Daemon Thread in Java

Several Java daemon threads, such as the finalizer and gc, run automatically in the background.

In simple words, a daemon thread is there to support and assist user threads with tasks running in the background. Apart from serving user threads, it doesn't have any specific purpose.

Multithreading in Java brings the capability of running multiple threads simultaneously, and the concept of daemon threads helps in managing and supporting these user threads efficiently.

Java Daemon Thread Properties

• Termination of JVM : When all user threads finish their execution, the JVM (Java Virtual Machine) can be stopped from running.
• Daemon Thread Handling : If the JVM identifies the presence of an active daemon thread, it will terminate that thread and subsequently shut down the daemon. The status of the daemon thread, whether it is actively running or not, is not important to the JVM.
• Lowest Priority : Daemon threads are assigned the lowest possible priority among all threads.

Daemon Thread vs User Threads

• Priority : When a process consists only of daemon threads, the interpreter terminates. This makes sense because when there are only daemon threads remaining, there are no other threads available for the daemon thread to provide services to.
• Usage : User threads receive services from daemon threads to handle supporting tasks in the background.

Why does JVM kill the daemon thread when there is no user thread running?

• The purpose of a daemon thread is to assist the user thread with background supporting tasks. Therefore, if there are no user threads present, it is unnecessary for the JVM to continue running the daemon thread. Consequently, if there are no user threads, the JVM terminates the daemon thread.

Methods of Daemon Thread

• boolean isDaemon() : This method is used to determine whether the currently active thread is a daemon thread. It returns true if the thread is a daemon, and false otherwise.

public final boolean isDaemon()

• void setDaemon(boolean status) : By using this method, the current thread can be marked as either a user thread or a daemon thread.

public final void setDaemon(boolean on)

Simple example of Daemon thread in java

import java.io.*;
public class Main extends Thread{  
	 public void run(){  
		  if(Thread.currentThread().isDaemon()){ //checking for daemon thread  
		   System.out.println("Daemon thread work");  
		  }  
		  else{  
		  System.out.println("User thread work");  
		  }  
	 }  
	 public static void main(String[] args){  
	  Main thread1=new Main();//creating thread  
	  Main thread2=new Main();  
	  Main thread3=new Main();  
	  thread1.setDaemon(true);//now t1 is daemon thread    
	  thread1.start();//starting threads  
	  thread2.start();  
	  thread3.start();  
	}  
}

Daemon thread work
User thread work
User thread work 

Example 2 : Daemon thread in Java

public class Main extends Thread{  
    public void run(){  
		 System.out.println("Current Thread Name: "+Thread.currentThread().getName());  
		 System.out.println("Daemon Thread: "+Thread.currentThread().isDaemon());  
	 }  
	 public static void main(String[] args){  
	  Main thread1=new Main();//creating thread  
	  Main thread2=new Main();  
	  thread1.start();//starting threads 
	  thread1.setDaemon(true);//now t1 is daemon thread    
	  thread2.start();  
	}  
}

Current Thread Name: Thread-0
Exception in thread "main" Daemon Thread: false
java.lang.IllegalThreadStateException
	at java.base/java.lang.Thread.setDaemon(Thread.java:1403)
	at HELLO/DockerTpoint.Main.main(Main.java:15)