NIO網絡編程中重復觸發讀（寫）事件

一、前言

　　公司最近要基於Netty構建一個TCP通訊框架， 因Netty是基於NIO的，為了更好的學習和使用Netty，特意去翻了之前記錄的NIO的資料，以及重新實現了一遍NIO的網絡通訊，不試不知道，一試發現好多細節沒注意，導致客戶端和服務端通訊的時候出現了一些非常莫名其妙的問題，這邊我記錄下耗了我一晚上的問題~

二、正文

　　廢話不多說，先上問題代碼~

　　服務端：

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("連接准備就緒");
                    ServerSocketChannel server = (ServerSocketChannel)key.channel();
                    System.out.println("等待客戶端連接中........................");
                    SocketChannel channel = server.accept();
                    channel.configureBlocking(false);
                    channel.register(selector,SelectionKey.OP_READ);
                }
                else if(key.isReadable()){
                    System.out.println("讀准備就緒，開始讀.......................");
                    SocketChannel channel = (SocketChannel)key.channel();
                    System.out.println("客戶端的數據如下：");
                    
                    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(("客戶端，你傳過來的數據是："+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);
    }
}

　　客戶端：

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("服務端返回的數據："+new String(bf.array()));
                    }
                }
                selector.selectedKeys().clear();
            }
        }
    }
}

　　服務端運行結果：

 

 　　客戶端運行結果：

　　這邊我們可以看到，客戶端輸出了兩次，筆者調試的時候發現，服務端只往客戶端寫過一次數據，但是客戶端卻打印了兩次數據，而且兩次的數據不一樣，挺詭異的！然后我就各種查，折磨了我一夜，今早一來，又想着怎么解決問題，不經意間發現了一篇文章：java nio使用的是水平觸發還是邊緣觸發？，文章中指出Nio的Selector.select()是“水平觸發”（也叫“條件觸發”），只要條件一直滿足，那么就會一直觸發，至此我如醍醐灌頂：是不是我通道里面的數據第一次沒有讀取干凈？導致客戶端觸發了多次讀取？后來驗證之后，發現確實是這個問題，讀者可以看我服務器端的代碼，我返回的是數據字節數是："服務端返回的數據：".length()+bf.array().length=26+1024，而客戶端只是將這個數據讀入1024大小的ByteBuffer中，還有26字節沒有讀取干凈，所以就觸發了第二次的讀事件！！！

　　既然問題找到了，現在就是要解決如何將通道內的數據讀取干凈了，修改之后的代碼如下， 特別注意紅色部分：

　　服務端：

　　

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("連接准備就緒");
                    ServerSocketChannel server = (ServerSocketChannel)key.channel();
                    System.out.println("等待客戶端連接中........................");
                    SocketChannel channel = server.accept();
                    channel.configureBlocking(false);
                    channel.register(selector,SelectionKey.OP_READ);
                }
                else if(key.isReadable()){
                    System.out.println("讀准備就緒，開始讀.......................");
                    SocketChannel channel = (SocketChannel)key.channel();
                    System.out.println("客戶端的數據如下：");
                    
                    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(("客戶端，你傳過來的數據是："+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);
    }
}

　　客戶端：

　　

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("服務端返回的數據："+bos.toString());
                    }
                }
                selector.selectedKeys().clear();
            }
        }
    }
}

　　客戶端的輸出：

　　 

三、參考鏈接

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

四、聯系本人

　　為方便沒有博客園賬號的讀者交流，特意建立一個企鵝群（純公益，非利益相關），讀者如果有對博文不明之處，歡迎加群交流：261746360，小杜比亞-博客園。