     * allow pending updates to be processed, ignoring errors. The errors are
     * harmless as the last update for the file descriptor is guaranteed to
     * be EPOLL_CTL_DEL.
     */
    if (res < 0 && errno != EBADF && errno != ENOENT && errno != EPERM) {
        JNU_ThrowIOExceptionWithLastError(env, "epoll_ctl failed");
    }
}

而epollCtl实际通过JNI调用了系统方法epoll_ctl(epfd, (int)opcode, (int)fd, &event) 来将需要监听的句柄加入到epoll的数据结构中
epoll_ctl函数用于管理文件描述符的事件集 使用此函数可以注册、修改、删除一个或多个事件

Selector.select()

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public int select(long timeout)
    throws IOException
{
    if (timeout < 0)
        throw new IllegalArgumentException("Negative timeout");
    return lockAndDoSelect((timeout == 0) ? -1 : timeout);
}
private int lockAndDoSelect(long timeout) throws IOException {
    synchronized (this) {
        if (!isOpen())
            throw new ClosedSelectorException();
        synchronized (publicKeys) {
            synchronized (publicSelectedKeys) {
                return doSelect(timeout);
            }
        }
    }
}
protected int doSelect(long timeout) throws IOException {
    if (closed)
        throw new ClosedSelectorException();
    //处理已经不监听的事件(文件描述符或Channel)
    processDeregisterQueue();
    try {
        //标记开始一个可能被中断的IO操作
        begin();
        //通过调用native方法epoll_wait获取已经就绪的pollfd
        pollWrapper.poll(timeout);
    } finally {
        end();
    }
    //处理已经不监听的事件
    processDeregisterQueue();
    //更新有事件变化的selectorKeys
    int numKeysUpdated = updateSelectedKeys();
    if (pollWrapper.interrupted()) {
        // Clear the wakeup pipe
        pollWrapper.putEventOps(pollWrapper.interruptedIndex(), 0);
        synchronized (interruptLock) {
            pollWrapper.clearInterrupted();
            IOUtil.drain(fd0);
            interruptTriggered = false;
        }
    }
    return numKeysUpdated;
}

Selector.select()最终调用了pollArrayWrapper.poll()方法 将已经就绪的fd添加到pollWrapper里的数组中

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int poll(long timeout) throws IOException {
    updateRegistrations();
    updated = epollWait(pollArrayAddress, NUM_EPOLLEVENTS, timeout, epfd);
    for (int i=0; i<updated; i++) {
        if (getDescriptor(i) == incomingInterruptFD) {
            interruptedIndex = i;
            interrupted = true;
            break;
        }
    }
    return updated;
}

PollArrayWrapper.poll()方法通过JNI调用系统方法epoll_wait来实现 获取准备就绪的句柄
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JNIEXPORT jint JNICALL
Java_sun_nio_ch_EPollArrayWrapper_epollWait(JNIEnv *env, jobject this,
                                            jlong address, jint numfds,
                                            jlong timeout, jint epfd)
{
    struct epoll_event *events = jlong_to_ptr(address);
    int res;

    if (timeout <= 0) {           /* Indefinite or no wait */
        RESTARTABLE(epoll_wait(epfd, events, numfds, timeout), res);
    } else {                      /* Bounded wait; bounded restarts */
        res = iepoll(epfd, events, numfds, timeout);
    }

    if (res < 0) {
        JNU_ThrowIOExceptionWithLastError(env, "epoll_wait failed");
    }
    return res;
}

epoll_wait函数负责检测事件

Selector.select()最终通过EPollArrayWrapper.poll()方法获取准备就绪的fd并添加到pollWrapper数组中，然后调用updateSelectedKeys()方法更新Selector持有的Map<Integer,SelectionKeyImpl> fdToKey中的
SelectionKeyImpl中事件状态

