NIO网络编程中重复触发读（写）事件

一、前言

　　公司最近要基于Netty构建一个TCP通讯框架，因Netty是基于NIO的，为了更好的学习和使用Netty，特意去翻了之前记录的NIO的资料，以及重新实现了一遍NIO的网络通讯，不试不知道，一试发现好多细节没注意，导致客户端和服务端通讯的时候出现了一些非常莫名其妙的问题，这边我记录下耗了我一晚上的问题~

二、正文

　　废话不多说，先上问题代码~

　　服务端：

package com.nio.server;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;public class NIOServer {private static Selector selector;private static ServerSocketChannel serverSocketChannel;private static ByteBuffer bf = ByteBuffer.allocate(1024);public static void main(String[] args) throws Exception{init();while(true){selector.select();Iterator<SelectionKey> it = selector.selectedKeys().iterator();while(it.hasNext()){SelectionKey key = it.next();if(key.isAcceptable()){System.out.println("连接准备就绪");ServerSocketChannel server = (ServerSocketChannel)key.channel();System.out.println("等待客户端连接中........................");SocketChannel channel = server.accept();channel.configureBlocking(false);channel.register(selector,SelectionKey.OP_READ);}else if(key.isReadable()){System.out.println("读准备就绪，开始读.......................");SocketChannel channel = (SocketChannel)key.channel();System.out.println("客户端的数据如下：");int readLen = 0;bf.clear();StringBuffer sb = new StringBuffer();while((readLen=channel.read(bf))>0){sb.append(new String(bf.array()));bf.clear();}if(-1==readLen){channel.close();}channel.write(ByteBuffer.wrap(("客户端，你传过来的数据是："+sb.toString()).getBytes()));}it.remove();}}}private static void init() throws Exception{selector = Selector.open();serverSocketChannel = ServerSocketChannel.open();serverSocketChannel.configureBlocking(false);serverSocketChannel.socket().bind(new InetSocketAddress(8080));serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);}
}

　　客户端：

package com.nio.client;import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;public class NIOClient {private static Selector selector;public static void main(String[]args) throws Exception{selector = Selector.open();SocketChannel sc = SocketChannel.open();sc.configureBlocking(false);sc.connect(new InetSocketAddress("127.0.0.1",8080));sc.register(selector,SelectionKey.OP_READ);ByteBuffer bf = ByteBuffer.allocate(1024);bf.put("Hi,server,i'm client".getBytes());if(sc.finishConnect()){bf.flip();while(bf.hasRemaining()){sc.write(bf);}while(true){selector.select();Iterator<SelectionKey> it = selector.selectedKeys().iterator();while(it.hasNext()){SelectionKey key = it.next();if(key.isReadable()){bf.clear();SocketChannel othersc  = (SocketChannel)key.channel();othersc.read(bf);System.out.println("服务端返回的数据："+new String(bf.array()));}}selector.selectedKeys().clear();}}}
}

　　服务端运行结果：

　　客户端运行结果：

　　这边我们可以看到，客户端输出了两次，笔者调试的时候发现，服务端只往客户端写过一次数据，但是客户端却打印了两次数据，而且两次的数据不一样，挺诡异的！然后我就各种查，折磨了我一夜，今早一来，又想着怎么解决问题，不经意间发现了一篇文章：java nio使用的是水平触发还是边缘触发？，文章中指出Nio的Selector.select()是“水平触发”（也叫“条件触发”），只要条件一直满足，那么就会一直触发，至此我如醍醐灌顶：是不是我通道里面的数据第一次没有读取干净？导致客户端触发了多次读取？后来验证之后，发现确实是这个问题，读者可以看我服务器端的代码，我返回的是数据字节数是："服务端返回的数据：".length()+bf.array().length=26+1024，而客户端只是将这个数据读入1024大小的ByteBuffer中，还有26字节没有读取干净，所以就触发了第二次的读事件！！！

　　既然问题找到了，现在就是要解决如何将通道内的数据读取干净了，修改之后的代码如下，特别注意红色部分：

　　服务端：

package com.nio.server;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;public class NIOServer {private static Selector selector;private static ServerSocketChannel serverSocketChannel;private static ByteBuffer bf = ByteBuffer.allocate(1024);public static void main(String[] args) throws Exception{init();while(true){selector.select();Iterator<SelectionKey> it = selector.selectedKeys().iterator();while(it.hasNext()){SelectionKey key = it.next();if(key.isAcceptable()){System.out.println("连接准备就绪");ServerSocketChannel server = (ServerSocketChannel)key.channel();System.out.println("等待客户端连接中........................");SocketChannel channel = server.accept();channel.configureBlocking(false);channel.register(selector,SelectionKey.OP_READ);}else if(key.isReadable()){System.out.println("读准备就绪，开始读.......................");SocketChannel channel = (SocketChannel)key.channel();System.out.println("客户端的数据如下：");int readLen = 0;bf.clear();StringBuffer sb = new StringBuffer();while((readLen=channel.read(bf))>0){  bf.flip();byte [] temp = new byte[readLen];bf.get(temp,0,readLen);sb.append(new String(temp));bf.clear();}if(-1==readLen){channel.close();}　　　　　　　　　　　　System.out.println(sb.toString());channel.write(ByteBuffer.wrap(("客户端，你传过来的数据是："+sb.toString()).getBytes()));}it.remove();}}}private static void init() throws Exception{selector = Selector.open();serverSocketChannel = ServerSocketChannel.open();serverSocketChannel.configureBlocking(false);serverSocketChannel.socket().bind(new InetSocketAddress(8080));serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);}
}

　　客户端：

package com.nio.client;import java.io.ByteArrayOutputStream;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;public class NIOClient {private static Selector selector;public static void main(String[]args) throws Exception{selector = Selector.open();SocketChannel sc = SocketChannel.open();sc.configureBlocking(false);sc.connect(new InetSocketAddress("127.0.0.1",8080));sc.register(selector,SelectionKey.OP_READ);ByteBuffer bf = ByteBuffer.allocate(1024);bf.put("Hi,server,i'm client".getBytes());if(sc.finishConnect()){bf.flip();while(bf.hasRemaining()){sc.write(bf);}while(true){selector.select();Iterator<SelectionKey> it = selector.selectedKeys().iterator();while(it.hasNext()){SelectionKey key = it.next();if(key.isReadable()){ByteArrayOutputStream bos = new ByteArrayOutputStream();bf.clear();SocketChannel othersc  = (SocketChannel)key.channel();while(othersc.read(bf)>0){bf.flip();while(bf.hasRemaining()){bos.write(bf.get());}bf.clear();};System.out.println("服务端返回的数据："+bos.toString());}}selector.selectedKeys().clear();}}}
}

　　客户端的输出：

三、参考链接

https://www.zhihu.com/question/22524908

四、联系本人

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转载于:https://www.cnblogs.com/xdouby/p/8942083.html