这篇文章介绍了NIO的基本概念：

Java NIO提供了与标准IO不同的IO工作方式： 

• • Channels and Buffers（通道和缓冲区）：标准的IO基于字节流和字符流进行操作的，而NIO是基于通道（Channel）和缓冲区（Buffer）进行操作，数据总是从通道读取到缓冲区中，或者从缓冲区写入到通道中。
  • Asynchronous IO（异步IO）：Java NIO可以让你异步的使用IO，例如：当线程从通道读取数据到缓冲区时，线程还是可以进行其他事情。当数据被写入到缓冲区时，线程可以继续处理它。从缓冲区写入通道也类似。
  • Selectors（选择器）：Java NIO引入了选择器的概念，选择器用于监听多个通道的事件（比如：连接打开，数据到达）。因此，单个的线程可以监听多个数据通道。

 

基于这篇文章 

写了一个NIO Server和 Client的代码。其中也有一些需要注意的地方。

 

首先是在代码里面有一些写就绪的情况，这种情况有一些特殊：

一般来说，你不应该注册写事件。写操作的就绪条件为底层缓冲区有空闲空间，而写缓冲区绝大部分时间都是有空闲空间的，所以当你注册写事件后，写操作一直是就绪的，选择处理线程全占用整个CPU资源。所以，只有当你确实有数据要写时再注册写操作，并在写完以后马上取消注册。

 

下面代码里面可以看到，有一个处理写就绪的函数，是使用了SelectionKey的attachment来处理，并且根据attachment是否仍有数据，来选择使用 interestOps与否。查了一些资料，可能是因为intestOps不会清空attachment吧，需要再研究。

 

下面是server的代码：

package com.myapp.nio;import org.apache.log4j.LogManager;import org.apache.log4j.Logger;import java.io.IOException;import java.net.InetSocketAddress;import java.nio.ByteBuffer;import java.nio.channels.SelectionKey;import java.nio.channels.Selector;import java.nio.channels.ServerSocketChannel;import java.nio.channels.SocketChannel;import java.util.Iterator;import java.util.Set;/** * Created by baidu on 16/11/17. */public class NioServer extends Thread{    private static final Logger logger = LogManager.getLogger(NioServer.class);    private InetSocketAddress inetSocketAddress;    private Handler handler = new ServerHandler();    public NioServer(String hostname, int port) {        inetSocketAddress = new InetSocketAddress(hostname, port);    }    // 用Override校验继承合法性    @Override    public void run() {        try {            Selector selector = Selector.open(); // 打开选择器            ServerSocketChannel serverSocketChannel = ServerSocketChannel.open(); // 打开通道            serverSocketChannel.configureBlocking(false); // 非阻塞            serverSocketChannel.socket().bind(inetSocketAddress);            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);            logger.info("Server: socket server stated on port " + inetSocketAddress.getPort());            while(true) {                int nKeys = selector.select();                if (nKeys > 0) {                    Set
     
       selectionKeySet = selector.selectedKeys();                    Iterator
      
        iterator = selectionKeySet.iterator();                    while (iterator.hasNext()) {                        SelectionKey selectionKey = iterator.next();                        // 以下两种写法是等价的                        //if ((selectionKey.readyOps() & SelectionKey.OP_ACCEPT) != 0) {
                            if (selectionKey.isAcceptable()) {                            logger.info("Server: accepted");                            handler.handleAccept(selectionKey);                        }                        //else if ((selectionKey.readyOps() & SelectionKey.OP_READ) != 0) {
                            else if (selectionKey.isReadable()) {                            logger.info("Server: readable");                            handler.handleRead(selectionKey);                        }                        //else if ((selectionKey.readyOps() & SelectionKey.OP_WRITE) != 0) {
                            else if (selectionKey.isWritable()) {                            logger.info("Server: writable");                            handler.handleWrite(selectionKey);                        }                        // Is below necessary?                        iterator.remove();                    }                }            }        } catch (IOException e) {            e.printStackTrace();        }    }    interface Handler {        void handleAccept(SelectionKey selectionKey) throws IOException;        void handleRead(SelectionKey selectionKey) throws IOException;        void handleWrite(SelectionKey selectionKey) throws IOException;    }    class ServerHandler implements Handler {        public void handleAccept(SelectionKey selectionKey) throws IOException {            ServerSocketChannel serverSocketChannel = (ServerSocketChannel) selectionKey.channel();            SocketChannel socketChannel = serverSocketChannel.accept();            logger.info("Server: accept client socket " + socketChannel);            socketChannel.configureBlocking(false);            socketChannel.register(selectionKey.selector(), SelectionKey.OP_READ);        }        public void handleRead(SelectionKey selectionKey) throws IOException {            ByteBuffer byteBuffer = ByteBuffer.allocate(512);            SocketChannel socketChannel = (SocketChannel) selectionKey.channel();            // 改了原文中的一个小问题，重复循环忙等的问题            while (true) {                int readBytes = socketChannel.read(byteBuffer);                if (readBytes > 0) {                    logger.info("Server: readBytes: " + readBytes + ", data: "                        + new String(byteBuffer.array(), 0, readBytes));                    // 这个flip是一定需要的, 会把limit和position重置,这样才能重新读到数据                    byteBuffer.flip();                    socketChannel.write(byteBuffer);                }                else {                    break;                }            }            socketChannel.close();        } // handle Write这一块，其实需要再多研究一下        public void handleWrite(SelectionKey selectionKey) throws IOException {            ByteBuffer byteBuffer = (ByteBuffer) selectionKey.attachment();            byteBuffer.flip();            SocketChannel socketChannel = (SocketChannel) selectionKey.channel();            socketChannel.write(byteBuffer);            if (byteBuffer.hasRemaining()) {                selectionKey.interestOps(SelectionKey.OP_READ);            }        }    }    public static void main(String[] args) {        String hostname = "localhost";        int port = 8106;        NioServer nioServer = new NioServer(hostname, port);        nioServer.start();    }}
      
