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:          https://www.zhihu.com/question/22524908四、联系本人  为方便没有博客园账号的读者交流,特意建立一个企鹅群(纯公益,非利益相关),读者如果有对博文不明之处,欢迎加群交流:261746360,小杜比亚-博客园。

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