Unable to lock files on Linux using java.nio.channels.FileLock

Question

I was creating an application in Java for which I want only one instance running. For this purpose I created a file and got a lock while my application is running.

I

Answer 1

Recently i encountered the same kind of problem, but in my case i had an advantage: my application polled some directory only after some timeout. As my application did not immediately poll for directory i wrote special class that creates lock file with his own PID inside in init method, after that before it tries to work with directory it needs to call ownedLock() - if it returns true then we can work otherwise exit(code is in Kotlin but you will get the main idea):

import java.io.File
import java.lang.management.ManagementFactory
class DirectoryLocker(private val directory: String, private val lockName: String) {
   private val lockFile by lazy { File("$directory/$lockName.lock") }

   // Will try to acquire lock to directory, whoever last writes its pid to file owns the directory
   fun acquireLock() = with(lockFile) {
      createNewFile()
      writeText(currentPID())
   }

   fun ownedLock(): Boolean = lockFilePid() == currentPID()

   fun releaseOwnedLock() {
      if(lockFilePid() == currentPID()) lockFile.delete()
   }

   private fun currentPID(): String {
      val processName = ManagementFactory.getRuntimeMXBean().name
      return processName.split("@".toRegex()).first()
   }

   private fun lockFilePid(): String? {
      return if(lockFile.exists()) lockFile.readLines().first() else null
   }
}