简单的聊天程序，主要用到的是Socket

服务端：

 import java.io.*;

 import java.net.*;

 import java.util.*;

 public class ChatServer {

 boolean stat = false;

 ServerSocket ss = null;

 List<Client> clients = new ArrayList<Client>();//用于存客户端

 public static void main(String[] args) {

   new ChatServer().start();

 }

 public void start(){

   try {

    ss = new ServerSocket(8888);

    stat = true;

   } catch(BindException e){  //Sever端已经运行，当重复运行时抛异常

    System.out.println("端口正在使用中。。。。");

    System.out.println("请关掉相关程序并重新运行服务器！"); //还会抛别的异常，所以直接关闭窗口

    System.exit(0);

   } catch(IOException e) {

    e.printStackTrace();

   }

   try{

    while(stat){

     Socket s = ss.accept();

 System.out.println("a client connected!" );  //测试语句写在最左边，以后没用可以删除或注掉

     Client c = new Client(s);    //每建立一个客户端，就new一个客户端对象，启动一个线程

     new Thread(c).start();

     clients.add(c);  //勿忘写，将每个客户端加入到容器里

    }

   } catch (IOException e) {

    e.printStackTrace();

   } finally {

    try {

     ss.close();

    } catch (IOException e) {

     e.printStackTrace();

    }

   }

 }

 class Client implements Runnable {

   private Socket s;

   private DataInputStream dis;

   private DataOutputStream dos;

   private boolean cont = false;

   public Client(Socket s){

    this.s = s; 

    try {

     dis = new DataInputStream(s.getInputStream());//初始化

     dos = new DataOutputStream(s.getOutputStream());

     cont = true;

    } catch (IOException e) {

     e.printStackTrace();

    }

   }

   public void send(String str){  //用于发送给客户端

    try {

     dos.writeUTF(str);

    } catch (IOException e) {

     clients.remove(this);  //移除那个退出的对象

     System.out.println("一个客户退出了");

     //e.printStackTrace();

    }

   }

   public void run() {

    try{

     while(cont){

      String str = dis.readUTF(); //阻塞式方法

 System.out.println(str);

      for(int i=0; i<clients.size(); i++){

       Client c = clients.get(i);  //取客户端

       c.send(str); 

      }

      /*     另外两种方法，但不适用，它会锁定服务端

      for(Iterator<Client> it = clients.iterator(); it.hasNext();){

       Client c = it.next();

       c.send(str);

      }

      Iterator<Client> it = clients.iterator();

      while(it.hasNext()){

       Client c = it.next();

       c.send(str);

      }

      */

     }

    } catch (EOFException e){   //readUTF()阻塞式方法，所以关闭客户端会抛异常

     System.out.println("Client closed!");

    } catch (IOException e) {

     e.printStackTrace();

    } finally {

     try {

      if(dis != null) dis.close();

      if(dos != null) dos.close();

      if(s != null) {

       s.close();

       s = null;//更严格的方法，等于空就没人去用了，垃圾收集器就回收走

      }

     } catch (IOException e) {

      e.printStackTrace();

     }

    }

   }

 }

 }

客户端：

import java.awt.*;

import java.awt.event.*;

import java.io.*;

import java.net.*;

public class ChatClient extends Frame {

Socket s = null;

DataOutputStream dos = null;

DataInputStream dis = null;

private boolean cont = false;

TextField tfTxt = new TextField();

TextArea taContent = new TextArea();

Thread tRecv = new Thread(new RecvThread());

public static void main(String[] args) {

  new ChatClient().launchFrame();

}

public void launchFrame() {

  setLocation(400, 300);

  this.setSize(300, 300);

  add(tfTxt,BorderLayout.SOUTH);

  add(taContent,BorderLayout.NORTH);

  pack(); //包在一起，去掉中间空着的

  this.addWindowListener(new WindowAdapter(){

   public void windowClosing(WindowEvent e){

    disconnect();

    System.exit(0);

   }

  });

  tfTxt.addActionListener(new TfListent());

  setVisible(true);

  connect();

  tRecv.start();   //启动线程

}

public void connect(){

  try {

   s = new Socket("127.0.0.1",8888);//注意不要定义成Socket s,这就成了局部变量而不是成员变量了

System.out.println("connected!");

   dos = new DataOutputStream(s.getOutputStream());

   dis = new DataInputStream(s.getInputStream());

   cont = true;

  } catch (UnknownHostException e) {

   e.printStackTrace();

  } catch (IOException e) {

   e.printStackTrace();

  }

}

public void disconnect(){

  try {

   dos.close();

   dis.close();

   s.close();

  } catch (IOException e) {

   e.printStackTrace();

  }

  /*//无法解决readUTF阻塞式方法

  try {

   cont = false;  //关闭线程

   tRecv.join();  //合并线程，彻底让他停止

  } catch (InterruptedException  e) {

   e.printStackTrace();

  } finally {

   try {

    dos.close(); //线程停止之后才能关流，不然抛SocketException异常

    dis.close();

    s.close();

   } catch (IOException e) {

    e.printStackTrace();

   }

  }

  */

}

private class TfListent implements ActionListener {

  public void actionPerformed(ActionEvent e) {

   String str = tfTxt.getText().trim();

   tfTxt.setText("");

   try {

    dos.writeUTF(str);

    dos.flush();

   } catch (IOException e1) {

    e1.printStackTrace();

   }

  }

}

private class RecvThread implements Runnable{

  public void run() {

   try {

    while(cont){

     String str = dis.readUTF();

     taContent.setText(taContent.getText() + str + '\n');

    }

   } catch (SocketException e){

    System.out.println("退出了，bye!");

   } catch (IOException e) {  

    e.printStackTrace();

   }

  }

}

}

简单的聊天程序，主要用到的是Socket，线程以及流

如果没看懂可以去我博客

http://www.cnblogs.com/huidaoli/p/3252924.html