Introduction

• A Daemon Thread in Java is a special type of thread that runs in the background and provides support services to user threads.
• If all user threads are finished, the JVM will automatically stop/kill all daemon threads and shut down.
• Use of Daemon Threads:
  • Perform background tasks without blocking program termination.
  • Provide supporting services like memory cleanup and resource management.
  • Help in running periodic monitoring or housekeeping tasks in applications.
• Real-World Examples where Daemon Threads are used:
  • Garbage Collection: JVM’s garbage collector is a daemon thread that automatically frees memory.
  • Background Logging: Threads writing logs asynchronously in applications.
  • Monitoring Services: Threads checking for system health, performance or connections in servers.
  • Auto-Save Functionality: In editors/IDEs where changes are saved in the background without disturbing the user’s work.
• For Daemon Threads, Java has provided two important methods:
  1. public final void setDaemon(boolean on)
     • Used to mark a thread as daemon thread. Must be called before start().
  2. public final boolean isDaemon()
     • Returns true if the thread is a daemon thread, otherwise false.
• Program (Spell Checker Daemon Thread):

  class SpellCheckerDaemon extends Thread
  {
      @Override
      public void run()
      {
          if (isDaemon())
          {
              System.out.println("Spell Checker Daemon is running in the background...");
          }
  
          // Simulating background spell-check task
          while (true)
          {
              System.out.println("Checking spelling in background...");
              try
              {
                  Thread.sleep(1000); // Pause for demonstration
              }
              catch (Exception e)
              {
                  e.printStackTrace();
              }
          }
      }
  }
  
  public class MainApp
  {
      public static void main(String[] args) throws Exception
      {
          SpellCheckerDaemon spellChecker = new SpellCheckerDaemon();
  
          // Mark as daemon before starting
          spellChecker.setDaemon(true);
  
          spellChecker.start();
  
          System.out.println("Main thread: User is typing in the editor...");
          Thread.sleep(3000); // Simulate user typing
  
          System.out.println("Main thread work finished. JVM will exit now.");
      }
  }

  Output (sample):

  Main thread: User is typing in the editor...
  Spell Checker Daemon is running in the background...
  Checking spelling in background...
  Checking spelling in background...
  Checking spelling in background...
  Main thread work finished. JVM will exit now.

• Key Points about Daemon Threads:
  • They are low-priority background threads.
  • JVM terminates daemon threads automatically when all user threads finish.
  • They are created by calling setDaemon(true) on a thread before starting it.