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private int updateSelectedKeys() {
    int entries = pollWrapper.updated;
    int numKeysUpdated = 0;
    for (int i=0; i<entries; i++) {
        int nextFD = pollWrapper.getDescriptor(i);
        SelectionKeyImpl ski = fdToKey.get(Integer.valueOf(nextFD));
        // ski is null in the case of an interrupt
        if (ski != null) {
            int rOps = pollWrapper.getEventOps(i);
            if (selectedKeys.contains(ski)) {
                if (ski.channel.translateAndSetReadyOps(rOps, ski)) {
                    numKeysUpdated++;
                }
            } else {
                ski.channel.translateAndSetReadyOps(rOps, ski);
                if ((ski.nioReadyOps() & ski.nioInterestOps()) != 0) {
                    selectedKeys.add(ski);
                    numKeysUpdated++;
                }
            }
        }
    }
    return numKeysUpdated;
}

从pollWrapper中拿到准备就绪的fd，根据Map<Integer,SelectionKeyImpl> fdToKey的映射关系查到对应的SelectionKeyImpl并更新其持有Channel中事件的状态SelectionKeyImpl.channel.translateAndSetReadyOps(rOps, ski)，
准备就绪的fd对应的SelectionKeyImpl会被放入SelectorImpl中的Set<SelectionKey> selectedKeys中，这样用户级别的API selector.selectedKes()就可以拿到准备就绪的fd 进行业务处理

ServerSocketChannel.register()

ServerSocketChannel.open()会初始化ServerSocketChannelImpl对象

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public final SelectionKey register(Selector sel, int ops,
                                       Object att)
    throws ClosedChannelException
{
    synchronized (regLock) {
        if (!isOpen())
            throw new ClosedChannelException();
        if ((ops & ~validOps()) != 0)
            throw new IllegalArgumentException();
        if (blocking)
            throw new IllegalBlockingModeException();
        SelectionKey k = findKey(sel);
        //如果channel和selector已经注册过  则直接添加感兴趣的事件和附件
        if (k != null) {
            k.interestOps(ops);
            k.attach(att);
        }
        //如果没有注册过 先通过selector.register注册
        if (k == null) {
            // New registration
            synchronized (keyLock) {
                if (!isOpen())
                    throw new ClosedChannelException();
                k = ((AbstractSelector)sel).register(this, ops, att);
                addKey(k);
            }
        }
        return k;
    }
}

接下来我们来看下selector.register的注册过程

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protected final SelectionKey register(AbstractSelectableChannel ch,
                                      int ops,
                                      Object attachment)
{
    if (!(ch instanceof SelChImpl))
        throw new IllegalSelectorException();
    SelectionKeyImpl k = new SelectionKeyImpl((SelChImpl)ch, this);
    k.attach(attachment);
    synchronized (publicKeys) {
        implRegister(k);
    }
    k.interestOps(ops);
    return k;
}

protected void implRegister(SelectionKeyImpl ski) {
    if (closed)
        throw new ClosedSelectorException();
    SelChImpl ch = ski.channel;
    int fd = Integer.valueOf(ch.getFDVal());
    fdToKey.put(fd, ski);
    pollWrapper.add(fd);
    keys.add(ski);
}

如果未注册过调用selector.register() 构建SelectionKey 注册感兴趣事件和attachment附件 并将新建的SelectionKey添加到pollWrapper的数组中

Selector.wakeup

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public Selector wakeup() {
    synchronized (interruptLock) {
        if (!interruptTriggered) {
            pollWrapper.interrupt();
            interruptTriggered = true;
        }
    }
    return this;
}
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//EPollArrayWrapper
public Selector wakeup() {
    synchronized (interruptLock) {
        if (!interruptTriggered) {
            pollWrapper.interrupt();
            interruptTriggered = true;
        }
    }
    return this;
}
public void interrupt() {
    interrupt(outgoingInterruptFD);
}
private static native void interrupt(int fd);

EPollArrayWrapper.c

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JNIEXPORT void JNICALL
Java_sun_nio_ch_EPollArrayWrapper_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");
    }
}

linux的poll实现

linux中有系统调用poll方法，定义如下：

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int poll (struct pollfd *fds, unsigned int nfds, int timeout);

