How to stop a thread in java and let the same thread to do another thing?

Question

I start one thread to download contents from the internet, at one time, the thread is downloading one file, and then i want it stop and begin to download another file, what should i do? should i count on the concurrency mechanism in java?

You can't magic a thread into another state, but you can write the code that runs on the thread to do something else. However, I suspect the proper answer to this question is "Why do you care what thread does what?" — Mike Caron, Jun 10 '11 at 01:55
Could you clarify your question? Do you want to start multiple threads each downloading simultaneously while the main thread waits for them to complete, or do you want to have multiple threads downloading each of which you can halt/restart when the main thread wants? — Bohemian, Jun 10 '11 at 02:11

trutheality · Accepted Answer · 2011-06-12T18:55:52.453

4

You can start your thread using a single-thread executor, then when you kill it (gracefully I hope) and start a new one it will ensure that it's using the same thread.

// Suppose you have a DownloadFile class that implements Runnable
DownloadFile task1 = new DownloadFile();

...

ExecutorService exec = Executors.newSingleThreadExecutor();

Future<Boolean> future = exec.submit( task1, Boolean.TRUE );

...

// Cancel the task
future.cancel();

// Give the executor another task
DownloadFile task2 = new DownloadFile();
...
exec.submit( task2, Boolean.TRUE );

Other useful docs:

edited Jun 12 '11 at 18:55

answered Jun 10 '11 at 01:55

trutheality

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killing it gracefully == getting the Future and calling `Future.cancel`. – jtahlborn Jun 10 '11 at 03:08
http://download.oracle.com/javase/6/docs/api/java/util/concurrent/ExecutorService.html Has a general example on using an `ExecutorService`. I'll add in a simple example to the answer. – trutheality Jun 12 '11 at 18:47

score 0 · Answer 2 · edited May 23 '17 at 11:48

0

Stopping thread using stop() method is not recommended.

I start one thread to download contents from the internet, at one time, the thread is downloading one file, and then i want it stop and begin to download another file, what should i do?

You need to use the sleep( ) method of Thread class to pause the same thread for some specified milliseconds before you can continue the work with the same thread.

Note : Once the thread has been stopped, it can't resume its work. It will result in IllegalThreadStateException.

See :

edited May 23 '17 at 11:48

Community

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answered Jun 10 '11 at 01:53

Saurabh Gokhale

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That doesn't seem to address the question at all. – trutheality Jun 10 '11 at 01:57
@trutheality : Why not ? It address the question correctly. We can't stop the thread and re-start the same stopped thread again. This is what OP is trying to achieve .... – Saurabh Gokhale Jun 10 '11 at 01:59
I'm pretty sure that what the OP meant by "stopping" isn't the same as "using `stop()`". I read it as "I have one thread doing work, I want to tell it to not do that anymore because I want to do something else, how do I make sure that I'm reusing the available resources, rather than having hanging do-nothing background threads when I do that?" – trutheality Jun 10 '11 at 02:04
@trutheality : In that case, let the OP come here and speak what does he exactly mean ? .... and yes, in your case, you need to start a **new** thread – Saurabh Gokhale Jun 10 '11 at 02:07

Swaranga Sarma · Answer 3 · 2011-06-10T06:43:39.737

To interrupt a thread from a current processing and redirect it to something else is a classic synchronization problem. This is done as follows:

Have a job queue. The jobs in the job queue [or more specifically the objects in the job queue] should have a method [say process()] that the thread will execute. The thread generally does not know the details of the process() method. It only cares that it has to run the process() method.

The thread must wait on the job queue. This is done as follows:

Job j = null;

synchronized(jobQueueInstance)
{
    if(jobQueueInstance.isEmpty())
    {
        jobQueueInstance.wait();
    }

    j = jobQueueInstance.poll(); //get the job from the head of the queue
}

try
{
    j.process();
}
catch(InterruptedException ex)
{
    //thread is interrupted, means it needs to abandon this job and fetch a new one from the queue
}

I hope this helps. However I have omitted a few things to oversimplify the problem:

I have omitted the Job class declaration, I guess it will either be an interface or an abstract class.
I have also omitted the part where some other thread will add a new Job() and notify() on the jobQueueInstance.
Also omitted the part about the thread that will interrupt() the "busy" job-running thread.

NOTE Threads 3 and 4 can be the same thread.

score 0 · Answer 4 · answered Jun 10 '11 at 07:31

You should place a timeout on your download using the software for downloading. This usually results in an Exception which you can catch, clean up what you are doing and continue on.

Unless the software you are using supports interrupts or time out settings, there is no safe way to force this to happen externally. You can use stop() safely provided your intention is to shutdown at soon as possible. (In which case System.exit() is a better option)

If you have a library which you have no choice but to kill, you need to run it in a separate process and kill the whole process. This will ensure any resources used will be cleaned up by the OS. (Assuming it hasn't left temporary files etc. ;)

How to stop a thread in java and let the same thread to do another thing?

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