What is a daemon thread in Java?

Question

Can anybody tell me what daemon threads are in Java?

Answer 1

Any Java thread can be a daemon thread.

Daemon threads are service providers for other threads running in the same process as the daemon thread. For example, the HotJava browser uses up to four daemon threads named "Image Fetcher" to fetch images from the file system or network for any thread that needs one. The run() method for a daemon thread is typically an infinite loop that waits for a service request. When the only remaining threads in a process are daemon threads, the interpreter exits. This makes sense because when only daemon threads remain, there is no other thread for which a daemon thread can provide a service.

To specify that a thread is a daemon thread, call the setDaemon method with the argument true. To determine if a thread is a daemon thread, use the accessor method isDaemon.

Hope this may help!!!!!!

Answer 2

Traditionally daemon processes in UNIX were those that were constantly running in background, much like services in Windows.

A daemon thread in Java is one that doesn't prevent the JVM from exiting. Specifically the JVM will exit when only daemon threads remain. You create one by calling the setDaemon() method on Thread.

Have a read of Daemon threads.

Answer 3

Here is an example to test behavior of daemon threads in case of jvm exit due to non existence of user threads.

Please note second last line in the output below, when main thread exited, daemon thread also died and did not print finally executed9 statement within finally block. This means that any i/o resources closed within finally block of a daemon thread will not be closed if JVM exits due to non existence of user threads.

public class DeamonTreadExample {

public static void main(String[] args) throws InterruptedException {

    Thread t = new Thread(() -> {
        int count = 0;
        while (true) {
            count++;
            try {
                System.out.println("inside try"+ count);
                Thread.currentThread().sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } finally {
                System.out.println("finally executed"+ count);
            }
        }
    });
    t.setDaemon(true);
    t.start();

    Thread.currentThread().sleep(10000);
    System.out.println("main thread exited");
  }
}

Output

inside try1
finally executed1
inside try2
finally executed2
inside try3
finally executed3
inside try4
finally executed4
inside try5
finally executed5
inside try6
finally executed6
inside try7
finally executed7
inside try8
finally executed8
inside try9
finally executed9
inside try10
main thread exited

Answer 4

Daemon threads 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.

For example, the HotJava browser uses up to four daemon threads named "Image Fetcher" to fetch images from the file system or network for any thread that needs one.

Daemon threads are typically used to perform services for your application/applet (such as loading the "fiddley bits"). The core difference between user threads and daemon threads is that the JVM will only shut down a program when all user threads have terminated. Daemon threads are terminated by the JVM when there are no longer any user threads running, including the main thread of execution.

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.

Eg:

public class DaemonThread extends Thread {
    public void run() {
        System.out.println("Entering run method");

        try {
            System.out.println("In run Method: currentThread() is" + Thread.currentThread());

            while (true) {
                try {
                    Thread.sleep(500);
                } catch (InterruptedException x) {}

                System.out.println("In run method: woke up again");
            }
        } finally {
            System.out.println("Leaving run Method");
        }
    }
    public static void main(String[] args) {
        System.out.println("Entering main Method");

        DaemonThread t = new DaemonThread();
        t.setDaemon(true);
        t.start();

        try {
            Thread.sleep(3000);
        } catch (InterruptedException x) {}

        System.out.println("Leaving main method");
    }

}

OutPut:

C:\java\thread>javac DaemonThread.java

C:\java\thread>java DaemonThread
Entering main Method
Entering run method
In run Method: currentThread() isThread[Thread-0,5,main]
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
Leaving main method

C:\j2se6\thread>

Answer 5

Daemon thread is just like a normal thread except that the JVM will only shut down when the other non daemon threads are not existing. Daemon threads are typically used to perform services for your application.

Answer 6

Daemon threads are generally known as "Service Provider" thread. These threads should not be used to execute program code but system code. These threads run parallel to your code but JVM can kill them anytime. When JVM finds no user threads, it stops it and all daemon threads terminate instantly. We can set non-daemon thread to daemon using :

setDaemon(true)