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：

1package com.nio.server;
2
3import java.net.InetSocketAddress;
4import java.nio.ByteBuffer;
5import java.nio.channels.SelectionKey;
6import java.nio.channels.Selector;
7import java.nio.channels.ServerSocketChannel;
8import java.nio.channels.SocketChannel;
9import java.util.Iterator;
10
11public class NIOServer {
12    private static Selector selector;
13    private static ServerSocketChannel serverSocketChannel;
14    private static ByteBuffer bf = ByteBuffer.allocate(1024);
15    public static void main(String[] args) throws Exception{
16        init();
17        while(true){
18            selector.select();
19            Iterator<SelectionKey> it = selector.selectedKeys().iterator();
20            while(it.hasNext()){
21                SelectionKey key = it.next();
22                if(key.isAcceptable()){
23                    System.out.println("连接准备就绪");
24                    ServerSocketChannel server = (ServerSocketChannel)key.channel();
25                    System.out.println("等待客户端连接中........................");
26                    SocketChannel channel = server.accept();
27                    channel.configureBlocking(false);
28                    channel.register(selector,SelectionKey.OP_READ);
29                }
30                else if(key.isReadable()){
31                    System.out.println("读准备就绪，开始读.......................");
32                    SocketChannel channel = (SocketChannel)key.channel();
33                    System.out.println("客户端的数据如下：");
34                    
35                    int readLen = 0;
36                    bf.clear();
37                    StringBuffer sb = new StringBuffer();
38                    while((readLen=channel.read(bf))>0){
39                        sb.append(new String(bf.array()));
40                        bf.clear();
41                    }
42                    if(-1==readLen){
43                        channel.close();
44                    }
45                    channel.write(ByteBuffer.wrap(("客户端，你传过来的数据是："+sb.toString()).getBytes()));
46                }
47                it.remove();
48            }
49        }
50    }
51    private static void init() throws Exception{
52        selector = Selector.open();
53        serverSocketChannel = ServerSocketChannel.open();
54        serverSocketChannel.configureBlocking(false);
55        serverSocketChannel.socket().bind(new InetSocketAddress(8080));
56        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
57    }
58}

　　客户端：

1package com.nio.client;
2
3import java.net.InetSocketAddress;
4import java.nio.ByteBuffer;
5import java.nio.channels.SelectionKey;
6import java.nio.channels.Selector;
7import java.nio.channels.SocketChannel;
8import java.util.Iterator;
9
10public class NIOClient {
11    private static Selector selector;
12    public static void main(String[]args) throws Exception{
13        selector = Selector.open();
14        SocketChannel sc = SocketChannel.open();
15        sc.configureBlocking(false);
16        sc.connect(new InetSocketAddress("127.0.0.1",8080));
17        sc.register(selector,SelectionKey.OP_READ);
18        
19        ByteBuffer bf = ByteBuffer.allocate(1024);
20        bf.put("Hi,server,i'm client".getBytes());
21        
22        
23        if(sc.finishConnect()){
24            bf.flip();
25            while(bf.hasRemaining()){
26                sc.write(bf);
27            }
28            
29            while(true){
30                selector.select();
31                Iterator<SelectionKey> it = selector.selectedKeys().iterator();
32                while(it.hasNext()){
33                    SelectionKey key = it.next();
34                    
35                    if(key.isReadable()){
36                        bf.clear();
37                        SocketChannel othersc  = (SocketChannel)key.channel();
38                        othersc.read(bf);
39                        System.out.println("服务端返回的数据："+new String(bf.array()));
40                    }
41                }
42                selector.selectedKeys().clear();
43            }
44        }
45    }
46}

