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Do java threads get deleted when they finish
(3 answers)
Closed 7 years ago.
I want to know if a thread in java closes itself when run method ends.
I mean, I have a new thread declaration:
new Thread(new SubmitDataOnBackground(handler.getIDValue(), data, this.context)).start();
And then, in SubmitDataOnBackground I have this run method:
public void run() {
SubmitDataHandler submit = new SubmitDataHandler(ID, data, this.context);
submit.buildAndSubmitData();
}
After buildandSubmitData finishes, does the thread close itself or I have to add any code somewhere?
I am not sure if I am leaving a new thread opened each time I call this method or it is ok.
My application is a server so it will never ends because it is active the whole time. I just want to know the amount of threads is not outnumbered because it just creates new ones without closing the others when finish.
Threads close themselves after the run method has been called. Read this for further information https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html
EDIT: If you want to avoid this behaviour, I recommend using ThreadPools.
Yes, a thread finishes when run() method execution ends. You can read more on threads and concurency in general here
One tip here - when using multiple threads that are started and finished all the time, it is a good idea to use a thread pool. That is because creating a thread is quite a heavy operation.
Threads are terminated after finishing their jobs (when the execution of run() ends). If you want to check, use isAlive().
Yes, threads are terminated after finishing their specified jobs (sequence of instructions) in run() method.
However, the thread object that has been created still exists, allowing you call it again with Thread.start() to create a new Thread.
If you want to be sure that your thread run method ends before continuing doing something more, try to use the method Thread.join() in the same place where you are working with threads.
Read this for further information about that:
https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#join%28%29
In order to actually make a Thread stop itself, the process is quite simple. All you need to do is simply let the run method run out and return.
public void run(){
// implement your code
// Just about to return and the Thread will then stop soon after
}
Note that the thread will not necessarily be declared finished immediately after the run method has finished, as the Java Virtual Machine (JVM) still needs to finish it off in the background, but it should terminate completely soon after.
In other words, when a normal Thread (also referred to as a user Thread) is created, it is expected that it will complete its work and not shut down permanetly. The JVM will not terminate until all user Threads have finished, or until a call is made to the System.exit() method, which terminates the JVM abruptly.
EDIT: System.exit() does not stop the JVM abruptly, it executes all the shutdown hooks first. Runtime.getRuntime().halt() stops the JVM without any further processing.
Related
So I have some code, I am creating 6 threads, in my main thread, that run some code. I start the threads. I then call join() on the threads, so that the main thread waits for them all to die before continuing with execution.
Now, I am using some really basic and most likely inaccurate way to measure how long my code takes to run. Just calls to get the system time at the start, the end, and then print the difference.
Lets say it is taking, for example, around 500ms to run all of my code.
I decided to remove the calls to join() for each thread, and instead I just told my main thread to sleep for 20ms. This resulted in my code finishing in around 200ms, and the main thread managed to continue with execution with the proper data from the worker threads - i.e. the 6 worker threads must have finished in that 20ms wait.
THEREFORE, why is it taking so much longer when I use .join on each worker thread? Naturally, I cannot keep the call in the main method to sleep(20), and would rather use something like join()'s
The problem with multi-threaded bugs is you can appear to be working when it is not working reliably. It is possible your threads are doing something you don't need at the end, or you joining thread doesn't use the results right away. In any case, I suggest you wait for the result correctly.
BTW I would use an ExecutorService as this allows you to recycle your threads and wait for just the results you need in the form of a Future<MyResult> note this also captures and Exception/Error thrown as well.
It is also possible that your code is simply not getting finished if you take out the joins. If your main function exits without joining on all of its threads, then it is possible some are getting set as daemon threads via setDaemon(), which would stop the program cleanup from waiting on them.
Do you own all the code involved?
Java Thread can have begining,end and sequence,What does that mean?
I think all it means is that a thread executes a sequence of actions. It's expressing that concept pretty badly, to be honest.
In other words:
You create a Thread, ideally passing it a Runnable. (You can extend Thread instead and override its run method but that's generally frowned upon.)
You call start on it
The thread which called start continues executing the next statement in its program
The run method executes in the separate thread, independently of the thread that started it. The behaviour in here is what I believe is meant by the "sequence"
The new thread eventually ends due to one of the following conditions:
Its run method completes normally
Its run method completes with an exception
If it's a daemon thread, it can terminate as part of the JVM terminating due to all the non-daemon thread exiting
I'm trying to implement a sort of interrupt process into my java program so that if an operation takes longer than 5 minutes, i can kill it.
Is there any sort of generic way I can do this? I'm using an external API to carry out very processor intensive calculations and it already multithreads the process so can I still use the executor class to do this?
-edit-
Ended up solving it by using a bash script wrapper function. It kills the PID after a timeout.
It's considered unsafe to kill or forcefully stop a Thread because it may leave the program in an undetermined state, which will later cause a crash or other more serious problem. Instead, you should design your worker thread to periodically check the interrupt flag via Thread#isInterrupted or Thread#interrupted and exit if it is set. Then, using another thread, you can signal to the worker thread that it should stop by calling interrupt() on the worker thread, which will result in the worker thread detecting the interrupt or possibly receiving an InterruptedException if it is blocking inside your code or the third party code.
Depending on how your thread is coded (ie. whether it would properly terminate when interrupted), you could use the provided Thread.join(millis) or Thread.join(mills, nanos) method calls.
Something like this:
Thread myThread
// ... start myThread
myThread.join(300000); // 5mins in millis
if (myThread.isAlive()) {
myThread.interrupt();
}
Inside the thread itself, you would want to ensure that you .yield() at relevant points and properly handle an InterruptedException to allow this kind of logic to work.
Of course this is an "ideal" kinda situation - if the thread is blocked due to some outside process, and cannot handle the .interrupt(), then it will not work very well.
HTH
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 & 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?
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
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.
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.
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.
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.
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.
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();
}
}
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.
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.
This question already has answers here:
How to abort a thread in a fast and clean way in java?
(15 answers)
Closed 2 years ago.
I'm in the process of writing a piece of code that connects to a server spawns a bunch of threads using that connection and does a bunch of "stuff".
There are certain instances where the connection fails and I need to stop everything and start from scratch with a new object.
I wanted to clean up after the object but calling thread.stop on the threads, but this method is seemingly deprecated.
What is the recommended alternative to doing this? Should I write my own cleanup and exit method for each of the threads? Set the thread to null? or something else?
Assuming your threads are reasonably under your control - i.e. they're not calling anything which is going to potentially wait forever without your code executing - I would shut it down with a simple (but thread-safe - use volatile!) flag.
See this article for an example in C# - the Java equivalent should be easy to work out. Calling interrupt won't have any effect until the thread next waits, and stop can leave your app in a hard-to-predict state. Wherever possible, go for a clean, orderly shutdown instead.
Use your_thread.interrupt and check in your thread if Thread.interrupted() return true. If so, close your thread properly.
private Thread m_CleanupThread = null;
public void threadCleanUp(){
m_CleanupThread = new Thread(this);
m_CleanupThread.Start();
}
This thread will terminate and garbage collector will do the rest.