Related
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.
A few more points (Reference: Java Concurrency in Practice)
When a new thread is created it inherits the daemon status of its
parent.
When all non-daemon threads finish, the JVM halts, and any remaining daemon threads are abandoned:
finally blocks are not executed,
stacks are not unwound - the 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.
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
}
}
}
}
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 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: d(isk) a(nd) e(xecution) mon(itor) or from de(vice) mon(itor)
Definition of Daemon (Computing):
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
What is Daemon thread in Java?
Daemon threads can shut down any time in between their flow, Non-Daemon i.e. user thread executes completely.
Daemon threads are threads that run intermittently in the background as long as other non-daemon threads are running.
When all of the non-daemon threads complete, daemon threads terminates automatically.
Daemon threads are service providers for user threads running in the same process.
The JVM does not care about daemon threads to complete when in Running state, not even finally block also let execute. JVM do give preference to non-daemon threads that is created by us.
Daemon threads acts as services in Windows.
The JVM stops the daemon threads when all user threads (in contrast to the daemon threads) are terminated. Hence daemon threads can be used to implement, for example, a monitoring functionality as the thread is stopped by the JVM as soon as all user threads have stopped.
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.
One misconception I would like to clarify:
Assume that if daemon thread (say B) is created within user thread (say
A); then ending of this user thread/parent thread (A) will not end
the daemon thread/child thread (B) it has created; provided user thread is the only
one currently running.
So there is no parent-child relationship on thread ending. All daemon threads (irrespective of where it is created) will end once there is no single live user thread and that causes JVM to terminate.
Even this is true for both (parent/child) are daemon threads.
If a child thread created from a daemon thread then that is also a daemon thread. This won't need any explicit daemon thread flag setting.
Similarly if a child thread created from a user thread then that is also a user thread, if you want to change it, then explicit daemon flag setting is needed before start of that child thread.
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.
Java has a special kind of thread called daemon thread.
Very low priority.
Only executes when no other thread of the same program is running.
JVM ends the program finishing these threads, when daemon threads are
the only threads running in a program.
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...
You only call the setDaemon() method before you call the start() method. Once the thread is running, you can’t modify its daemon status.
Use isDaemon() method to check if a thread is a daemon thread or a user thread.
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.
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.
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
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.
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.
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 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.
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.
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 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)
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
There already are numerous answers; however, maybe I could shed a bit clearer light on this, as when I was reading about Daemon Threads, initially, I had a feeling, that I understood it well; however, after playing with it and debugged a bit, I observed 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:
I created two threads from the Main Thread, and I only set one of those as a diamond;
After orderly completing execution of the Main Thread, none of those newly created threads exited, but I expected, that Daemon thread should have been exited;
I surfed over many blogs and articles, and the best and clearest definition I found so far, comes from the Java Concurrency In Practice book, which very clearly states, that:
7.4.2 Daemon threads
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.
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.
Daemon threads are those threads which provide general services for user threads (Example : clean up services - garbage collector)
Daemon threads are running all the time until kill by the JVM
Daemon Threads are treated differently than User Thread when JVM terminates , finally blocks are not called JVM just exits
JVM doesn't terminates unless all the user threads terminate. JVM terminates if all user threads are dies
JVM doesn't wait for any daemon thread to finish before existing and finally blocks are not called
If all user threads dies JVM kills all the daemon threads before stops
When all user threads have terminated, daemon threads can also be terminated and the main program terminates
setDaemon() method must be called before the thread's start() method is invoked
Once a thread has started executing its daemon status cannot be changed
To determine if a thread is a daemon thread, use the accessor method isDaemon()
Java daemon thread
[Daemon process]
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.
Thread inherits it's properties from parent thread. To set it externally you can use setDaemon() method before starting it or check it via isDaemon()
Daemon Thread
Threads that run in the background are called daemon threads.
Example of Daemon Threads:
Garbage Collector.
Signal Dispatcher.
Objective of Daemon Thread:
The main objective of the daemon threads is to provide support to the non-daemon threads.
Additional information about Daemon Thread:
Generally, Daemon threads run in the MIN_PRIORITY however, it is possible to run daemon threads with MAX_PRIORITY as well.
Example: Usually the GC runs with a MIN_PRIORITY priority, however, once there is a requirement for additional memory. JVM increases the priority of the GC from MIN_PRIORITY to MAX_PRIORITY.
Once the Thread has been started, it can't be changed from Daemon Thread to Non-Daemon Thread that will result in IllegalThreadStateException.
