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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