　　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：

1package com.nio.server;
2
3import java.net.InetSocketAddress;
4import java.nio.ByteBuffer;
5import java.nio.channels.SelectionKey;
6import java.nio.channels.Selector;
7import java.nio.channels.ServerSocketChannel;
8import java.nio.channels.SocketChannel;
9import java.util.Iterator;
10
11public class NIOServer {
12    private static Selector selector;
13    private static ServerSocketChannel serverSocketChannel;
14    private static ByteBuffer bf = ByteBuffer.allocate(1024);
15    public static void main(String[] args) throws Exception{
16        init();
17        while(true){
18            selector.select();
19            Iterator<SelectionKey> it = selector.selectedKeys().iterator();
20            while(it.hasNext()){
21                SelectionKey key = it.next();
22                if(key.isAcceptable()){
23                    System.out.println("连接准备就绪");
24                    ServerSocketChannel server = (ServerSocketChannel)key.channel();
25                    System.out.println("等待客户端连接中........................");
26                    SocketChannel channel = server.accept();
27                    channel.configureBlocking(false);
28                    channel.register(selector,SelectionKey.OP_READ);
29                }
30                else if(key.isReadable()){
31                    System.out.println("读准备就绪，开始读.......................");
32                    SocketChannel channel = (SocketChannel)key.channel();
33                    System.out.println("客户端的数据如下：");
34                    
35                    int readLen = 0;
36                    bf.clear();
37                    StringBuffer sb = new StringBuffer();
38                    while((readLen=channel.read(bf))>0){
39                        bf.flip();
40                        byte [] temp = new byte[readLen];
41                        bf.get(temp,0,readLen);
42                        sb.append(new String(temp));
43                        bf.clear();
44                    }
45                    if(-1==readLen){
46                        channel.close();
47                    }　　　　　　　　　　　　System.out.println(sb.toString());
48                    channel.write(ByteBuffer.wrap(("客户端，你传过来的数据是："+sb.toString()).getBytes()));
49                }
50                it.remove();
51            }
52        }
53    }
54    private static void init() throws Exception{
55        selector = Selector.open();
56        serverSocketChannel = ServerSocketChannel.open();
57        serverSocketChannel.configureBlocking(false);
58        serverSocketChannel.socket().bind(new InetSocketAddress(8080));
59        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
60    }
61}

　　客户端：

1package com.nio.client;
2
3import java.io.ByteArrayOutputStream;
4import java.net.InetSocketAddress;
5import java.nio.ByteBuffer;
6import java.nio.channels.SelectionKey;
7import java.nio.channels.Selector;
8import java.nio.channels.SocketChannel;
9import java.util.Iterator;
10
11public class NIOClient {
12    private static Selector selector;
13    public static void main(String[]args) throws Exception{
14        selector = Selector.open();
15        SocketChannel sc = SocketChannel.open();
16        sc.configureBlocking(false);
17        sc.connect(new InetSocketAddress("127.0.0.1",8080));
18        sc.register(selector,SelectionKey.OP_READ);
19        
20        ByteBuffer bf = ByteBuffer.allocate(1024);
21        bf.put("Hi,server,i'm client".getBytes());
22        
23        
24        if(sc.finishConnect()){
25            bf.flip();
26            while(bf.hasRemaining()){
27                sc.write(bf);
28            }
29            
30            while(true){
31                selector.select();
32                Iterator<SelectionKey> it = selector.selectedKeys().iterator();
33                while(it.hasNext()){
34                    SelectionKey key = it.next();
35                    
36                    
37                    if(key.isReadable()){
38                        ByteArrayOutputStream bos = new ByteArrayOutputStream();
39                        bf.clear();
40                        SocketChannel othersc  = (SocketChannel)key.channel();
41                        while(othersc.read(bf)>0){
42                            bf.flip();
43                            while(bf.hasRemaining()){
44                                bos.write(bf.get());
45                            }
46                            bf.clear();
47                        };
48                        System.out.println("服务端返回的数据："+bos.toString());
49                    }
50                }
51                selector.selectedKeys().clear();
52            }
53        }
54    }
55}

　　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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