Example:
public static void main(String[] args) {
Thread.currentThread().setDaemon(true);
}
Output:
Exception in thread "main" java.lang.IllegalThreadStateException
at java.base/java.lang.Thread.setDaemon(Thread.java:1403)
If we are branching off a thread, the child thread inherits the nature of the parent thread. If the parent thread is a non-daemon thread automatically the child thread will be non-daemon as well and if the parent thread is a daemon, the child thread will be a daemon as well.
class Scratch {
public static void main(String[] args) {
CustomThread customThread = new CustomThread();
customThread.start();
}
}
class CustomThread extends Thread{
#Override
public void run() {
System.out.println(currentThread().isDaemon());
}
}
Output:
false
When the last non-daemon thread terminates, all the daemon threads get terminated automatically.
class Scratch {
public static void main(String[] args) {
System.out.println("Main Thread Started.");
CustomThread customThread = new CustomThread();
customThread.setDaemon(true);
customThread.start();
System.out.println("Main Thread Finished.");
}
}
class CustomThread extends Thread{
#Override
public void run() {
System.out.println("Custom Thread Started.");
try {
sleep(2000);
} catch (InterruptedException ignore) {}
System.out.println("Custom Thread Finished."); //Won't get executed.
}
}
Output:
Main Thread Started.
Main Thread Finished.
Custom Thread Started.
User threads versus Daemon threads in java threads
Daemon Threads
this threads in Java are low-priority threads that runs in the background to perform tasks such as garbage collection. Daemon thread in Java is also a service provider thread that provides services to the user thread.
User Threads
this threads are high-priority threads. The JVM will wait for any user thread to complete its task before terminating it
"keep in mind both User and Daemon threads wrapped upon OS threads"
Recently OpenJdk proposed Virtual threads with in project Loom (which they are User based as well) you may find more on Fibers and Continuations for the Java Virtual Machine threads in here.
I read this statement:
The main thread must be the last thread to finish execution. When the main thread stops, the program terminates.
Is it true?
I also came to know "Even if the main thread dies, the program keeps running".
This is my current understanding:
When you start a program, the JVM creates one thread to run your program.
The JVM creates one user thread for running a program. This thread is called main thread.
The main method of the class is called from the main thread.
If a program spawns new threads from the main thread, the program waits until the last thread dies.
Which one is true?
The program terminates when all non-daemon threads die (a daemon thread is a thread marked with setDaemon(true); it's usually used for utility threads). From the documentation:
When a Java Virtual Machine starts up, there is usually a single non-daemon thread (which typically calls the method named main of some designated class). The Java Virtual Machine continues to execute threads until either of the following occurs:
The exit method of class Runtime has been called and the security manager has permitted the exit operation to take place.
All threads that are not daemon threads have died, either by returning from the call to the run method or by throwing an exception that propagates beyond the run method.
I read this statement: “The main thread must be the last thread to finish execution. When the main thread stops, the program terminates.”Is it true?
No, it is not. The virtual machine terminates if the last non-daemon thread has finished. It doesn't have to be the main thread.
Simple example:
public static void main(String[] args) {
System.out.println("Main thread started");
new Thread(new Runnable() {
#Override
public void run() {
System.out.println("Second thread started");
try {
Thread.sleep(2000); // wait two seconds
} catch(Exception e){}
System.out.println("Second thread (almost) finished");
}
}).start();
System.out.println("Main thread (almost) finished");
}
When the main thread stops, the program terminates.
The program terminates when there no longer is any non-daemon thread running (or someone called System.exit). The main thread can have finished long ago.
The JVM will exit when the main thread and all non-daemon threads finish execution.
When you create a new thread, you can call Thread.setDaemon(true) to make it a daemon thread. If you do this, then the JVM will not wait until this thread finishes before execution. This is useful for any threads you create which are made to run in the background until the program stops.
If you create a new thread and do not call Thread.setDaemon(true), then the JVM will delay exit until this thread is complete, even if the main thread is finished.
When the main thread was start it'll not wait for the another thread which was created by us until they if can't use the join() of the thread class to wait for this thread. So basically if the child thread or sub thread getting more time for processing the task and you don't use the join() then main thread may be stop. To keep with main thread you must use the join() so the main thread stop after only this related thread are stop
check this link
http://download.oracle.com/javase/1.5.0/docs/api/java/lang/Thread.html#join%28%29
The first statementis not exact.
The java program terminates when all non-daemon threads has been terminated or when System.exit() or Runtime.exit() is invoked.
Thread is terminated when it exited its run() method. Main thread is special because you do not explicitly implement its run() method, you implement main() instead and the main() is called from run(). So, main thread is terminated when main() is terminated.
But main thread is not neccessarely the last one.
This is the from the JVM specification 3rd Draft, so it's the most current I'm aware of:
5.7 Virtual Machine Exit
The Java virtual machine terminates all its activity and exits when either:
• All threads that are not daemon threads terminate.
