Multi Threading

[Garandi Garandi]

I have the following code. Wants the main thread to wait until all other threads finishes execution.

  
public void test(){
int numberOfRequest = 100;
long startTime = System.currentTimeMillis();
for(int i = 0; i < numberOfRequest; i++){
Thread th = new Thread(new Runnable(){
public void run(){
System.out.println("Hello World");
}

});
th.start();
}
long endTime = System.currentTimeMillis();
long totalTime = endTime - startTime;
System.out.println("Total Time to run "+numberOfRequest +" is "+ totalTime +" millis");
}

Thank you
Garandi

[David Weitzman]

The pure JFC solution is to store all the threads in a collection and have the main thread call join() on each one after they're all launched.

If you have util.concurrent in the classpath anyway you can use CountDown or in Java 5.0 CountDownLatch
[ August 04, 2004: Message edited by: David Weitzman ]

[Rovas Kram]

The following modification has the main thread join each sub thread which will cause the main thread to wait for each sub thread to complete.

 public void test() {
int numberOfRequest = 100;
long startTime = System.currentTimeMillis();
for(int i = 0; i < numberOfRequest; i++) {
Thread th = new Thread(new Runnable() {
public void run() {
System.out.println("Hello World");
}
});
th.start();
                        try {
th.join();
}
catch (InterruptedException ex) {
// who cares
}
 
}
long endTime = System.currentTimeMillis();
long totalTime = endTime - startTime;
System.out.println("Total Time to run "+numberOfRequest +" is "+ totalTime +" millis");
} 

[David Weitzman]

Rovas, calling Thread.start() on one line and Thread.join() on the same thread a line later is not the most useful of patterns. It's like running the code without starting a thread, only there's a lot more overhead. I'm having trouble imagining circumstances when you'd ever want to join() immediately after start()ing. I guess it would keep the code from mucking with your ThreadLocals or throwing unexpected Errors/RuntimeExceptions at you.

I've added a word to my response above to clarify.

[Rovas Kram]

Thanks for pointing that out, David. The code that I submitted is not multi-threaded at all. As stated in the first reply(I'd use the user name if i could see - come on javaranch), I should put each sub-thread in a collection and then call join on each thread.

[Garandi Garandi]

I should put each sub-thread in a collection and then call join on each thread.

I was wondering how to code it, if you can help me with it; that would be very useful.
Thank you in advance
Garandi

[David Weitzman]

Collection threads = new LinkedList();
 
for each thread you want to start {
   Thread thread = ...;
   thread.start();
   threads.add(thread);
}
 
// All threads are started
 
for (Iterator it = threads.iterator(); it.hasNext();)
   ((Thread)it.next()).join();
 
// All threads are stopped