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What do you think is the best way for obtaining the results of the work of a thread? Imagine a Thread which does some calculations, how do you warn the main program the calculations are done?
You could poll every X milliseconds for some public variable called "job finished" or something by the way, but then you'll receive the results later than when they would be available... the main code would be losing time waiting for them. On the other hand, if you use a lower X, the CPU would be wasted polling so many times.
So, what do you do to be aware that the Thread, or some Threads, have finished their work?
Sorry if it looks similar to this other question, that's probably the reason for the eben answer, I suppose. What I meant was running lots of threads and know when all of them have finished, without polling them.
I was thinking more in the line of sharing the CPU load between multiple CPU's using batches of Threads, and know when a batch has finished. I suppose it can be done with Futures objects, but that blocking get method looks a lot like a hidden lock, not something I like.
Thanks everybody for your support. Although I also liked the answer by erickson, I think saua's the most complete, and the one I'll use in my own code.
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If one thread has to wait for the output of another thread you should make use of a condition variable. The thread interested in the output of the other threat should call cv.wait (). This will cause the current thread to block.
A thread can be created by implementing the Runnable interface and overriding the run() method. The current thread is the currently executing thread object in Java. The method currentThread() of the Thread class can be used to obtain the current thread. This method requires no parameters. A program that demonstrates this is given as follows:
The parameter of the ThreadStart is the method that is executed by the new thread. Once a thread it created, it needs to call the Start method to actually start the thread. The following code snippet creates a new thread, workerThread, that will execute code in the Print method. Thread workerThread = new Thread (new ThreadStart (Print));
Pass in the function you want to run on a new thread to the target argument and any arguments that your function might need to the args argument and any keyword arguments to the kwargs argument. I think this is significantly easier to understand than the vast majority of answers, and this approach requires no extra imports!
Don't use low-level constructs such as threads, unless you absolutely need the power and flexibility.
You can use a ExecutorService such as the ThreadPoolExecutor to submit() Callables. This will return a Future object.
Using that Future object you can easily check if it's done and get the result (including a blocking get() if it's not yet done).
Those constructs will greatly simplify the most common threaded operations.
I'd like to clarify about the blocking get():
The idea is that you want to run some tasks (the Callables) that do some work (calculation, resource access, ...) where you don't need the result right now. You can just depend on the Executor to run your code whenever it wants (if it's a ThreadPoolExecutor then it will run whenever a free Thread is available). Then at some point in time you probably need the result of the calculation to continue. At this point you're supposed to call get(). If the task already ran at that point, then get() will just return the value immediately. If the task didn't complete, then the get() call will wait until the task is completed. This is usually desired since you can't continue without the tasks result anyway.
When you don't need the value to continue, but would like to know about it if it's already available (possibly to show something in the UI), then you can easily call isDone() and only call get() if that returns true).
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