Can anybody tell me what daemon threads are in Java?
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java
multithreading
Daemon thread is like daemon process which is responsible for managing resources,a daemon thread is created by the Java VM to serve the user threads. example updating system for unix,unix is daemon process. child of daemon thread is always daemon thread,so by default daemon is false.you can check thread as daemon or user by using "isDaemon()" method. so daemon thread or daemon process are basically responsible for managing resources. for example when you starting jvm there is garbage collector running that is daemon thread whose priority is 1 that is lowest,which is managing memory. jvm is alive as long as user thread is alive,u can not kill daemon thread.jvm is responsible to kill daemon threads.
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 threads are threads that run in the background as long as other non-daemon threads of the process are still running. Thus, when all of the non-daemon threads complete, the daemon threads are terminated. An example for the non-daemon thread is the thread running the Main.
A thread is made daemon by calling the setDaemon() method before the thread is started
For More Reference : Daemon thread in Java
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 existing.
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.
A daemon thread is a thread that is considered doing some tasks in the background like handling requests or various chronjobs that can exist in an application.
When your program only have daemon threads remaining it will exit. That's because usually these threads work together with normal threads and provide background handling of events.
You can specify that a Thread is a daemon one by using setDaemon method, they usually don't exit, neither they are interrupted.. they just stop when application stops.
Daemon threads die when the creator thread exits.
Non-daemon threads (default) can even live longer than the main thread.
if ( threadShouldDieOnApplicationEnd ) {
thread.setDaemon ( true );
}
thread.start();
Daemon threads are low-priority threads whose only role is to provide services to user threads.
Daemon threads are generally created by JVM for background tasks while user threads are created by application for foreground tasks.
Since daemon threads are meant to serve user threads and are only needed while user threads are running, they won't prevent the JVM from exiting once all user threads have finished their execution.
We can set non-daemon thread to daemon using :
setDaemon(true)
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)
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.
Java uses user thread and daemon tread concepts.
JVM flow
1. If there are no `user treads` JVM starts terminating the program
2. JVM terminates all `daemon threads` automatically without waiting when they are done
3. JVM is shutdown
As you see daemon tread is a service thread for user treads.
daemon tread is low priority thread.setDaemon() method before starting it or check it via isDaemon()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.
JVM will accomplish the work when a last non-daemon thread execution is completed. By default, JVM will create a thread as nondaemon but we can make Thread as a daemon with help of method setDaemon(true). A good example of Daemon thread is GC thread which will complete his work as soon as all nondaemon threads are completed.
Let's talk only in code with working examples. I like russ's answer above but to remove any doubt I had, I enhanced it a little bit. I ran it twice, once with the worker thread set to deamon true (deamon thread) and another time set it to false (user thread). It confirms that the deamon thread ends when the main thread terminates.
public class DeamonThreadTest {
public static void main(String[] args) {
new WorkerThread(false).start(); //set it to true and false and run twice.
try {
Thread.sleep(7500);
} catch (InterruptedException e) {
// handle here exception
}
System.out.println("Main Thread ending");
}
}
class WorkerThread extends Thread {
boolean isDeamon;
public WorkerThread(boolean isDeamon) {
// 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.
this.isDeamon = isDeamon;
setDaemon(isDeamon);
}
public void run() {
System.out.println("I am a " + (isDeamon ? "Deamon Thread" : "User Thread (none-deamon)"));
int counter = 0;
while (counter < 10) {
counter++;
System.out.println("\tworking from Worker thread " + counter++);
try {
sleep(5000);
} catch (InterruptedException e) {
// handle exception here
}
}
System.out.println("\tWorker thread ends. ");
}
}
result when setDeamon(true)
=====================================
I am a Deamon Thread
working from Worker thread 0
working from Worker thread 1
Main Thread ending
Process finished with exit code 0
result when setDeamon(false)
=====================================
I am a User Thread (none-deamon)
working from Worker thread 0
working from Worker thread 1
Main Thread ending
working from Worker thread 2
working from Worker thread 3
working from Worker thread 4
working from Worker thread 5
working from Worker thread 6
working from Worker thread 7
working from Worker thread 8
working from Worker thread 9
Worker thread ends.
Process finished with exit code 0
Daemon threads are as everybody explained, will not constrain JVM to exit, so basically its a happy thread for Application from exit point of view.
