这里主要围绕着Java NIO展开，从Java NIO的基本使用，到介绍Linux下NIO API，再到Java Selector其底层的实现原理。

    Java NIO基本使用

    Linux下的NIO系统调用介绍

    Selector原理

    Channel和Buffer之间的堆外内存

Java NIO基本使用

从JDK NIO文档里面可以发现，Java将其划分成了三大块：Channel，Buffer以及多路复用Selector。Channel的存在，封装了对什么实体的连接通道（如网络/文件）；Buffer封装了对数据的缓冲存储，最后对于Selector则是提供了一种可以以单线程非阻塞的方式，来处理多个连接。

基本应用示例

NIO的基本步骤是，创建Selector和ServerSocketChannel，然后注册channel的ACCEPT事件，调用select方法，等待连接的到来，以及接收连接后将其注册到Selector中。下面的为Echo Server的示例：

public class SelectorDemo {

 public static void main(String[] args) throws IOException {

        Selector selector = Selector.open();

        ServerSocketChannel socketChannel = ServerSocketChannel.open();

        socketChannel.bind(new InetSocketAddress(8080));

        socketChannel.configureBlocking(false);

        socketChannel.register(selector, SelectionKey.OP_ACCEPT);

        while (true) {

            int ready = selector.select();

            if (ready == 0) {

                continue;

            } else if (ready < 0) {

                break;

            }

            Set keys = selector.selectedKeys();

            Iterator iterator = keys.iterator();

            while (iterator.hasNext()) {

                SelectionKey key = iterator.next();

                if (key.isAcceptable()) {

                    ServerSocketChannel channel = (ServerSocketChannel) key.channel();

                    SocketChannel accept = channel.accept();

                    if (accept == null) {

                        continue;

                    }

                    accept.configureBlocking(false);

                    accept.register(selector, SelectionKey.OP_READ);

                } else if (key.isReadable()) {

                    // 读事件

                    deal((SocketChannel) key.channel(), key);

                } else if (key.isWritable()) {

                    // 写事件

                    resp((SocketChannel) key.channel(), key);

                }

                // 注：处理完成后要从中移除掉

                iterator.remove();

            }

        }

        selector.close();

        socketChannel.close();

    }

private static void deal(SocketChannel channel, SelectionKey key) throws IOException {

        ByteBuffer buffer = ByteBuffer.allocate(1024);

        ByteBuffer responseBuffer = ByteBuffer.allocate(1024);

        int read = channel.read(buffer);

        if (read > 0) {

            buffer.flip();

            responseBuffer.put(buffer);

        } else if (read == -1) {

            System.out.println("socket close");

            channel.close();

            return;

        }

        key.interestOps(SelectionKey.OP_READ | SelectionKey.OP_WRITE);

        key.attach(responseBuffer);

    }

 private static void resp(SocketChannel channel, SelectionKey key) throws IOException {

        ByteBuffer buffer = (ByteBuffer) key.attachment();

        buffer.flip();

        channel.write(buffer);

        if (!buffer.hasRemaining()) {

            key.attach(null);

            key.interestOps(SelectionKey.OP_READ);

        }

    }

}

Linux下的NIO系统调用介绍

在Linux环境下，提供了几种方式可以实现NIO，如epoll，poll，select等。对于select/poll，每次调用，都是从外部传入FD和监听事件，这就导致每次调用的时候，都需要将这些数据从用户态复制到内核态，就导致了每次调用代价比较大，而且每次从select/poll返回回来，都是全量的数据，需要自行去遍历检查哪些是READY的。对于epoll，则为增量式的，系统内部维护了所需要的FD和监听事件，要注册的时候，调用epoll_ctl即可，而每次调用，不再需要传入了，返回的时候，只返回READY的监听事件和FD。下面作个简单的伪代码：

// 1. 创建server socket

// 2. 绑定地址

// 3. 监听端口

// 4. 创建epoll

int epollFd = epoll_create(1024);

// 5. 注册监听事件

struct epoll_event event;

event.events = EPOLLIN | EPOLLRDHUP | EPOLLET;

event.data.fd = serverFd;

epoll_ctl(epollFd, EPOLL_CTL_ADD, serverFd, &event);

while(true) {

    readyNums = epoll_wait( epollFd, events, 1024, -1 );

    if ( readyNums < 0 )

     {

         printf("epoll_wait error\n");

         exit(-1);

     }

     for ( i = 0; i <  readyNums; ++i)

     {

         if ( events[i].data.fd == serverFd )

         {

             clientFd = accept( serverFd, NULL, NULL );

             // 注册监听事件

             ...

         }else if ( events[i].events & EPOLLIN )

         {

            // 处理读事件

         }else if ( events[i].events & EPOLLRDHUP )