上述pollfd结构体定义如下：

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typedef struct pollfd {
	int fd;
	short events;
	short revents;
} pollfd_t;

int fd：一个文件描述句柄，代表一个Channel连接
short events:该文件描述符感兴趣的事件，如POLLIN表示该文件描述符有读事件，POLLOUT表示该文件描述符可写。
short revents：代表该文件描述符当前已有的事件，如有读事件则值为POLLIN，有读写事件则为POLLIN和POLLOUT的并集
整体的意思就是：你指定了结构体列表的起始地址和要监控的结构体个数，linux系统就会为你在timeout时间内监控上述结构体列表中的文件描述符的相关事件，并把发生的事件写入到上述的short revents属性中。
所以我们在执行一次poll之后，要想获取所有的发生了事件的文件描述符，则需要遍历整个pollfd列表，依次判断上述的short revents是否不等于0，不等于0代表发生了事件。

  1. jdk的poll实现概述
    jdk要做的事情就是准备参数数据，然后去调用上述poll方法，这就要用到JNI来实现。jdk使用PollSelectorImpl来实现上述poll调用。
    3.1 pollfd参数
    jdk需要将java层面接收到的一个Channel连接映射到一个pollfd结构体，PollSelectorImpl针对此创建了一个AllocatedNativeObject 对象，该对象不是在堆中，它内部使用Unsafe类直接操作内存地址。它就是专门用来存放上述一个个pollfd结构体的内容，通过固定的offset来获取每个结构体的数据内容。
    所以在调用上述poll方法的时候，直接传递的是AllocatedNativeObject对象的内存地址
    注册Channel要做的事：其实就是将Channel的相关数据填充到上述AllocatedNativeObject的内存地址上，下次调用poll的时候，自然就会被监控
    取消Channel注册要做的事：其实就是从上述AllocatedNativeObject的内存地址上移除该Channel代表的pollfd结构体

  2. PollSelectorImpl代码分析
    PollSelectorImpl的创建过程有如下2个内容
    1 创建了pipe，得到读写文件描述符，并注册到了PollArrayWrapper中
    2 创建了PollArrayWrapper
    PollArrayWrapper pollWrapper：内部创建了一个上述介绍的AllocatedNativeObject对象（用于存放注册的Channel），而pollWrapper则更像是一个工具类，来方便的用户操作AllocatedNativeObject对象，pollWrapper把普通的操作都转化成对内存的操作
    我们知道PollSelectorImpl在select过程的阻塞时间受控于所注册的Channel的事件，一旦有事件才会进行返回，没有事件的话就一直阻塞，为了可以允许手动控制这种局面的话，就额外增加了一个监控，即对pipe的读监控。对pipe的读文件描述符即fd0注册到PollArrayWrapper中的第一个位置，如果我们对pipe的写文件描述符fd1进行写数据操作，则pipe的读文件描述符必然会收到读事件，即可以使PollSelectorImpl不再阻塞，立即返回。
    来看下初始化注册f0的代码

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void initInterrupt(int fd0, int fd1) {
    interruptFD = fd1;
    putDescriptor(0, fd0);
    putEventOps(0, POLLIN);
    putReventOps(0, 0);
}

即将fd0存放到PollArrayWrapper的AllocatedNativeObject中，并关注POLLIN即读事件。并将pipe的写文件描述符保存到interruptFD属性中
Selector对外提供了wakeup方法，来看下PollSelectorImpl的实现

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public void interrupt() {
    interrupt(interruptFD);
}
private static native void interrupt(int fd);

这里就是对上述pipe的写文件描述符执行interrupt操作，来看看底层实现代码是：

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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");
    }
}

这里就是简单的对pipe的写文件描述符写入数据用来触发pipe的读文件描述符的读事件而已。
至此，PollSelectorImpl的初始化过程就完成了。

  1. 注册和取消注册Channel过程
    注册Channel其实就是向PollSelectorImpl中的PollArrayWrapper存放该Channel的fd、关注的事件信息，来看下实现代码
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protected void implRegister(SelectionKeyImpl ski) {
    synchronized (closeLock) {
        if (closed)
            throw new ClosedSelectorException();

