How to start stop restart a thread in Java

Question

I am having a real hard time finding a way to start  stop  and restart a thread in Java   Specifically  I have a class Task  currently implements Runnable  in a file Task java  My main application needs to be able to START this task on a thread  STOP  kill  the thread when it needs to  and sometimes KILL  amp  RESTART the thread     My first attempt was with ExecutorService but I can t seem to find a way for it restart a task  When I use  shutdownnow   any future call to  execute   fails because the ExecutorService is  shutdown      So  how could I accomplish this

User · Answer

As stated by Taylor L  you can t just  stop  a thread  by calling a simple method  due to the fact that it could leave your system in an unstable state as the external calling thread may not know what is going on inside your thread   With this said  the best way to  stop  a thread is to have the thread keep an eye on itself and to have it know and understand when it should stop

User · Answer

Sometimes if a Thread was started and it loaded a downside dynamic class which is processing with lots of Thread currentThread sleep while ignoring interrupted Exception catch es   one interrupt might not be enough to completely exit execution   In that case  we can supply these loop-based interrupts   while th isAlive         log trace  Still processing Internally  Sending Interrupt         th interrupt        try           Thread currentThread   sleep 100         catch  InterruptedException e            e printStackTrace

User · Answer

You can t restart a thread so your best option is to save the current state of the object at the time the thread was stopped and when operations need to continue on that object you can recreate that object using the saved and then start the new thread   These two articles Swing Worker and Concurrency may help you determine the best solution for your problem

User · Answer

Once a thread stops you cannot restart it  However  there is nothing stopping you from creating and starting a new thread   Option 1  Create a new thread rather than trying to restart   Option 2  Instead of letting the thread stop  have it wait and then when it receives notification you can allow it to do work again  This way the thread never stops and will never need to be restarted   Edit based on comment   To  kill  the thread you can do something like the following   yourThread setIsTerminating true      tell the thread to stop yourThread join       wait for the thread to stop

User · Answer

You can start a thread like       Thread thread new Thread new Runnable                      Override                 public void run                         try                             Do you task                      catch  Exception ex                           ex printStackTrace                                       thread start     To stop a Thread      thread join     it will kill you thread       if you want to know whether your thread is alive or dead you can use System out println  Thread is   thread isAlive       Its advisable to create a new thread rather than restarting it

User · Answer

There s a difference between pausing a thread and stopping killing it  If stopping for you mean killing the thread  then a restart simply means creating a new thread and launching   There are methods for killing threads from a different thread  e g   your spawner   but they are unsafe in general  It might be safer if your thread constantly checks some flag to see if it should continue  I assume there is some loop in your thread   and have the external  controller  change the state of that flag    You can see a little more in  http   java sun com j2se 1 4 2 docs guide misc threadPrimitiveDeprecation html  May I ask why you want to kill the thread and restart it  Why not just have it wait until its services are needed again  Java has synchronization mechanisms exactly for that purpose  The thread will be sleeping until the controller notifies it to continue executing

User · Answer

It is impossible to terminate a thread unless the code running in that thread checks for and allows termination   You said   Sadly I must kill restart it     I don t have complete control over the contents of the thread and for my situation it requires a restart   If the contents of the thread does not allow for termination of its exectuion then you can not terminate that thread   In your post you said   My first attempt was with ExecutorService but I can t seem to find a way for it restart a task  When I use  shutdownnow        If you look at the source of  shutdownnow  it just runs through and interrupts the currently running threads   This will not stop their execution unless the code in those threads checks to see if it has been ineterrupted and  if so  stops execution itself   So shutdownnow is probably not doing what you think   Let me illustrate what I mean when I say that the contents of the thread must allow for that thread to be terminated   myExecutor execute new Runnable      public void run       while  true        System out println  running                myExecutor shutdownnow      That thread will continue to run forever  even though shutdownnow was called  because it never checks to see if it has been terminated or not   This thread  however  will shut down   myExecutor execute new Runnable      public void run       while   Thread interrupted          System out println  running                myExecutor shutdownnow      Since this thread checks to see whether or not it has been interrupted   shut down   terminated   So if you want a thread that you can shut down  you need to make sure it checks to see if it has been interrupted   If you want a thread that you can  shut down  and  restart  you can make a runnable that can take new tasks as was mentioned before   Why can you not shut down a running thread   Well I actually lied  you can call  yourThread stop    but why is this a bad idea   The thread could be in a synchronized  or other critical section  but we will limit ourselves to setions guarded by the syncrhonized key word here  section of code when you stop it   synch blocks are supposed to be executed in their entirity and only by one thread before being accessed by some other thread   If you stop a thread in the middle of a synch block  the protection put into place by the synch block is invalidated and your program will get into an unknown state   Developers make put stuff in synch blocks to keep things in synch  if you use threadInstance stop   you destroy the meaning of synchronize  what the developer of that code was trying to accomplish and how the developer of that code expected his synchronized blocks to behave

User · Answer

If your task is performing some kind of action in a loop there is a way to pause restart processing  but I think it would have to be outside what the Thread API currently offers  If its a single shot process I am not aware of any way to suspend restart without running into API that has been deprecated or is no longer allowed   As for looped processes  the easiest way I could think of is that the code that spawns the Task instantiates a ReentrantLock and passes it to the task  as well as keeping a reference itself  Every time the Task enters its loop it attempts a lock on the ReentrantLock instance and when the loop completes it should unlock  You may want to encapsulate all this try finally  making sure you let go of the lock at the end of the loop  even if an exception is thrown   If you want to pause the task simply attempt a lock from the main code  since you kept a reference handy   What this will do is wait for the loop to complete and not let it start another iteration  since the main thread is holding a lock   To restart the thread simply unlock from the main code  this will allow the task to resume its loops   To permanently stop the thread I would use the normal API or leave a flag in the Task and a setter for the flag  something like stopImmediately   When the loop encountered a true value for this flag it stops processing and completes the run method

User · Answer

Review java lang Thread   To start or restart  once a thread is stopped  you can t restart that same thread  but it doesn t matter  just create a new Thread instance       Create your Runnable instance Task task   new Task           Start a thread and run your Runnable Thread t   new Thread task     To stop it  have a method on your Task instance that sets a flag to tell the run method to exit  returning from run exits the thread  If your calling code needs to know the thread really has stopped before it returns  you can use join      Tell Task to stop task setStopFlag true       Wait for it to do so t join      Regarding restarting  Even though a Thread can t be restarted  you can reuse your Runnable instance with a new thread if it has state and such you want to keep  that comes to the same thing  Just make sure your Runnable is designed to allow multiple calls to run

[java] How to start/stop/restart a thread in Java?

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