• Some thread invokes the exit method of class Runtime or class System, and
the exit operation is permitted by the security manager.
There is no distinction made about the main thread, so we shouldn't assume that is the only one that it applies to.
As I was learning multithreading in java, i came to know that there is no execution order for threads.
As per my understanding is below statement true?
A user thread (which is not a Daemon Thread) it should terminate before termination of main thread.
I have read similar links:
if main method completes the execution, what happens to any long running thread?
When does the main thread stop in Java?
I have a program to demontrate, please correct me.
class ThreadDemo {
public static void main(String args[]) {
Thread t = new Thread(new Runnable(){
#Override
public void run() {
System.out.println("Within 'Child Thread' # "+System.currentTimeMillis());
}
}, "Child Thread");
//t.setDaemon(false);
t.start();
System.out.println(Thread.currentThread()+" thread is alive:"+Thread.currentThread().isAlive());
System.out.println(t+" thread is alive:"+t.isAlive());
System.out.println("'Main' thread exiting # "+System.currentTimeMillis());
}
}
Most of the times output to this program on my system is
'Main' thread exiting # 1406971862950
Within 'Child Thread' # 1406971862952
Does this mean that main thread exits before child thread? If yes, then why is this happening?
That's true. When you execute your program, a main thread is created which executes the main method. Since you are creating another thread in main, both the created thread and main thread will execute parallerly irrespective of each other.
However if you require your created thread to complete executing and than main finishing the execution jst add followind code after starting your thread.
t.join();
This would force your main thread to stop until your created thread finishes execution.
Daemon threads are the threads that are pulled automatically by the JVM as and when required. Garbage collector is one among.
Main thread is pulled by user. So it ia non-daemon thread.
JVM shuts down only when all non-daemon threads complete execution. It does not wait for daemon threads to complete.
I am creating two threads within the main method say t1 and t2.
t1 is user thread and has a loop of 10 print statements
t2 is daemon thread and has a loop of 10 print statements
main starts both threads and has a single print statement.
however even after main exits t2 still continues to run parallel to t1 even after main has ended.
can a daemon thread run even after the thread from which it is created has exited.
kindly update
thanks
tejinder
can a daemon thread run even after the thread from which it is created has exited.
Yes. A daemon thread will run until either it exits or all non-daemon threads finish and the JVM terminates. The operation or termination of the parent thread that started the child thread does not affect the child thread at all.
If you want to stop a child thread then the parent thread should interrupt() it and then join() with it. Something like:
Thread child = new Thread(new MyRunnable());
child.start();
...
child.interrupt();
child.join();
Remember that interrupt() does not cancel the thread. It just makes methods like Thread.sleep(), Object.wait(), and others throw InterruptedException. Your child thread should do something like:
while (!Thread.currentThread().isInterrupted()) {
...
try {
Thread.sleep(100);
} catch (InterruptedException e) {
// catching the exception clears the interrupt bit so we need to set it again
Thread.currentThread().interrupt();
// we probably want to quit the thread if we were interrupted
return;
}
}
Yes, from where ever you start the threads, threads run independently.
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.
A few more points (Reference: Java Concurrency in Practice)
When a new thread is created it inherits the daemon status of its
parent.
When all non-daemon threads finish, the JVM halts, and any remaining daemon threads are abandoned:
finally blocks are not executed,
stacks are not unwound - the 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.
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
}
}
}
}
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 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: d(isk) a(nd) e(xecution) mon(itor) or from de(vice) mon(itor)
Definition of Daemon (Computing):
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
What is Daemon thread in Java?
Daemon threads can shut down any time in between their flow, Non-Daemon i.e. user thread executes completely.
Daemon threads are threads that run intermittently in the background as long as other non-daemon threads are running.
When all of the non-daemon threads complete, daemon threads terminates automatically.
Daemon threads are service providers for user threads running in the same process.
The JVM does not care about daemon threads to complete when in Running state, not even finally block also let execute. JVM do give preference to non-daemon threads that is created by us.
Daemon threads acts as services in Windows.
The JVM stops the daemon threads when all user threads (in contrast to the daemon threads) are terminated. Hence daemon threads can be used to implement, for example, a monitoring functionality as the thread is stopped by the JVM as soon as all user threads have stopped.
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.
One misconception I would like to clarify:
Assume that if daemon thread (say B) is created within user thread (say
A); then ending of this user thread/parent thread (A) will not end
the daemon thread/child thread (B) it has created; provided user thread is the only
one currently running.
So there is no parent-child relationship on thread ending. All daemon threads (irrespective of where it is created) will end once there is no single live user thread and that causes JVM to terminate.
Even this is true for both (parent/child) are daemon threads.