        // Check to see if the array is large enough
        if (channelArray.length == totalChannels) {
            // Make a larger array
            int newSize = pollWrapper.totalChannels * 2;
            SelectionKeyImpl temp[] = new SelectionKeyImpl[newSize];
            // Copy over
            for (int i=channelOffset; i<totalChannels; i++)
                temp[i] = channelArray[i];
            channelArray = temp;
            // Grow the NativeObject poll array
            pollWrapper.grow(newSize);
        }
        channelArray[totalChannels] = ski;
        ski.setIndex(totalChannels);
        pollWrapper.addEntry(ski.channel);
        totalChannels++;
        keys.add(ski);
    }
}

将channel存储到PollArrayWrapper中的AllocatedNativeObject中

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/**
 * Prepare another pollfd struct for use.
 */
void addEntry(SelChImpl sc) {
    putDescriptor(totalChannels, IOUtil.fdVal(sc.getFD()));
    putEventOps(totalChannels, 0);
    putReventOps(totalChannels, 0);
    totalChannels++;
}

存储的信息是：Channel的fd，关注的事件（初始是0）而channel的关注事件是后来才设置到PollArrayWrapper的AllocatedNativeObject中的

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void putEventOps(int i, int event) {
    int offset = SIZE_POLLFD * i + EVENT_OFFSET;
    pollArray.putShort(offset, (short)event);
}

不同的Selector实现，上述实现过程也是不一样的。
再来看看取消注册Channel

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protected void implDereg(SelectionKeyImpl ski) throws IOException {
    // Algorithm: Copy the sc from the end of the list and put it into
    // the location of the sc to be removed (since order doesn't
    // matter). Decrement the sc count. Update the index of the sc
    // that is moved.
    int i = ski.getIndex();
    assert (i >= 0);
    if (i != totalChannels - 1) {
        // Copy end one over it
        SelectionKeyImpl endChannel = channelArray[totalChannels-1];
        channelArray[i] = endChannel;
        endChannel.setIndex(i);
        pollWrapper.release(i);
        PollArrayWrapper.replaceEntry(pollWrapper, totalChannels - 1,
                                      pollWrapper, i);
    } else {
        pollWrapper.release(i);
    }
    // Destroy the last one
    channelArray[totalChannels-1] = null;
    totalChannels--;
    pollWrapper.totalChannels--;
    ski.setIndex(-1);
    // Remove the key from keys and selectedKeys
    keys.remove(ski);
    selectedKeys.remove(ski);
    deregister((AbstractSelectionKey)ski);
    SelectableChannel selch = ski.channel();
    if (!selch.isOpen() && !selch.isRegistered())
        ((SelChImpl)selch).kill();
}

其实就是将最后一个直接覆盖到要删除的那个，以及更新相关数据的变化。

  1. doSelect实现过程
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protected int doSelect(long timeout)
    throws IOException
{
    if (channelArray == null)
        throw new ClosedSelectorException();
    processDeregisterQueue();
    try {
        begin();
        pollWrapper.poll(totalChannels, 0, timeout);
    } finally {
        end();
    }
    processDeregisterQueue();
    int numKeysUpdated = updateSelectedKeys();
    if (pollWrapper.getReventOps(0) != 0) {
        // Clear the wakeup pipe
        pollWrapper.putReventOps(0, 0);
        synchronized (interruptLock) {
            IOUtil.drain(fd0);
            interruptTriggered = false;
        }
    }
    return numKeysUpdated;
}

第一步：就是处理那些取消了的Channel,即遍历Selector的Set cancelledKeys，依次调用他们的取消注册和其他逻辑
第二步：就是使用pollWrapper执行poll过程，该过程即是准备好参数，然后调用linux的系统调用poll方法，如下

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int poll(int numfds, int offset, long timeout) {
    return poll0(pollArrayAddress + (offset * SIZE_POLLFD),
                 numfds, timeout);
}
private native int poll0(long pollAddress, int numfds, long timeout);

这里将AllocatedNativeObject的内存地址作为pollAddress，已注册的所有的Channel的数量作为numfds，timeout是用户传递的参数，然后就开始JNI调用
再看下native方法实现
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JNIEXPORT jint JNICALL
Java_sun_nio_ch_PollArrayWrapper_poll0(JNIEnv *env, jobject this,
                                       jlong address, jint numfds,
                                       jlong timeout)
{
    struct pollfd *a;
    int err = 0;

    a = (struct pollfd *) jlong_to_ptr(address);

    if (timeout <= 0) {           /* Indefinite or no wait */
        RESTARTABLE (poll(a, numfds, timeout), err);
    } else {                     /* Bounded wait; bounded restarts */
        err = ipoll(a, numfds, timeout);
    }

    if (err < 0) {
        JNU_ThrowIOExceptionWithLastError(env, "Poll failed");
    }
    return (jint)err;
}

