Java array synchronization

Question

I have a kind of async task managing class, which has an array like this:

public static int[][][] tasks;

Mostly I access the cells like this:

synchronized(tasks[A][B]) {
  // Doing something with tasks[A][B][0]
}

My question is, if I do this:

synchronized(tasks[A]) {
  // ...
}

will it also block threads trying to enter synchronized(tasks[A][B])? In other words, does a synchronized access to an array also synchronizes the accsess to it's cells? If not, how to I block the WHOLE array of tasks[A] for my thread?


Edit: the answer is NO. When someone is in a synchronized block on tasks[A] someone else can simultaniously be in a synchronized block on tasks[A][B] for example - because it's a different object. So when talking about accessing objects from one place at a time, arrays are no different: to touch object X from one place at a time you need to surround it by synchronized(X) EVERYWHERE you touch it.

Answer 1

No. Each array is an Object (with a monitor) unto itself; and the array tasks[A] is a distinct object from the array tasks[A][B]. The solution, if you want to synchronize all accesses to "sub" arrays of tasks[A] is simply that you must do synchronized (tasks[A]). If all accesses into the descendant objects (say, tasks[A][B]) do this, then any further synchronization is redundant.'

It appears your underlying question is actually something like "how can I safely modify the structure as well as the contents of a data structure while retaining the best concurrency possible?" If you augment your question a bit more about the problem space, we might be able to delve deeper. A three-dimensional array may not be the best solution.