What is a daemon thread in Java

Question

Can anybody tell me what daemon threads are in Java

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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

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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

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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

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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

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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

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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

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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  quot Main Thread ending quot               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  quot Hello from Worker  quot  count                  try                   sleep 5000                 catch  InterruptedException e                       handle exception here

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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  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

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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  lt  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

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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

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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    - gt            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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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 gt javac DaemonThread java  C  java thread gt 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 gt

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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

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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

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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

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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

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