先运行服务端，如果不先连接服务端，就不监听，那客户端不知道连接谁

服务端

 
 
import java.io.*;
import java.net.*;
import java.util.ArrayList;
public class QLFWD{
    public static ServerSocket server_socket;
    public static ArrayList<Socket> socketList=new ArrayList<Socket>();  
    public static void main(String []args){
        try{
            server_socket = new ServerSocket(5000);
            while(true){
                Socket socket = server_socket.accept();
                socketList.add(socket); //把sock对象加入sock集合
                ServerBO_Thread st=new ServerBO_Thread(socket,socketList); //初始化多线程
                st.start();//启动多线程
            }
        }catch(Exception ex){
            ex.printStackTrace();
        }finally{
            try{
                if(server_socket!=null){
                    server_socket.close();
                }
            }catch(Exception ex){
                ex.printStackTrace();
            }
        }
    }
    public void encryptWrite(String src,DataOutputStream output)throws IOException{
        //将一个字符串转化为字符数组
        //System.out.println(src);
        char[] char_arr = src.toCharArray();
        //加密操作
        for(int i = 0;i<char_arr.length;i++){
            output.writeChar(char_arr[i]+13);
        }
        //用作结束标志符
        output.writeChar(2333);
        output.flush();
    }
    //读取并解密
    public String readDecrypt(DataInputStream input)throws IOException{
        String rtn="";
        while(true){
            int char_src =input.readChar();
            if(char_src!=2333){
                rtn=rtn+(char)(char_src-13);
            }else{
                break;
            }
        }
        return rtn;
    }
}
class ServerBO_Thread extends Thread{
    Socket client = null;
    ArrayList<Socket> clients;
    ServerBO_Thread(Socket s,ArrayList<Socket> ss){//初始化
        client=s;
        clients=ss; 
    }
    public void run(){
        DataInputStream input = null;
        DataOutputStream output =null;
        try{
            input = new DataInputStream(client.getInputStream());
            QLFWD bo = new QLFWD();
            String receive=null;
            String send=null;
            while(true){//监视当前客户端有没有发来消息
                if(!client.isClosed()){
                    receive=bo.readDecrypt(input);
                    clients.trimToSize();
                    String[] param = receive.split("&");
                    if(")start".equals(param[1])){    //分析客户端发来的内容
                        send = param[0]+"进入聊天室";
                    }else{
                        send = param[0]+"说:    "+param[1];
                    }
                    if(!("3333".equals(param[1]))){//3333为退出聊天室信号
                        for(Socket socket:clients){ //遍历socke集合 
                            //把读取到的消息发送给各个客户端  
                            if(!socket.isClosed()){
                                output = new DataOutputStream(socket.getOutputStream());
                                bo.encryptWrite(send,output);
                            }
                        }  
                    }else{//如果有客户端退出
                        for(Socket socket:clients){ //遍历socke集合 
                                if(socket!=client){//告诉其他人此人退出聊天室
                                    if(!(socket.isClosed())){
                                        output = new DataOutputStream(socket.getOutputStream());
                                        bo.encryptWrite(param[0]+"已退出聊天室",output);
                                    }
                                }
                            }
                        output = new DataOutputStream(client.getOutputStream());
                        bo.encryptWrite("3333",output);//返回信号给要退出的客户端，然后关闭线程
                        client.close();
                        input.close();
                        output.close();
                    }
                }else{
                    break;
                }
            }
        }catch(Exception ex){
            ex.printStackTrace();
        }
    }
}

多次运行客户端增加用户

结果：