Question

Say you have the following class

public class AccessStatistics {
  private final int noPages, noErrors;
  public AccessStatistics(int noPages, int noErrors) {
    this.noPages = noPages;
    this.noErrors = noErrors;
  }
  public int getNoPages() { return noPages; }
  public int getNoErrors() { return noErrors; }
}

and you execute the following code

private AtomicReference<AccessStatistics> stats =
  new AtomicReference<AccessStatistics>(new AccessStatistics(0, 0));

public void incrementPageCount(boolean wasError) {
  AccessStatistics prev, newValue;
  do {
    prev = stats.get();
    int noPages = prev.getNoPages() + 1;
    int noErrors = prev.getNoErrors;
    if (wasError) {
      noErrors++;
    }
    newValue = new AccessStatistics(noPages, noErrors);
  } while (!stats.compareAndSet(prev, newValue));
}

In the last two lines

newValue = new AccessStatistics(noPages, noErrors);
while (!stats.compareAndSet(prev, newValue)) 

Does it means the new created AccessStatistics instance has same reference as the current AccessStatistics instance. how could it be ? Can Anyone explain it . Thanks a lot.

Answer 1

stats.compareAndSet(prev, newValue) will fail and return false if the current reference held by stats is not prev.

Typically, in a multi-threaded environment, it is very possible that between prev = stats.get(); and stats.compareAndSet(prev, newValue); another thread would have modified the reference held by stats.

stats.compareAndSet(prev, newValue); really says:

• if stats still holds a reference to prev, as it was 5 lines before, update it to hold a reference to newValue
• if however another thread has already changed the reference held by stats since I last checked 5 lines ago, discard my calculation and loop to recalculate a new newValue.