Question

I want to know if calculation of the function value and assignment of the result to a variable is an atomic operation in Java.

For example:

I have a thread-safe priority queue q. In q I save elements, and each element has a rank according to which it is placed in the queue. In addition I have a shared variable topRank that should always contain the rank of the topmost element in q. The following code is executed in parallel by number of threads:

element = q.remove(); // do something with element
topRank = q.peek();

could it happen that threadA would remove one elements from q and calculate value of q.peek() and just before assigning it to topRank would be interrupted by threadB, then threadB would remove one more element from q and update topRank. And then threadA would resume to assignment of incorrect value to the topRank.

A link to official literature would be very appreciated.

Thanks.

Answer 1

The short answer is that these operations are NOT atomic and you need to write the synchronization into your code. This is a HUGE topic and there is a lot to learn about writing thread-safe programs. Google for “java thread safety” and “java multithreading”.