TYPES OF THREAD IN JAVA

Java allows us to create two types of threads –

§  Non-Daemon threads.

§  Daemon Threads

A thread that executes main logic of the project is called non-daemon thread. A thread that is running in background to provide services to non-daemon threads is called daemon thread. Hence, we can say daemon threads are service threads. We can’t change the nature of the main thread to Daemon because it is started by JVM. Hence, it is impossible. Main thread is always Non-Daemon and it isn’t possible to change the nature of main thread to Daemon, as it is already started by the JVM before starting our program.

By default main thread is non-daemon and for all the remaining threads daemon nature will be inherited from parent to child thread. Whenever the last non-daemon thread terminates, automatically all the daemon threads will be terminated. Garbage collector is a daemon thread. Since garbage collector provides service destroying of unreferenced objects. It is a low priority thread so we cannot guarantee its execution and it is also terminated if all non-daemon threads execution is completed.

Daemon Thread creation –

To create user defined thread as daemon thread, thread class has the following method –

§  public final void setDaemon(Boolean on) – if on value is true – thread is created as daemon, else it is created as non-daemon thread. Hence, the daemon thread property default value is false. To check thread is daemon or non-daemon, thread class provides a method public final Boolean isDaemon() returns true if thread is daemon, else returns false.

Note – setDaemon() method cannot be called after start() method call, it leads to RE:java.lang.IllegalThreadStateException, because once thread is created as non-daemon thread then it cannot be converted as daemon.

Example –

public class Daemon extends Thread{

          public void run()

          {

                   for(int i=0;i<5;i++)

                   {

                             System.out.println("LAzy thread");

                             try{

                                      Thread.sleep(100);

                             }

                             catch(InterruptedException e){

                                      System.out.println("Interrupted exception");

                             }

                   }

          }

          public static void main(String[] args) {

                   Daemon d = new Daemon();

                   d.setDaemon(true); //statement-1

                   d.start();

                   for(int i=0;i<5;i++)

                   {

                             System.out.println("main thread "+i);

                   }

          }

}

Note – If we comment statement-1, then both main and child threads are non-daemon threads. Hence, both the threads will be executed, until their execution is completed. If we aren’t commenting statement-1, then main thread is non-daemon and child thread is daemon thread. Whenever main thread terminates, automatically child thread will also be terminated.