CODEWITHSUNDEEP
javaarraysmultithreading
Numeron
Numeron

Reputation: 8823

Efficient multithreaded array building in Java

I have many threads adding result-like objects to an array, and would like to improve the performance of this area by removing synchronization.

To do this, I would like for each thread to instead post their results to a ThreadLocal array - then once processing is complete, I can combine the arrays for the following phase. Unfortunately, for this purpose ThreadLocal has a glaring issue: I cannot combine the collections at the end, as no thread has access the collection of another.

I can work around this by additionally adding each ThreadLocal array to a list next to the ThreadLocal as they are created, so I have all the lists available later on (this will require synchronization but only needs to happen once for each thread), however in order to avoid a memory leak I will have to somehow get all the threads to return at the end to clean up their ThreadLocal cache... I would much rather the simple process of adding a result be transparent, and not require any follow up work beyond simply adding the result.

Is there a programming pattern or existing ThreadLocal-like object which can solve this issue?

Upvotes: 4

Views: 108

Answers (1)

dimo414
dimo414

Reputation: 48824

You're right, ThreadLocal objects are designed to be only accessible to the current thread. If you want to communicate across threads you cannot use ThreadLocal and should use a thread-safe data structure instead, such as ConcurrentHashMap or ConcurrentLinkedQueue.

For the use case you're describing it would be easy enough to share a ConcurrentLinkedQueue between your threads and have them all write to the queue as needed. Once they're all done (Thread.join() will wait for them to finish) you can read the queue into whatever other data structure you need.

Upvotes: 1

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