Looking for an approach to solve a multi threading problem. I have N number of tasks say 100. I need to run this 100 tasks using limited number of threads say 4. Task size is huge , so I dont want to create all the tasks together. Each task will be created only when a free thread is available from the pool. Any recommended solution for the same.
You could use a BlockingQueue to define the tasks. Have one thread create the tasks and add them to the queue using put, which blocks until there's space in the queue. Then have each worker thread just pull the next task off of the queue. The queue's blocking nature will basically force that first thread (that's defining the tasks) to not get too far ahead of the workers.
This is really just a case of the producer-consumer pattern, where the thing being produced and consumed is a request to do some work.
You'll need to specify some way for the whole thing to finish once all of the work is done. One way to do this is to put N "poison pills" on the queue when the generating thread has created all of the tasks. These are special tasks that just tell the worker thread to exit (rather than doing some work and then asking for the next item). Since each thread can only read at most one poison pill (because it exits after it reads it), and you put N poison pills in the queue, you'll ensure that each of your N threads will see exactly one poison pill.
Note that if the task-generating thread consumes resources, like a database connection to read tasks from, those resources will be held until all of the tasks have been generated -- which could be a while! That's not generally a good idea, so this approach isn't a good one in those cases.