Question

If I have a common thread pool that queues runnable tasks and the runnable task is the same object every time.

What happens when it reaches a block of code (within the runnable task) that has a synchronised lock? and say 5 threads are waiting on the same lock?

When it unlocks, is the synchronised block a queue line waiting to be executed and new threads join the queue, or whoever managed to grab the block first?

If there isn’t a queue line, assuming I want an ordered execution, what is the best method to do so?

Answer 1

synchronized blocks do not have any defined order. you can use a java.util.concurrent.locks.ReentrantLock with “fair” locking to achieve the same affect as a synchronized block, but with guaranteed ordering.