Want to add that daemon threads can be used when say I'm providing an API like pushing data to a 3rd party server / or JMS, I might need to aggregate data at the client JVM level and then send to JMS in a separate thread. I can make this thread as daemon thread, if this is not a mandatory data to be pushed to server. This kind of data is like log push / aggregation.
Regards, Manish
Daemon threads are like assistants. Non-Daemon threads are like front performers. Assistants help performers to complete a job. When the job is completed, no help is needed by performers to perform anymore. As no help is needed the assistants leave the place. So when the jobs of Non-Daemon threads is over, Daemon threads march away.
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 non daemon thread),
// the WorkerThread continues to run.
// When true, (i.e. when it's a daemon thread),
// the WorkerThread terminates when the main
// thread or/and user defined thread(non daemon) 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
}
}
}
}
Daemon Thread and User Threads. Generally all threads created by programmer are user thread (unless you specify it to be daemon or your parent thread is a daemon thread). User thread are generally meant to run our programm code. JVM doesn't terminates unless all the user thread terminate.
For me, daemon thread it's like house keeper for user threads. If all user threads finished , the daemon thread has no job and killed by JVM. I explained it in the YouTube video.
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>
I can see, that there already are numerous answers; however, I'd like to shed a bit clearer light on this, because, when I was reading about Daemon Threads, initially, I had a feeling, that I understood it well; however, after I played with it and debugged a bit, I saw a strange (to me) behaviour.
I was taught, that if I want the thread to die right after the main thread orderly finishes its execution, I should set it as *Diamond.
What I tried was, that I created two threads from the Main Thread, and then I only set one of those as Diamond. What happened was, that after the orderly finishing the execution of the Main Thread, none of those newly created threads exited. But I expected, that Daemon thread should've been exited. I surfed over many blogs and articles, but the best and clearest definition I found so far comes from the Java Concurrency In Practice book, by Brian Goetz and few others.
It very clearly says, that:
Sometimes you want to create a thread that performs some helper function but you don’t want the existence of this thread to prevent the JVM from shutting down. This is what daemon threads are for. Threads are divided into two types: normal threads and daemon threads. When the JVM starts up, all the threads it creates (such as garbage collector and other housekeeping threads) are daemon threads, except the main thread. When a new thread is created, it inherits the daemon status of the thread that created it, so by default any threads created by the main thread are also normal threads. Normal threads and daemon threads differ only in what happens when they exit. When a thread exits, the JVM performs an inventory of running threads, and if the only threads that are left are daemon threads, it initiates an orderly shutdown. When the JVM halts, any remaining daemon threads are abandoned— finally blocks are not executed, stacks are not unwound—the JVM just exits. Daemon threads should be used sparingly—few processing activities can be safely abandoned at any time with no cleanup. In particular, it is dangerous to use daemon threads for tasks that might perform any sort of I/O. Daemon threads are best saved for “housekeeping” tasks, such as a background thread that periodically removes expired entries from an in-memory cache.
Java has a special kind of thread called daemon thread.
What are daemon threads used for?
Normally used as service providers for normal threads. Usually have an infinite loop that waits for the service request or performs the tasks of the thread. They can’t do important jobs. (Because we don't know when they are going to have CPU time and they can finish any time if there aren't any other threads running. )
A typical example of these kind of threads is the Java garbage collector.
There's more...
setDaemon() method before you call the start() method. Once the thread is running, you can’t modify its daemon status.isDaemon() method to check if a thread is a daemon thread or a user thread.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");
}
}
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
A background process that handles requests for services such as print spooling and file transfers, and is dormant when not required.
—— Source: English by Oxford Dictionaries
A few more points (Reference: Java Concurrency in Practice)
When all non-daemon threads finish, the JVM halts, and any remaining daemon threads are abandoned:
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.
In Java, Daemon Threads are one of the types of the thread which does not prevent Java Virtual Machine (JVM) from exiting. The main purpose of a daemon thread is to execute background task especially in case of some routine periodic task or work. With JVM exits, daemon thread also dies.
By setting a thread.setDaemon(true), a thread becomes a daemon thread. However, you can only set this value before the thread start.
One misconception I would like to clarify:
Source: Stackoverflow.com