JAVA Thread Concurrency

Question

I am using ExecutorService to launch multiple threads.

• Thread A
• Thread B
• Thread C and
• Thread D
• ....
• .... Thread N

All thread started concurrently. I am using a ConcurrentHashMap to store result of Thread A , Thread B , Thread C which can be used by Thread D in future.

Now it may happen that Thread A is still under execution when Thread D is looking for the output from Thread A.

This is applicable for Thread C which may be dependent on output of Thread D

Or Thread N which may be dependent on output of any of threads like Thread N-1 , or Thread N- 4 etc ...

What can be the best approach to handle this situation or to make thread A wait until other threads are ready with there output

Answer 1

I suggest having three tasks producing results which are queued into the same Executor or another executor. This way Thread D only does work when there is something to do.

A better option might be for Thread A to process its own work immediately (and the same for Thread B & C) in which case you don't need Thread D.

Answer 2

Sounds like you want to use join. Joining threads after starting will effectively make your program run in a liniear fashion. Thread A runs, Then B, then C then D will look for the results of the first 3 threads once thread C has finished.