how to use multiple threads, in sync?

Question

i have multiple threads, who all run from one object. i want the "main thread" to run alone until a certain point, then it waits and all the other threads run together, then the main thread wakes, etc..... i am having trouble synchronizing my threads. i either get an Illegal Monitor State Exception, or it gets stuck in one of the "wait" loops that are suppose to receive a "notify" that never arrives.

more specifically, i have an object with an array. each cell in the array has a thread that checks the adjacent cells and then changes it's cell with that information. in order to make the changes orderly, i want all the cells to first make the check of their adjacent cells and keep the value they produced, then wait. when all of them are done, the main thread will wake all of them up and they will update their respective cells.

i looked up how "wait" and "notify" work, but i still don't understand how they sync. from what i understand i need to connect them all to one object, and then that object is the "lock", so if i use "synchronize" on its methods only one thread can approach it at a time? how can i make sure a "wait" method will always have a "notify" to end it?

Edit: the method basically runs Conway's game of life. the main orientation of the code is like so: the class LifeMatrix extends JPanel. it had an array of panels, each is either "dead or alive" (true/false). the class RunMatrixThread extends thread, and is the "main thread" that coordinates the code. the class CellThead extends thread, and a CellThread is made for every cell in the matrix. so my idea was to give all the threads the "LifeMatrix" as an observer, but if i try to notify the LifeMatrix Object (with matrix.notify()) it gives me the Illigal Monitor State Exception, and if i try to use "notify all" it gets stuck in RunMatrixThread's wait() command. also, do i notify an object? or do i notify the threads that are waiting?

Answer 1

Don't use parallelization. Before using threads think if you really can parallelize your job because if all of your tasks have to be sync with each other use threads won't give you better perfomance in terms of execution time. Say that you have an array of objects [a,b] if a must waiting for some changes on b, you can't treat a and b separately so you can't parallelize your job. On the contrary if you need to process a, b and all the elements of your array and at the end perform some computation on them you can Join the threads with join() method. When you call join method you basically join threads branches in one (the main thread). A new thread will fork your main thread and join will join these threads.

Answer 2

If you're trying to get "worker threads" to do parcels of work that are authorized/initiated/doled-out by a "main" thread, then you probably should be using a thread pool (eg, https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html )

A thread pool takes care of creating the worker threads and "synchronizing" their activity with the main thread, and it lets you focus on the task (or tasks) that the workers perform.

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each cell in the array has a thread that...

For Conway's Life, that's way too many worker threads. If the work is purely compute-bound, then there's no point in having many more threads than your host has processors to execute them.

If I was coding life for a host with N processors, I would use a thread pool that had N threads. And, In each generation, I would have the main thread submit N tasks to the pool: Each task would do one horizontal stripe of the board.

Answer 3

Ok, first of all i want to thank all of you for trying to help, some of the links you gave me were very helpful.

I found out what my problem was: i was trying to use wait/notify methods from 2 different types of threads at the same time, on the same object. i had the 'CellThread' that used wait and 'notifyAll', and i had the 'RunMatrixThread' that did the same. they of course had "synchronized" methods, but because they were 2 different TYPES of threads, the two types weren't in sync with EACH OTHER. what solved the problem was that i made 2 new synchronized methods within the 'RunMatrixThread' class, one for waiting and one for notifying, and then just called those methods from all threads (from both thread classes) whenever i wanted to wait/notify. in this way, there was a unified object that had a lock on everything.

PS: i know its a bad idea to use so many threads. it was the coarse's assignment, and they required we do it this way.