telnet 连接测试
telnet 127.0.0.1 8000



NioEchoServer start,port=8000
client socketChannel java.nio.channels.SocketChannel[connected local=/127.0.0.1:8000 remote=/127.0.0.1:56357]
receiveData,len=3,data=a

receiveData,len=3,data=b

receiveData,len=3,data=c

收到的数据 带了 回车换行符 ，因此长度多了2


完整代码
https://gitee.com/dyyx/hellocode/blob/master/src/nio/echo/NioEchoServer.java


import java.net.InetSocketAddress;
import java.net.ServerSocket;
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.nio.charset.Charset;
import java.util.Iterator;

public class NioEchoServer{

	private static final Charset UTF8 = Charset.forName("UTF-8");
 
    public static void main(String[] args) throws Exception{
    	//-Dport=8000
    	int port = getInt(System.getProperty("port"));
    	if(port <= 0) {
    		port=8000;
    	}
    	
    	ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.configureBlocking(false);
        ServerSocket serverSocket = serverSocketChannel.socket();
        serverSocket.bind(new InetSocketAddress(port));
        System.out.println("NioEchoServer start,port="+port);

        Selector selector = Selector.open();
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
        
        while(selector.select()>0){

            Iterator< SelectionKey> iterator = selector.selectedKeys().iterator();
            while(iterator.hasNext()){
                SelectionKey selectionKey = iterator.next();
                iterator.remove();
                
                if(selectionKey.isAcceptable()){
                    ServerSocketChannel server = (ServerSocketChannel)selectionKey.channel();
                    SocketChannel socketChannel  = server.accept();
                    if(socketChannel==null){
                        continue;
                    }
                    socketChannel.configureBlocking(false);
                    socketChannel.register(selector, SelectionKey.OP_READ);
 
                    System.out.println("client socketChannel "+socketChannel);         
                }

                
                if(selectionKey.isReadable()){
                    SocketChannel socketChannel = (SocketChannel)selectionKey.channel();
                    ByteBuffer readBuffer = ByteBuffer.allocateDirect(1024);
                    socketChannel.read(readBuffer);
                    readBuffer.flip();

                    String receiveData= UTF8.decode(readBuffer).toString();
                    System.out.println("receiveData,len="+receiveData.length()+",data="+receiveData);

                    // selectionKey.attach(receiveData);
                    // 直接写 不用注册写事件
                    ByteBuffer writeBuffer = ByteBuffer.allocateDirect(1024);
                    writeBuffer.put(receiveData.getBytes(UTF8));
                    writeBuffer.flip();
                    while(writeBuffer.hasRemaining()){
                        socketChannel.write(writeBuffer);
                    }
                }

                if(selectionKey.isWritable()){
                    SocketChannel socketChannel = (SocketChannel)selectionKey.channel();
                    String message = (String) selectionKey.attachment();
                    if(message==null){
                        continue;
                    }
                    selectionKey.attach(null);
                    ByteBuffer writeBuffer = ByteBuffer.allocateDirect(1024);
                    writeBuffer.put(message.getBytes());
                    writeBuffer.flip();
                    while(writeBuffer.hasRemaining()){
                        socketChannel.write(writeBuffer);
                    }
                }
            }
        }
    }
    
    private static int getInt(String str) {
    	try {
    		return Integer.parseInt(str);
    	}catch(Throwable e) {
    		return 0;
    	}
    }    

}