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实现思路
首先需要知道java里如何创建一个Socket服务器端。

    //创建一个服务器端对象ServerSocket server = new ServerSocket(); //绑定启动的ip和端口号server.bind(new InetSocketAddress("127.0.0.1",8082));//启动成功后，调用accept()方法阻塞，//当有客户端成功连接时会生成一个Socket对象用于通讯Socket socket = server.accept();

提示：注意server.accept()方法调用会阻塞，只有新的客户端连接后才返回一个新的socket对象。如果一直未连接那么会一直处于阻塞状态

1、了解了如何创建一个socket服务器端后。那么如何实现给指定的连接客户端发送消息呢？首先我们可以知道只有有客户端连接服务器就会生成一个socket对象，socket对象中获取对应的输入输出流读取和写入就可以实现发送消息；也就是只要咱们把每次连接的socket保存起来，发送和接收数据就直接获取对应的socket就可以实现。
2、总结一下。也就是需要循环调用 server.accept()阻塞方法，对每个连接的socket保存起来。并对每一个socket实现读取写入操作。为了可用性。对于服务器阻塞方法需要单独创建一个线程进行阻塞等待连接操作。并且对于每一个连接的客户端单独创建一个线程进行读写操作。
好了话不多说直接上代码

项目源码
服务器端接口ISocketServer

/*** @description: 启用socket服务* @author * @date 2024/2/19 13:51* @version 1.0*/
public interface ISocketServer {/*** @description: 启动服务* @author liangxuelong* @date 2023/6/19 13:53* @version 1.0*/public void startServer();/*** @description: 停止服务* @author liangxuelong* @date 2023/6/19 13:53* @version 1.0*/public void stopServer();/*** @description: 判断是否启动* @author liangxuelong* @date 2023/6/19 14:01* @version 1.0*/public boolean isStart();}

服务器端实现一个抽象类AbstractSocketServer，定义启动、停止。查看连接状态等方法

/*** @author ** @version 1.0* @description:* @date 2023/6/19 14:04*/
public abstract class AbstractSocketServer implements ISocketServer{//服务器端口protected int port;//默认ip地址protected String ipAddress = "127.0.0.1";//默认最大连接数protected int maxConnectSize = 1;//当前连接状态protected boolean isConn = false;//java ServerSocket 对象private ServerSocket server;//保存所有连接通讯类protected ConcurrentHashMap<String,ICommunication> communications = new ConcurrentHashMap<>();public AbstractSocketServer(String ipAddress, int port) {this.ipAddress = ipAddress;this.port = port;}public AbstractSocketServer(int port) {this.port = port;}/*** @description: 启动服务* @author liangxuelong* @date 2023/6/19 16:14* @version 1.0*/@Overridepublic void startServer() {new Thread(() -> {try {if (isConn) {stopServer();}server = new ServerSocket();//绑定数据连接地址端口号server.bind(new InetSocketAddress(ipAddress,port));//绑定成功设置当前服务器状态为trueisConn = true;//循环等待客户端连接while(true){//阻塞 等待socket client 连接Socket socket = server.accept();//生成连接通讯对象SocketCommunication socketCommunication = new SocketCommunication(socket,this);new Thread(() -> {try {addCommunication(socketCommunication);socketCommunication.handle();} catch (Exception e) {e.printStackTrace();}removeCommunication(socket.getInetAddress().getHostAddress());}).start();}} catch (IOException ioException) {ioException.printStackTrace();}}).start();}/*** @description: 停止服务* @author liangxuelong* @date 2023/6/19 16:14* @version 1.0*/@Overridepublic void stopServer(){if (server != null) {try {server.close();} catch (IOException ioException) {ioException.printStackTrace();}isConn = false;removeAllCommunication();}}/*** @description: 给所有连接发送* @author liangxuelong* @date 2023/6/19 16:51* @version 1.0*/public void sendToALl(byte[] bytes){for (String ipAddress : communications.keySet()) {send(ipAddress,bytes);}}/*** @description: 给某个ip发送* @author liangxuelong* @date 2023/6/19 16:53* @version 1.0*/public boolean send(String ip,byte[] bytes) {if (communications.get(ip) == null)return false;try {communications.get(ip).send(bytes);return true;} catch (Exception e) {e.printStackTrace();return false;}}/*** @description: 当前是否启动 socket 服务端* @author liangxuelong* @date 2023/6/19 16:14* @version 1.0*/@Overridepublic boolean isStart() {return isConn;}/*** @description: 设置最大连接数量* @author * @date 2023/6/19 16:19* @version 1.0*/public void setMaxConnectSize(int maxConnectSize) {this.maxConnectSize = maxConnectSize;}/*** @description: 获取当前连接数据* @param:* @return: void* @author liangxuelong* @date: 2023/6/19*/public int getConnectSize(){return communications.size();}/*** @description: 获取当前所有连接的Ip地址* @param:* @return: void* @author liangxuelong* @date: 2023/6/19*/public Set<String> getConnectIpAddress(){return communications.keySet();}/*** @description: 添加连接对象* @author liangxuelong* @date 2023/6/19 15:14* @version 1.0*/protected void addCommunication(ICommunication communication) throws Exception {Socket socket = communication.getSocket();//判断是否超出最大连接数量，超出后断开连接if (maxConnectSize > communications.size()) {ICommunication iCommunication = communications.get(socket.getInetAddress().getHostAddress());if (iCommunication != null) {try {iCommunication.disconnect();} catch (Exception e) {e.printStackTrace();}communications.remove(socket.getInetAddress().getHostAddress());}communications.put(socket.getInetAddress().getHostAddress(),communication);} else {socket.close();throw new ConnectException(maxConnectSize);}}/*** @description: 移除连接对象* @author liangxuelong* @date 2023/6/19 15:14* @version 1.0*/public void removeCommunication(String ip){ICommunication iCommunication = communications.get(ip);if (iCommunication != null) {try {iCommunication.disconnect();} catch (Exception e) {e.printStackTrace();}communications.remove(ip);}}/*** @description: 移除所有连接对象* @author * @date 2023/6/19 15:14* @version 1.0*/public void removeAllCommunication(){for (String ipAddress : communications.keySet()) {removeCommunication(ipAddress);}}}

