小弟用jbuilder5.0学习java网络编程，编了一个c/s程序， 
server用 
ServerSocket soc=new ServerSocket(8000); 
Socket server_socket=soc.accept(); 
DataInputStream in=new DataInputStream(server_socket.getInputStream()); 
DataOutputStream out=new DataOutputStream(server_socket.getOutputStream()); 
client端用 
Socket client_socket=new Socker("localhost",8000); 
DataInputStream in=new DataInputStream(client_socket.getInputStream()); 
DataOutputStream out=new DataOutputStream(client_socket.getOutputStream()); 
二者可以连接上，但是当client段调用out.writeUTF(string)， 
server端调用in.readUTF(string)时, 
为何只有当client端调用两次out.writeUTF(string)时，server端 
才能收到一次数据，而且时两次传输中前面的那一次的数据。 

这究竟是为什么？

按你的要求，小弟做了这个程序，发送两次，均送到。试一试把
import java.io.*;
import java.net.*;

public class TestTCP{
    public static void main(String args[]){
        Server s = new Server(8000);
        try{
            Thread.sleep(100);
        }catch(Exception e){
            e.printStackTrace();
        }
        Client c = new Client("localhost",8000);
        c.send("Hello");
        c.send("World!");
        s.print();
    }
}
class Server implements Runnable{
    private ServerSocket server;
    private Socket client;
    public Server(int port){
        try{
            System.out.println("constructing server...");
            server = new ServerSocket(port);
            Thread t = new Thread(this);
            t.start();
        }catch(Exception e){
            e.printStackTrace();
        }
        
    }
    public void print(){
        try{
            BufferedReader br = new BufferedReader
                (new InputStreamReader(client.getInputStream()));
            
            String str;
            str = br.readLine();
            while(str!=null){
                System.out.println("server get msg:""+str+""");
                str = br.readLine();
            }
        }catch(Exception e){
            e.printStackTrace();
        }        
    }
    public void run(){
        try{
            System.out.println("listening for client...");
            client = server.accept();
            }catch(Exception e){
            e.printStackTrace();
        }
    }
}
class Client{
    private Socket client;
    public Client(String host,int port){
        try{        
            System.out.println("Constructing client...");
            client = new Socket(host,port);
        }catch(Exception e){
            e.printStackTrace();
        }
        
    }
    public void send(String str){
        try{
            OutputStreamWriter osw = new OutputStreamWriter
                            (client.getOutputStream());
            System.out.println("Client sending msg:""+str+""");
            osw.write(str+"n");     
            osw.flush(); 
        }catch(Exception e){
            e.printStackTrace();
        }
    }
}