客户端：
多个客户可以正常收发信息，因为可以同时发送和接受信息，不是发送完信息后等待
返回信息，所以要加入多线程

public class Client {public static void main(String[]args) throws UnknownHostException, IOException{ System.out.println("客户端启动中..."); Socket client =new Socket("localhost",9999); new Thread(new Send(client)).start(); new Thread(new Receive(client)).start();}}

客户端的发送端封装了：
发送消息
从键盘读取消息
run()
释放资源

public class Send implements Runnable{private BufferedReader br;private DataOutputStream dos;private Socket client;private boolean flag=true;public Send(Socket client){ this.client=client; br=new BufferedReader(new InputStreamReader(System.in)); try { dos=new DataOutputStream(client.getOutputStream()); } catch (IOException e) { this.release(); }}public void run(){ while(flag) { System.out.println("请输入消息:"); String msg=getstr(); send(msg); }}private void release(){ this.flag=false; utils.close(dos,client);}//从控制台获取消息private String getstr(){ try { String msg=br.readLine(); return msg; } catch (IOException e) { e.printStackTrace(); } return null;}private void send(String msg){ try { dos.writeUTF(msg); dos.flush(); } catch (IOException e) { release(); }}}

客户端的接收端封装了:
接收消息
run()
释放资源

public class Receive implements Runnable {private DataInputStream dis;private Socket client;private boolean flag=true;public Receive(Socket client){ this.client=client; try { dis=new DataInputStream(client.getInputStream()); } catch (IOException e) { release(); }}private String receive(){ String msg; try { msg = dis.readUTF(); return msg; } catch (IOException e) { release(); } return null;}public void run(){ while(flag) { String msg=receive(); System.out.println(msg); }}private void release(){ this.flag=false; utils.close(dis,client);}}

作为释放资源工具类

public class utils {public static void close(Closeable... target ){ for(Closeable it:target) { try { if(null!=it) { it.close(); } } catch (IOException e) { e.printStackTrace(); } } }}

在线聊天室
服务器：

public class Chat {public static void main(String[]args) throws IOException{ System.out.println("服务器启动中..."); ServerSocket server=new ServerSocket(9999); while(true) { Socket client =server.accept(); System.out.println("一个客户端建立了连接"); new Thread(new channel(client)).start(); }}static class channel implements Runnable{ private DataInputStream dis; private DataOutputStream dos; private BufferedReader br; private Socket client; private boolean flag; public channel(Socket client) { this.client=client; try { dis=new DataInputStream(client.getInputStream()); dos=new DataOutputStream(client.getOutputStream()); flag=true; } catch (IOException e) { release(); } } //接收消息 private String receive() { try { String msg=dis.readUTF(); return msg; } catch (IOException e) { release(); } return null; } //发送给消息 private void send(String msg) { try { dos.writeUTF(msg); dos.flush(); } catch (IOException e) { release(); } } private void release() { this.flag=false; utils.close(dis,dos,client); //写一个工具类,使用Closeable...可变参数 } public void run() { while(flag) { String msg=receive(); send(msg); } release(); }}}