Home  Synchronization and concurrency  wait/notify  final  volatile  synchronized keyword  Java threading  Deadlock (and avoiding it)  Java 5: ConcurrentHashMap  Atomic variables  Explicit locks  Queues  Semaphores  CountDownLatch  CyclicBarrier

Volatile arrays in Java

A slight complication of Java volatile fields, and one sometimes overlooked, is that declaring an array volatile does not give volatile access to its fields!. At least, it doesn't when elements of the array are accessed with "normal" Java syntax. In other words:

  • it is unsafe to call arr[x] = y on an array (even if declared volatile) in one thread and then expect arr[x] to return y from another thread;
  • on the other hand, it is safe to call arr = new int[] (or whatever) and expect another thread to then read the new array as that referenced by arr: this is what is meant by declaring the array reference volatile.

So, what do we do if we want a truly volatile array in Java, where the array's individual fields have volatile semantics?

Solution 1: use AtomicIntegerArray or AtomicLongArray

The AtomicIntegerArray class implements an int array whose individual fields can be accessed with volatile semantics, via the class's get() and set() methods. Calling arr.set(x, y) from one thread will then guarantee that another thread calling arr.get(x) will read the value y (until another value is read to position x).

Solution 2: re-write the array reference after each field write

This is slightly kludgy and slightly inefficient (since what would be one write now involves two writes) but I believe it is theoretically correct. After setting an element of the array, we re-set the array reference to be itself:

volatile int[] arr = new int[...];
...
arr[4] = 100;
arr = arr;

The marginal benefit of this technique could be:

  • it may allow you to fix some broken code with minimal changes if you have code already (incorrectly) using a volatile array and expecting thread-safe element access;
  • it saves us creating the wrapper object around the array (which is what happens with IntegerArray and LongArray);
  • it's more or less the only option for array types with no available atomic wrapper (e.g. a float array).

Article written by Neil Coffey (@BitterCoffey).

Software

 LetterMeister (word puzzle game for iPhone)
 Currency Quoter (currency converter/predictor)
 French Vocab Games for iPhone/iPad
 Vocabularium: create Spanish vocab podcasts


Java programming articles and tutorials on this site are written by Neil Coffey (@BitterCoffey). Suggestions are always welcome if you wish to suggest topics for Java tutorials or programming articles, or if you simply have a programming question that you would like to see answered on this site. Most topics will be considered. But in particular, the site aims to provide tutorials and information on topics that aren't well covered elsewhere, or on Java performance information that is poorly described or understood. Suggestions may be made via the Javamex blog (see the site's front page for details).
Copyright © Neil Coffey 2014. All rights reserved.