Step by Step solution about How to resolve the algorithm Determine if only one instance is running step by step in the Kotlin programming language

Single Instance Program in Kotlin

This Kotlin program ensures that only one instance of the application can run at a time.

Explanation:

1. ServerSocket and Port: The program uses a ServerSocket with a private port number (65000) to check if another instance is already running. Every machine has a unique private IP address, and a port is assigned to each running application. Using a private port reduces the chances of port conflict.

2. alreadyRunning() Function:

   • This function attempts to create a ServerSocket on port 65000 with a backlog of 10.
   • If it fails with an IOException, it means that another instance is already using that port, so it returns true.
   • If it succeeds, no other instance is running, and it returns false.

3. close() Function:

   • This function closes the ServerSocket gracefully when the application is ready to terminate.

4. main Function:

   • It checks if another instance is already running by calling SingleInstance.alreadyRunning().
   • If another instance is running, it prints a message and terminates the current instance.
   • If this is the only instance, it prints a message and sleeps for 10 seconds.
   • As the main thread terminates, the close() function is called automatically to close the server socket.

// version 1.0.6

import java.io.IOException
import java.net.*

object SingleInstance {
    private var ss: ServerSocket? = null  

    fun alreadyRunning(): Boolean {
        try {
            ss = ServerSocket(65000, 10, InetAddress.getLocalHost()) // using private port 65000        
        }
        catch (e: IOException) {
            // port already in use so an instance is already running
            return true   
        }
        return false
    }

    fun close() {
        if (ss == null || ss?.isClosed() == true) return
        ss?.close()
    }
}

fun main(args: Array<String>) {
    if (SingleInstance.alreadyRunning()) {
        println("Application is already running, so terminating this instance")
        System.exit(0)
    }
    else { 
        println("OK, only this instance is running but will terminate in 10 seconds")
        Thread.sleep(10000)
        SingleInstance.close()  
    }
}