     

 

然后启动之后，用telnet作为客户端来访问一下：

$ telnet 127.0.0.1 8106Trying 127.0.0.1...Connected to localhost.Escape character is '^]'.hihihihihihiConnection closed by foreign host.如果没有byteBuffer.flip() 这个函数，那么不会有字符串返回。

 

打包： Project Structure-> Artifacts-> + -> Create Module with dependacies -> extract to the target JAR -> MANIFEST.MF路径最后的src改成resources.

然后 build->build artifact，就能在out目录里面有一个jar包，运行 java -jar xxx.jar

 

然后开始写客户端的代码：

package com.myapp.nio;import org.apache.log4j.LogManager;import org.apache.log4j.Logger;import java.io.IOException;import java.net.InetSocketAddress;import java.nio.ByteBuffer;import java.nio.channels.SocketChannel;/** * Created by baidu on 16/11/17. */public class NioClient {    private static final Logger logger = LogManager.getLogger(NioClient.class);    private InetSocketAddress inetSocketAddress;    public NioClient(String hostname, int port) {        inetSocketAddress = new InetSocketAddress(hostname, port);    }    public void send(String requestData) {        try {            SocketChannel socketChannel = SocketChannel.open(inetSocketAddress);            socketChannel.configureBlocking(false);            ByteBuffer byteBuffer = ByteBuffer.allocate(512);            socketChannel.write(ByteBuffer.wrap(requestData.getBytes()));            while(true) {                byteBuffer.clear();                int readBytes = socketChannel.read(byteBuffer);                if (readBytes > 0) {                    byteBuffer.flip();                    String getStr = new String(byteBuffer.array(), 0, readBytes);                    logger.info("Client: bytes: " + readBytes +                        "data: " + getStr);                    System.out.printf("Get return str: %s", getStr);                    socketChannel.close();                    break;                }            }        } catch (IOException e) {            e.printStackTrace();        }    }    public static void main(String[] args) {        String hostname = "localhost";        int port = 8106;        String requestData = "HIHIHI here~~~";        new NioClient(hostname, port).send(requestData);    }}

启动运行之后，命令行会返回：

log4j:WARN No appenders could be found for logger (com.myapp.nio.NioClient).log4j:WARN Please initialize the log4j system properly.log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.Get return str: HIHIHI here~~~Process finished with exit code 0

开始没有加log4j的properties，所以没有写日志，要加上。

log4j.properties

#log4j.rootLogger=INFO,Console,Filelog4j.rootLogger=INFO,File#控制台日志log4j.appender.Console=org.apache.log4j.ConsoleAppenderlog4j.appender.Console.Target=System.outlog4j.appender.Console.layout=org.apache.log4j.PatternLayoutlog4j.appender.Console.layout.ConversionPattern=[%p][%t][%d{yyyy-MM-dd HH\:mm\:ss}][%C] - %m%n#普通文件日志log4j.appender.File=org.apache.log4j.RollingFileAppenderlog4j.appender.File.File=logs/nio_client.loglog4j.appender.File.MaxFileSize=10MB#输出日志，如果换成DEBUG表示输出DEBUG以上级别日志log4j.appender.File.Threshold=ALLlog4j.appender.File.layout=org.apache.log4j.PatternLayoutlog4j.appender.File.layout.ConversionPattern=[%p][%t][%d{yyyy-MM-dd HH\:mm\:ss}][%C] - %m%nlog4j.appender.File.encoding=UTF-8

pom.xml也加上依赖：

     
         
      
       4.0.0
          
      
       com.myapp.nio
          
      
       nioClient
          
      
       1.0-SNAPSHOT
          
              
               
       
        1.2.17
           
          
              
               
                   
        
         log4j
                    
        
         log4j
                    
        
         ${log4j.version}
                
           
      
     

 

下次再写个线程池来处理看看。

（完）