Java Threads help !!! 1

[poolboy]

I'm a bit confused on join() in threads. I know what it does but it is the actual implemetation that's got me confused..I know it pauses the current thread to let the thread on which join() is called terminates...
so for example if I have threadA and threadB...

could you pleease provide a very simple line of code to show how to apply join() if threadA were to wait for threadB to finish !!

Regards
poolboy

 

[StevieK]

If you're trying to do something like ...

   threadA.join(threadB)   // No chance! [won't compile].

... then that's not the way it works. Instead, in the code for

threadA

(i.e. in it's

run()

method), you make a call like...

   threadB.join();

... (Assuming that threadA knows about threadB). Alternatively, if you had two threads created in you

main

program, for example...

   Thread threadA, threadB;
   threadA= new CountDownThread("Count-Down-Ten", 10);
   threadB= new CountDownThread("Count-Down-Twenty", 20);

... you could then start them both off,...

   threadA.start();
   threadB.start();

... and then make sure that they both finish before you continue any further in your

main

thread...

   try{
      threadA.join();
      threadB.join();
   }catch(InterruptedException ie){
      // Thread was Interrupted during join
   }

   blastOff();

... And it doesn't matter which one finishes first, because

blastOff()

wont get called until both threads finish (unless one of the threads happens to be Interrupted).

s-)
StevieK

 

[poolboy]

Hi Steviek
thanks a million !!
one more thing...in your code below :

***********"in the code for threadA (i.e. in it's run() method), you make a call like...

threadB.join();

... (Assuming that threadA knows about threadB). "**********

when you call the join() on threadB as above
does it imply that threadA must pause for threadB to complete or is it the other way round..

Thanks

 

[StevieK]

When

threadA

calls

threadB.join()

it then

wait

s (forever) until threadB dies (meaning that threadB has got to the end of its

run()

method).

So, if

threadA

was doing something like...

   lightMatch();

   try{
      threadB.join();
   }catch(InterruptedException ie){
   }

   igniteTouchPaper();

... Then threadA will not be able to call

igniteTouchPaper()

until either

threadB

dies or

threadB

is

interrupt

ed.

If, instead, you want

threadA

to be able to keep doing things until

threadB

dies (such as counting how long it's taking, and maybe letting the user know that something is happening), you can call

join()

will some arguments, telling it that it should only

wait

a maximum amount of milliseconds (and even nanoseconds, if you want).

So you could do something like...

   numberOfSecondsWeHaveWaitedForThreadBToDie= -1;

   for(;threadB.isAlive();)
   {
      numberOfSecondsWeHaveWaitedForThreadBToDie++;
      try{
         threadB.join(1000);
      }catch(InterruptedException ie){
      }
   }

... The only benefit of this over polling the

threadB.isAlive()

method is that when

threadB

eventually dies,

threadA

could potentially start executing immediately, as opposed to

sleep

ing until the next time it decides to poll.

Hope that was what you were after
StevieK s-)

 

[poolboy]

StevieK,
it was much more than I was after and I couldn't be more grateful to you. Thanks you very much !!

Regards
poolboy