服务器端实现抽象类创建SocketServer类

/*** @author * @version 1.0* @description: socket 服务启动类* @date 2023/6/19 15:53*/
public class SocketServer extends AbstractSocketServer {public SocketServer(String ipAddress, int port) {super(ipAddress, port);}public SocketServer(int port) {super(port);}
}

客户端通讯接口ICommunication主要用于读写socket交互数据

public interface ICommunication {/*** @description: 获取当前连接的socket对象* @author * @date 2023/6/19 14:28* @version 1.0*/public Socket getSocket();/*** @description: socket创建成功后读取数据* @author * @date 2023/6/19 14:29* @version 1.0*/public void handle() throws Exception;/*** @description: 将读取到的数据统一处理* @author * @date 2023/6/19 14:29* @version 1.0*/public void receive(Socket socket,byte[] data) throws Exception;/*** @description: 发送数据* @author * @date 2023/6/19 14:41* @version 1.0*/public void send(byte[] data) throws Exception;/*** @description: 断开连接* @author liangxuelong* @date 2023/6/19 14:42* @version 1.0*/public void disconnect() throws Exception;
}

客户端实现通讯接口ICommunication创建SocketCommunication对象用于读写处理接收发送和发送数据

/*** @author * @version 1.0* @description: socket 连接对象* 主要用于对已连接的客户端收发消息* @date 2023/6/19 14:43*/
public class SocketCommunication implements ICommunication{private Socket socket; //已经连接的客户端对象private AbstractSocketServer socketServer; //来自哪个服务器，创建的服务器对象public SocketCommunication(@NotNull Socket socket, AbstractSocketServer socketServer) {this.socket = socket;this.socketServer = socketServer;}@Overridepublic Socket getSocket() {return socket;}@Overridepublic void handle() throws Exception {try {System.out.println(socket.getInetAddress().getHostAddress()+" 已连接");InputStream in = socket.getInputStream();ByteArrayOutputStream output = new ByteArrayOutputStream();byte[] b = new byte[1024];int len;while ( (len = in.read(b)) != -1) {output.write(b, 0, len);try {Thread.sleep(10);} catch (InterruptedException e1) {e1.printStackTrace();}if(in.available() == 0) {this.receive(socket,output.toByteArray());output.reset();}b = new byte[1024];len = 0;}} catch (IOException ioException) {throw ioException;}}/*** @description: 接收数据，对客户端接收的数据进行统一处理* 可以编写相应的处理逻辑，我这里是服务器端收到消息后。回复当前连接数量、* @author * @date 2023/6/19 17:30* @version 1.0*/@Overridepublic void receive(Socket socket, byte[] data) throws Exception {//服务器接收客户端消息System.out.println(socket.getInetAddress().getHostAddress()+" 发送："+new String(data,"gbk"));//服务器回复客户端消息String msg = "当前连接数量："+socketServer.getConnectSize();socketServer.send(socket.getInetAddress().getHostAddress(),msg.getBytes("gbk"));}/*** @description: 发送数据* @author * @date 2023/6/19 17:30* @version 1.0*/@Overridepublic void send(byte[] data) throws Exception {socket.getOutputStream().write(data);}/*** @description: 断开连接* @author * @date 2023/6/19 17:30* @version 1.0*/@Overridepublic void disconnect() throws Exception {System.out.println(socket.getInetAddress().getHostAddress()+" 已断开");socket.close();}}

定义一个自定义异常ConnectException处理连接数超出最大限制

/*** @author * @version 1.0* @description:* @date 2023/6/19 16:25*/
public class ConnectException extends Exception{public ConnectException(int maxSize) {super("连接数量超出最大限制，连接失败！ 当前最大连接数："+maxSize);}
}

最后定义main方法进行测试

/*** @author * @version 1.0* @description: TODO* @date 2024/2/9 15:57*/
public class Main {public static void main(String[] args) {SocketServer server = new SocketServer("192.168.0.100",5032);server.setMaxConnectSize(2); //设置最大连接数量server.startServer();//每5秒钟获取一下当前连接，并发送数据new Thread(() -> {while (true) {//获取所有连接for (String ip : server.getConnectIpAddress()) {try {System.out.println("当前已连接ip:"+ip);String msg = "向指定客户端发送消息: "+ip+" 你好";//向指定ip发送消息if (server.send(ip,msg.getBytes("gbk"))) {System.out.println("向 "+ip+" 发送了 ["+msg+"]");}} catch (UnsupportedEncodingException e) {e.printStackTrace();}}try {Thread.sleep(5000);} catch (InterruptedException e) {e.printStackTrace();}}}).start();}
}