Java Socket例程2 Echo

EchoServer.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class EchoServer{
    public static void main(String args[]) throws IOException{
        ServerSocket serverSocket = null;
        PrintWriter out = null;
        BufferedReader in = null;
        try{
            serverSocket = new ServerSocket(1111);
        }
        catch(IOException e){
            System.err.println("Could not listen on port 1111.");
            System.exit(1);
        }
        
        Socket incoming = null;
        
        while(true){
            incoming = serverSocket.accept();
            out = new PrintWriter(incoming.getOutputStream());
            in = new BufferedReader(new InputStreamReader(incoming.getInputStream()));
            out.println("Hello!");
            out.println("Enter BYTE to exit");
            out.flush();
            while(true){
                String str = in.readLine();
                if(str == null){
                    break;
                }
                else{
                    out.println("Echo:"+str);
                    out.flush();
                    if(str.trim().equalsIgnoreCase("BYE"))
                        break;
                }
            }
            out.close();
            in.close();
            incoming.close();
            serverSocket.close();
        }    
    }
}

EchoClient.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;

public class EchoClient{
    public static void main(String args[]) throws IOException{
        Socket echoSocket = null;
        PrintWriter out = null;
        BufferedReader in = null;
        try {
            echoSocket = new Socket("localhost",1111);
            out = new PrintWriter(echoSocket.getOutputStream(),true);
            in  = new BufferedReader(new InputStreamReader(echoSocket.getInputStream()));
        }
        catch(UnknownHostException e){
            System.err.println("Don't know about host:localhost");
            System.exit(1);
        }
        System.out.println(in.readLine());
        System.out.print(in.readLine());
        BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
        String userInput;
        while((userInput = stdIn.readLine()) != null){
            out.println(userInput);
            System.out.println("Recv:"+in.readLine());
        }
        
        out.close();
        in.close();
        echoSocket.close();
    }
}