If a child thread created from a daemon thread then that is also a daemon thread. This won't need any explicit daemon thread flag setting.
Similarly if a child thread created from a user thread then that is also a user thread, if you want to change it, then explicit daemon flag setting is needed before start of that child thread.
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.
Java has a special kind of thread called daemon thread.
Very low priority.
Only executes when no other thread of the same program is running.
JVM ends the program finishing these threads, when daemon threads are
the only threads running in a program.
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...
You only call the setDaemon() method before you call the start() method. Once the thread is running, you can’t modify its daemon status.
Use isDaemon() method to check if a thread is a daemon thread or a user thread.
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.
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.
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
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.
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.
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 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.
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.
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 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)
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
There already are numerous answers; however, maybe I could shed a bit clearer light on this, as when I was reading about Daemon Threads, initially, I had a feeling, that I understood it well; however, after playing with it and debugged a bit, I observed 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:
I created two threads from the Main Thread, and I only set one of those as a diamond;
After orderly completing execution of the Main Thread, none of those newly created threads exited, but I expected, that Daemon thread should have been exited;
I surfed over many blogs and articles, and the best and clearest definition I found so far, comes from the Java Concurrency In Practice book, which very clearly states, that:
7.4.2 Daemon threads
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.
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.
Daemon threads are those threads which provide general services for user threads (Example : clean up services - garbage collector)
Daemon threads are running all the time until kill by the JVM
Daemon Threads are treated differently than User Thread when JVM terminates , finally blocks are not called JVM just exits
JVM doesn't terminates unless all the user threads terminate. JVM terminates if all user threads are dies
JVM doesn't wait for any daemon thread to finish before existing and finally blocks are not called
If all user threads dies JVM kills all the daemon threads before stops
When all user threads have terminated, daemon threads can also be terminated and the main program terminates
setDaemon() method must be called before the thread's start() method is invoked
Once a thread has started executing its daemon status cannot be changed
To determine if a thread is a daemon thread, use the accessor method isDaemon()
Java daemon thread
[Daemon process]
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.
Thread inherits it's properties from parent thread. To set it externally you can use setDaemon() method before starting it or check it via isDaemon()
Daemon Thread
Threads that run in the background are called daemon threads.
Example of Daemon Threads:
Garbage Collector.
Signal Dispatcher.
Objective of Daemon Thread:
The main objective of the daemon threads is to provide support to the non-daemon threads.
Additional information about Daemon Thread:
Generally, Daemon threads run in the MIN_PRIORITY however, it is possible to run daemon threads with MAX_PRIORITY as well.
Example: Usually the GC runs with a MIN_PRIORITY priority, however, once there is a requirement for additional memory. JVM increases the priority of the GC from MIN_PRIORITY to MAX_PRIORITY.
Once the Thread has been started, it can't be changed from Daemon Thread to Non-Daemon Thread that will result in IllegalThreadStateException.
Example:
public static void main(String[] args) {
Thread.currentThread().setDaemon(true);
}
Output:
Exception in thread "main" java.lang.IllegalThreadStateException
at java.base/java.lang.Thread.setDaemon(Thread.java:1403)
If we are branching off a thread, the child thread inherits the nature of the parent thread. If the parent thread is a non-daemon thread automatically the child thread will be non-daemon as well and if the parent thread is a daemon, the child thread will be a daemon as well.
class Scratch {
public static void main(String[] args) {
CustomThread customThread = new CustomThread();
customThread.start();
}
}
class CustomThread extends Thread{
#Override
public void run() {
System.out.println(currentThread().isDaemon());
}
}
Output:
false
When the last non-daemon thread terminates, all the daemon threads get terminated automatically.
class Scratch {
public static void main(String[] args) {
System.out.println("Main Thread Started.");
CustomThread customThread = new CustomThread();
customThread.setDaemon(true);
customThread.start();
System.out.println("Main Thread Finished.");
}
}
class CustomThread extends Thread{
#Override
public void run() {
System.out.println("Custom Thread Started.");
try {
sleep(2000);
} catch (InterruptedException ignore) {}
System.out.println("Custom Thread Finished."); //Won't get executed.
}
}
Output:
Main Thread Started.
Main Thread Finished.
Custom Thread Started.
User threads versus Daemon threads in java threads
Daemon Threads
this threads in Java are low-priority threads that runs in the background to perform tasks such as garbage collection. Daemon thread in Java is also a service provider thread that provides services to the user thread.
User Threads
this threads are high-priority threads. The JVM will wait for any user thread to complete its task before terminating it
"keep in mind both User and Daemon threads wrapped upon OS threads"
Recently OpenJdk proposed Virtual threads with in project Loom (which they are User based as well) you may find more on Fibers and Continuations for the Java Virtual Machine threads in here.