// 客户端TCPClient
import  java.io.*;
import  java.net.*;
import java.util.Scanner;
public class TCPClient {
       private  static  int  sort=3434;
       private  static  String host="localhost";  //本机ip地址
      public  static  void main(String avg[]){
          Scanner   input = new Scanner(System.in);
                 BufferedReader  br=null;
                 BufferedWriter  wr=null;
                 Socket          s =null;
            try{
             
                  
                  
            
                while(true){
                    System.out.print("客户端说:");
                     String  line = input.nextLine();
                    if(line.equals("byb"))  break;
                        s=new Socket(TCPClient.host,TCPClient.sort);
                    wr =new  BufferedWriter(new OutputStreamWriter( s.getOutputStream())); 
                     wr.write(line);
                     wr.close();
                    s.close();             //关闭会话
                     try {                   //暂停一会儿!
                        Thread.sleep(300);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                        s=new Socket(TCPClient.host,TCPClient.sort);
                    br =new  BufferedReader(new InputStreamReader( s.getInputStream()));
                    System.out.print("接收端:");
                    int   next=0;
                    StringBuffer li=new StringBuffer();
                    while((next=br.read()) != -1)  li.append((char)next);
                    System.out.println(new String(li.toString()));
                  
                    br.close();
                    s.close();            关闭会话!
                 }
                 
            }catch(UnknownHostException e){
                 System.out.println("服务器连接失败");
                 e.printStackTrace();
            }catch(IOException e){
                 e.printStackTrace();
            }
      }
}

------解决方案--------------------
 wr.write(line);后面加wr.flush();