In this example we shall show you how to make use JavaDaemon Thread, AThread is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads (User, Daemon) of execution running concurrently.

A Daemon Thread is typically used to provide a general service in the background as long as the program is running like the garbage collector thread. When a Java Virtual Machine starts up, there is usually a single user (non-daemon) thread which typically calls the method named main of some designated class. The Java Virtual Machine continues to execute threads until all threads that are not daemon threads have died, JVM doesn’t wait for daemon threads to finish their execution. As soon as last non daemon thread finished, JVM terminates no matter how many daemon thread exists or running.

1. Differences between User and Daemon thread:

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

2. Ways to create a thread:

There are two ways to create a new thread of execution:

1. Declare a class to be a subclass of Thread. This subclass should override therun() method of class Thread. An instance of the subclass can then be allocated and started.
2. Declare a class that implements theRunnable interface. That class then implements therun() method. An instance of the class can then be allocated and started.

3. Example:

3.1. Create a daemon thread:

We create a newDaemonThread class which extend Thread class and we override the run() method to print a simple messageDaemon thread is running on the console.

DaemonThread.java:

package com.jcg;/** * @author ashraf *  */public class DaemonThread extends Thread {@Overridepublic void run() {try {while (true) {System.out.println("Daemon thread is running");Thread.sleep(1000);}} catch (InterruptedException ie) {ie.printStackTrace();} finally {System.out.println("Daemon Thread exiting"); // never called}}}

3.2. Create a user thread:

This time, we create anotherUserThread class which implements implements Runnable interface, we override the run() method to print a simple messageUser thread is running five times on the console.

UserThread.java:

package com.jcg;/** * @author ashraf *  */public class UserThread implements Runnable {public void run() {try {for (int i = 0; i < 5; i++) {System.out.println("User thread is running");Thread.sleep(1000);}} catch (InterruptedException ie) {ie.printStackTrace();} finally {System.out.println("User Thread exiting");}}}

3.3. Run the example:

We create a newDaemonThreadTest class where we create a newDaemonThread and mark it as a daemon thread using setDaemon(true) then start it. Also, we create another user thread which will die after printing five messages. We will notice that when the user thread die, the JVM terminates the running daemon thread and it will die as well.

DaemonThreadTest.java:

package com.jcg;/** * @author ashraf *  */public class DaemonThreadTest {/** * @param args */public static void main(String[] args) {// Create a new daemon thread and start itDaemonThread daemonThread = new DaemonThread();daemonThread.setDaemon(true);daemonThread.start();// Create a new user thread and start itThread userThread = new Thread(new UserThread());userThread.start();}}

Output:

Daemon thread is runningUser thread is runningDaemon thread is runningUser thread is runningDaemon thread is runningUser thread is runningDaemon thread is runningUser thread is runningDaemon thread is runningUser thread is runningDaemon thread is runningUser Thread exiting

Now let’s see what will happen when mark the first daemon thread as non daemon user thread usingsetDaemon(false) and running the example again using the newUserThreadTest class. We will notice that the user thread has died and the JVM still waits for daemon thread to finish its execution, it doesn’t terminates the running daemon thread.

UserThreadTest.java:

package com.jcg;/** * @author ashraf *  */public class UserThreadTest {/** * @param args */public static void main(String[] args) {// Create a new daemon thread and start itDaemonThread daemonThread = new DaemonThread();daemonThread.setDaemon(false);daemonThread.start();// Create a new user thread and start itThread userThread = new Thread(new UserThread());userThread.start();}}

Output:

Daemon thread is runningUser thread is runningDaemon thread is runningUser thread is runningDaemon thread is runningUser thread is runningDaemon thread is runningUser thread is runningDaemon thread is runningUser thread is runningDaemon thread is runningUser Thread exitingDaemon thread is runningDaemon thread is runningDaemon thread is runningDaemon thread is runningDaemon thread is running

4. Download the Source Code of this example:

This was an example of Java Daemon Thread.

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