Question

In java’s Observer pattern classes Observer and Observable, are calls made to Observable objects’s notifyObservers(Object arg0), in different threads, thread-safe?

Example:
I have multiple threads, all Observables, that will be calling notifyObservers(…) ever so often. All these threads report to a single Observer object.

Will I encounter concurrency issues, and what would be a better way to solve this?
I am aware of a possible solution using event listeners. However I am unsure how to implement it and also, I would like to stick with the Observer pattern implementation, if possible.

Answer 1

From the source code (I have Java 5 source, but it should be the same for Java 6 and 7) it seems like you only have synchronization on the Observable itself.

From the notifyObservers(...) method (in Observable):

synchronized (this) {
  //the observers are notified in this block, with no additional synchronization
}

Thus if the Observer doesn’t change any shared data it should be fine. If it does – you could have multiple calls to update(Observable, Object) with different Observables – you’d need to add synchronization on that shared data yourself.