服务端代码如下：

package service;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.HashMap;
import java.util.Map;
public class Server implements Runnable {
Socket socket;
public Server(Socket socket) {
this.socket = socket;
}
public void run() {
BufferedReader bufferedreade = null;
PrintWriter printwrite = null;
Hasdsadlist hasd = new Hasdsadlist();
try {
// 读取用户输入的用户名，
bufferedreade = new BufferedReader(new InputStreamReader(socket.getInputStream()));
String name = bufferedreade.readLine();
printwrite = new PrintWriter(socket.getOutputStream()); // 发给客户
// 判断用户名是否重复
if (hasd.map.containsKey(name)) {
while (hasd.map.containsKey(name)) {
printwrite.println("用户名已存在，请重新输入用户名：");
printwrite.flush();
name = bufferedreade.readLine();
}
hasd.map.put(name, socket);
printwrite.println("欢迎登录"+name);
printwrite.flush();
} else {
printwrite.println("欢迎登录"+name);
printwrite.flush();
hasd.map.put(name, socket);
}
// 读取到的内容
String tem1;
while ((tem1 = bufferedreade.readLine()) != null) {
// 当接受到客户发端发的某个消息时，刷新列表显示所有的客户
if (tem1.equals("SX")) {
printwrite = new PrintWriter(socket.getOutputStream()); // 发给客户
for (Object key : hasd.map.keySet()) {
printwrite.println("所有的用户为：" + key);
printwrite.flush();
}
continue;
//判断输入的是否包含@
} else if(tem1.contains(Signal.signal)){
Socket values = hasd.map.get(tem1.substring(0, tem1.indexOf(Signal.signal))); // 通过key拿到value值
printwrite = new PrintWriter(values.getOutputStream());
printwrite.println("用户" + name + "发过来：" + (tem1.substring(tem1.indexOf(Signal.signal) + 1, tem1.length())));
printwrite.flush();
continue;
}else{
//如果不包含，发送给客户一句输入有误
printwrite.println("输入格式有误，请按照'用户+@+内容'格式输入");
printwrite.flush();
continue;
}

}

} catch (IOException e) {
e.printStackTrace();
} finally {
try {
bufferedreade.close();
} catch (IOException e1) {
e1.printStackTrace();
}
try {
socket.close();
} catch (IOException e) {
e.printStackTrace();
}
}

}
}

客户端代码如下：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;

import service.Hasdsadlist;

public class Clir2main {
public static void main(String[] args) {
Socket socket = null;
PrintWriter printwrite = null;
BufferedReader bufferedreade = null;
try {
socket = new Socket("127.0.0.1", 300);
bufferedreade = new BufferedReader(new InputStreamReader(socket.getInputStream()));
// 用户登录
Scanner sc = new Scanner(System.in);
System.out.println("输入用户名：");
String name = sc.next();
// 用户名写入到服务器
printwrite = new PrintWriter(socket.getOutputStream());
printwrite.println(name);
printwrite.flush();
// 用户读取服务器发来的判断用户名的内容
String temp;
while ((temp = bufferedreade.readLine()) != null) {
if (temp.contains("欢迎")) { // 说明用户名不重复 contains
System.out.println(temp); //直接打印，然后登录
break;
} else {
System.out.println(temp);
name = sc.next();
printwrite.println(name);
printwrite.flush();
}
}
// 用户的对话写到服务器
while (true) {
Clir1 cilr = new Clir1(socket);
Thread thred = new Thread(cilr);
thred.start(); //开启了写入服务端的线程
String temp1 = bufferedreade.readLine();
System.out.println(temp1);

while(true){ //while的作用，不会发送给其他用户
//如果读到输入有误，就直接打印出来，停止循环
if(temp1.contains("输入有误")){
System.out.println(temp1);
}
break;
}
}
} catch (IOException e) {
e.printStackTrace();
}
}
}

客户端 线程：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;
public class Clir1 implements Runnable {
PrintWriter printwrite = null;
BufferedReader bufferedreade = null;
private Socket socket;
public Clir1(Socket socket) {
this.socket = socket;
}
public void run() {
try {
Scanner sc = new Scanner(System.in);
printwrite = new PrintWriter(socket.getOutputStream());
while (true) {
String tem1 = sc.next();
printwrite.println(tem1);
printwrite.flush();

}
} catch (IOException e) {
e.printStackTrace();
}
}

}