What is a daemon thread in Java?

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Can anybody tell me what daemon threads are in Java?

A daemon thread is a thread that does not prevent the JVM from exiting when the program finishes but the thread is still running. An example for a daemon thread is the garbage collection.

You can use the setDaemon(boolean) method to change the Thread daemon properties before the thread starts.

Daemon thread in Java, Daemon threads are useful for background supporting tasks such as garbage collection, releasing memory of unused objects and removing� Daemon thread is a low priority thread that runs in background to perform tasks such as garbage collection. Properties: They can not prevent the JVM from exiting when all the user threads finish their execution.

Daemon Threads in Java, A daemon thread is a thread that does not prevent the JVM from exiting when the program finishes but the thread is still running. An example for� 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).

All the above answers are good. Here's a simple little code snippet, to illustrate the difference. Try it with each of the values of true and false in setDaemon.

public class DaemonTest {

    public static void main(String[] args) {
        new WorkerThread().start();

        try {
            Thread.sleep(7500);
        } catch (InterruptedException e) {
            // handle here exception
        }

        System.out.println("Main Thread ending") ;
    }

}

class WorkerThread extends Thread {

    public WorkerThread() {
        // 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.
        setDaemon(true); 
    }

    public void run() {
        int count = 0;

        while (true) {
            System.out.println("Hello from Worker "+count++);

            try {
                sleep(5000);
            } catch (InterruptedException e) {
                // handle exception here
            }
        }
    }
}

What is a daemon thread in Java?, Daemon thread in java is a service provider thread that provides services to the user thread. Its life depend on the mercy of user threads i.e. when all the user� 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).

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.

Daemon Thread in Java, A Daemon thread is a background service thread which runs as a low priority thread and performs background operations like garbage� Daemon thread in java. When a thread is marked as daemon thread, JVM doesn’t wait it to finish to terminate the program. As soon as all the user threads are finished, JVM terminates the program as well as all the associated daemon threads.

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>

Daemon thread in Java, Daemon thread in java. When a thread is marked as daemon thread, JVM doesn't wait it to finish to terminate the program. As soon as all the� Any thread inherits the daemon status of the thread that created it. Since the main thread is a user thread, any thread that is created inside the main method is by default a user thread. The method setDaemon() can only be called after the Thread object has been created and the thread has not been started.

Daemon Thread in Java, User thread vs Daemon threads in Java. As I told, the user threads are the normal threads you see and use in your day to day coding. Any thread you create from� Creating a thread as a daemon in Java is as simple as calling the setDaemon() method. A setting of true means the thread is a daemon; false means it is not. By default, all threads are created

Difference between Daemon Thread vs User Thread in Java?, Daemon thread in Java provides service to the user thread which runs in the background. It is considered to be a low priority thread which is used� Java Daemon Thread. Daemon threads is a low priority thread that provide supports to user threads. These threads can be user defined and system defined as well. Garbage collection thread is one of the system generated daemon thread that runs in background. These threads run in the background to perform tasks such as garbage collection.

Daemon Thread in Java | Daemon Thread vs User Thread, The Java runtime environment takes advantage of a special type of thread for background tasks. It's called a daemon thread, where the daemon� A thread is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads of execution running concurrently. Every thread has a priority. Threads with higher priority are executed in preference to threads with lower priority. Each thread may or may not also be marked as a daemon. When code running in some thread creates a new Thread object, the

Comments
  • The Thread javadoc describes what they are: java.sun.com/javase/6/docs/api/java/lang/Thread.html
  • answers.com/Q/What_is_a_daemon_thread
  • For Daemon Threads, when the JVM stops all daemon threads are exited Due to this reason daemon threads shouldn't be used often since cleanup might not be executed on them. For example any I/O would not gracefully exit and write/read all the way to the end.
  • For posterity, setDamon(boolean) can only be called before the thread has been started. By default the thread inherits the daemon status of its parent thread.
  • "does not prevent the JVM from exiting when the program finishes but the thread is still running" while @sateesh says that "JVM halts any remaining daemon threads are abandoned". So do daemon threads finish running when JVM exits?
  • @Gerald, ALL threads are killed when the JVM exits. B_erb said, "...when the program finishes." What that means is, if the program does not explicitly kill the JVM, then the JVM will automatically kill itself when the last non-daemon thread ends. Normal threads define "when the program exits." Daemon threads don't.
  • So this line thread that does not prevent the JVM from exiting when the program finishes but the thread is still running basically means the JVM process that started the thread doesn't care if the daemon thread finished executing or not, it will just end itself if all the normal threads have finished execution.
  • @SolomonSlow What are the consequences of killing a daemon thread (for example, a garbage collector) while it is still doing its job, when the JVM ends? Thanks.
  • Why shouldn't daemon threads be used for I/O? Is it a concern about BufferedWriters etc not being flushed?
  • @PaulCager Yeah, they can just get cut off at the knees in the middle of a write/read as well.
  • The second point is nonsense. When the JVM halts, all threads die and no finally blocks are executed, regardless of whether the threads are daemon or not. So don’t call System.exit(…) if you think there might be running threads doing I/O. The only difference is that the JVM will trigger its own termination when only daemon threads are left.
  • What is meant by "stacks are not unwound"?
  • @ɢʜʘʂʈʀɛɔʘɴ there are some explanations out there on "unwinding stacks," including this one: flylib.com/books/en/2.254.1.277/1
  • @russ Good code snippet! I had to define WorkerThread class as static though.