What is Daemon Thread in Java? Example Attached

(http://crunchify.com/what-is-daemon-thread-in-java-example-attached/)

Daemon threads in Java are like a service providers for other threads or objects running in the same process as the daemon thread. Daemon threads are used for background supporting tasks and are only needed while normal threads are executing. If normal threads are not running and remaining threads are daemon threads then the interpreter exits.

When a new thread is created it inherits the daemon status of its parent. Normal thread and daemon threads differ in what happens when they exit. When the JVM halts any remaining daemon threads are abandoned: finally blocks are not executed, stacks are not unwound – JVM just exits. Due to this reason daemon threads should be used sparingly and it is dangerous to use them for tasks that might perform any sort of I/O.

setDaemon(true/false) ?

This method is used to specify that a thread is daemon thread.

public boolean isDaemon() ?

This method is used to determine the thread is daemon thread or not.

Daemon thread in Java are those thread which runs in background and mostly created by JVM for performing background task like Garbage collection and other house keeping tasks.

Points to Note :

Any thread created by main thread, which runs main method in Java is by default non daemon because Thread inherits its daemon nature from the Thread which creates it i.e. parent Thread and since main thread is a non daemon thread, any other thread created from it will remain non-daemon until explicitly made daemon by calling setDaemon(true).

Thread.setDaemon(true) makes a Thread daemon but it can only be called before starting Thread in Java. It will throw IllegalThreadStateException if corresponding Thread is already started and running.

Difference between Daemon and Non Daemon thread in Java :

1. JVM doesn’t wait for any daemon thread to finish before exiting.
2. Daemon Thread are treated differently than User Thread when JVM terminates, finally blocks are not called, Stacks are not unwounded and JVM just exits.

Java Example:

CrunchifyDaemonThread.java

package com.crunchify.tutorials; /** * @author Crunchify.com */ public class CrunchifyDaemonThread extends Thread {public static void main(String[] args) {System.out.println("Main Method Entry"); CrunchifyDaemonThread t = new CrunchifyDaemonThread();t.setDaemon(true);// When false, (i.e. when it's a user thread), the Worker thread// continues to run.// When true, (i.e. when it's a daemon thread), the Worker thread// terminates when the main thread terminates.t.start(); try {Thread.sleep(3000);} catch (InterruptedException x) {} System.out.println("Main Method Exit");} public void run() {System.out.println("run Method Entry"); try {System.out.println("In run Method: currentThread() is"+ Thread.currentThread()); while (true) {try {Thread.sleep(1000);} catch (InterruptedException x) {} System.out.println("In run method.." + Thread.currentThread());}} finally {System.out.println("run Method Exit");}}}

Output1: (with t.setDaemon(true)):

with t.setDaemon(true);

Main Method Entry
run Method Entry
In run Method: currentThread() isThread[Thread-0,5,main]
In run method..Thread[Thread-0,5,main]
In run method..Thread[Thread-0,5,main]
Main Method Exit

When daemon thread is the only threads running in a program – as you see here JVM ends the program finishing the thread.

Output2: (with t.setDaemon(false)):

with t.setDaemon(false);

Main Method Entry
run Method Entry
In run Method: currentThread() isThread[Thread-0,5,main]
In run method..Thread[Thread-0,5,main]
In run method..Thread[Thread-0,5,main]
Main Method Exit
In run method..Thread[Thread-0,5,main]
In run method..Thread[Thread-0,5,main]
In run method..Thread[Thread-0,5,main]
In run method..Thread[Thread-0,5,main]
...
...
...

As you see here program doesn't exit when it’s not a daemon thread.