先将内存地址作为address转换成pollfd结构体地址，然后调用ipoll，在ipoll中我们就会见到linux的系统调用poll

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static int
ipoll(struct pollfd fds[], unsigned int nfds, int timeout)
{
    jlong start, now;
    int remaining = timeout;
    struct timeval t;
    int diff;

    gettimeofday(&t, NULL);
    start = t.tv_sec * 1000 + t.tv_usec / 1000;

    for (;;) {
        int res = poll(fds, nfds, remaining);
        if (res < 0 && errno == EINTR) {
            if (remaining >= 0) {
                gettimeofday(&t, NULL);
                now = t.tv_sec * 1000 + t.tv_usec / 1000;
                diff = now - start;
                remaining -= diff;
                if (diff < 0 || remaining <= 0) {
                    return 0;
                }
                start = now;
            }
        } else {
            return res;
        }
    }
}

至此linux系统开始为上述所有的Channel进行监控事件。
在发生了事件之后，会有2次遍历所有注册的Channel集合：
一次就是在linux底层poll调用的时候会遍历，将产生的事件值存放到pollfd结构体的revents地址中
另一次就是在java层面，获取产生的事件时，会遍历上述每一个结构体，拿到revents地址中的数据

第三步：一旦第二步返回就说明有事件或者超时了，一旦有事件，则linux的poll调用会把产生的事件遍历的赋值到poll调用指定的地址上，即我们指定的一个个pollfd结构体，映射到java对象就是PollArrayWrapper的AllocatedNativeObject，这时候我们获取事件就是遍历底层的每一个地址，拿到pollfd结构体中的revents，如果revents不为0代表发生了事件，还要与Channel关注的事件进行相&操作，不为0代表发生了Channel关注的事件了，并清空pollfd结构体中的revents数据供下次使用，代码如下

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/**
 * Copy the information in the pollfd structs into the opss
 * of the corresponding Channels. Add the ready keys to the
 * ready queue.
 */
protected int updateSelectedKeys() {
    int numKeysUpdated = 0;
    // Skip zeroth entry; it is for interrupts only
    for (int i=channelOffset; i<totalChannels; i++) {
        int rOps = pollWrapper.getReventOps(i);
        if (rOps != 0) {
            SelectionKeyImpl sk = channelArray[i];
            pollWrapper.putReventOps(i, 0);//清理已有事件
            if (selectedKeys.contains(sk)) {
                if (sk.channel.translateAndSetReadyOps(rOps, sk)) {
                    numKeysUpdated++;
                }
            } else {
                sk.channel.translateAndSetReadyOps(rOps, sk);
                if ((sk.nioReadyOps() & sk.nioInterestOps()) != 0) {
                    selectedKeys.add(sk);
                    numKeysUpdated++;
                }
            }
        }
    }
    return numKeysUpdated;
}

这里就是通过指针操作直接获取对应底层结构体的revents数据。

第四步：上面提到了Selector也会注册一个fd用于监听，并且注册的位置时第一个即0，这里会取出该fd的发生事件，然后读取内容忽略掉即可，不然后仍然会触发该事件。代码如下

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static native boolean drain(int fd) throws IOException;
JNIEXPORT jboolean JNICALL
Java_sun_nio_ch_IOUtil_drain(JNIEnv *env, jclass cl, jint fd)
{
    char buf[128];
    int tn = 0;

    for (;;) {
        int n = read(fd, buf, sizeof(buf));
        tn += n;
        if ((n < 0) && (errno != EAGAIN))
            JNU_ThrowIOExceptionWithLastError(env, "Drain");
        if (n == (int)sizeof(buf))
            continue;
        return (tn > 0) ? JNI_TRUE : JNI_FALSE;
    }
}

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