Java instance - see if port is in use

The following example demonstrates how to detect whether a port is already in use:

Example:

import java.net.*;
import java.io.*;
 
public class Main {
   public static void main(String[] args) {
      Socket Skt;
      String host = "localhost";
      if (args.length > 0) {
         host = args[0];
      }
      for (int i = 0; i < 1024; i++) {
         try {
            System.out.println("查看 "+ i);
            Skt = new Socket(host, i);
            System.out.println("端口 " + i + " 已被使用");
         }
         catch (UnknownHostException e) {
            System.out.println("Exception occured"+ e);
            break;
         }
         catch (IOException e) {
         }
      }
   }
}

---

The output of the above code running is:

……
查看 17
查看 18
查看 19
查看 20
查看 21
端口 21 已被使用
查看 22
查看 23
查看 24
……

You can also specify the host's port:
Main.java file:

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.net.SocketAddress;
import java.net.SocketTimeoutException;
 
 
 
public class Main {
 
    public static void main(String[] args) {
        // 检测本地 80 端口
        log(isSocketAliveUitlitybyCrunchify("localhost", 80));
 
        // 检测本地 8080 端口
        log(isSocketAliveUitlitybyCrunchify("localhost", 8080));
 
        // 检测本地 8081 端口
        log(isSocketAliveUitlitybyCrunchify("localhost", 8081));
 
        // 检测 runoob.com 的 80 端口
        log(isSocketAliveUitlitybyCrunchify("runoob.com", 80));
 
         // 检测 runoob.com 的 443 端口
        log(isSocketAliveUitlitybyCrunchify("runoob.com", 443));
 
        // 检测 runoob.com 的 81 端口
        log(isSocketAliveUitlitybyCrunchify("runoob.com", 81));
    }
 
    /**
     * 判断主机端口
     * 
     * @param hostName
     * @param port
     * @return boolean - true/false
     */
    public static boolean isSocketAliveUitlitybyCrunchify(String hostName, int port) {
        boolean isAlive = false;
 
        // 创建一个套接字
        SocketAddress socketAddress = new InetSocketAddress(hostName, port);
        Socket socket = new Socket();
 
        // 超时设置，单位毫秒
        int timeout = 2000;
 
        log("hostName: " + hostName + ", port: " + port);
        try {
            socket.connect(socketAddress, timeout);
            socket.close();
            isAlive = true;
 
        } catch (SocketTimeoutException exception) {
            System.out.println("SocketTimeoutException " + hostName + ":" + port + ". " + exception.getMessage());
        } catch (IOException exception) {
            System.out.println(
                    "IOException - Unable to connect to " + hostName + ":" + port + ". " + exception.getMessage());
        }
        return isAlive;
    }
 
    private static void log(String string) {
        System.out.println(string);
    }
 
    private static void log(boolean isAlive) {
        System.out.println("是否真正在使用: " + isAlive + "\n");
    }
 
}

The output of the above code running is:

hostName: localhost, port: 80
IOException - Unable to connect to localhost:80. Connection refused
是否真正在使用: true

hostName: localhost, port: 8080
IOException - Unable to connect to localhost:8080. Connection refused
是否真正在使用: false

hostName: localhost, port: 8081
IOException - Unable to connect to localhost:8081. Connection refused
是否真正在使用: false

hostName: runoob.com, port: 80
是否真正在使用: true

hostName: runoob.com, port: 443
是否真正在使用: true

hostName: runoob.com, port: 81
SocketTimeoutException runoob.com:81. connect timed out
是否真正在使用: false