Question

I have two threads modifying the same objects. The objects are custom, non-synchronized objects in an ArrayList (not a vector). I want to make these two threads work nicely together, since they are called at the same time.

Here is the only important method in thread 1.

public void doThread1Action() {
  //something...
  for(myObject x : MyArrayList){
    modify(x);
  }
}

Here is a method in thread 2:

public void doThread2Action() {   
  //something...
  for(myObject x : MyArrayList){
    modifyAgain(x);
  }
}

At the moment, when testing, I occasionally get `ConcurrentModificationExceptions“. (I think it depends on how fast thread 1 finishes its iterations, before thread 2 tries to modify the objects.)

Am I right in thinking that by simply appending synchronized to the beginning of these two methods, the threads will work together in a synchronized way and not try to access the ArrayList? Or should I change the ArrayList to a Vector?

Answer 1

A ConcurrentModificationException does not stem from modifying objects in a collection but from adding / removing from a collection while an iterator is active.

The shared resources is the collection and there must be a third method using and add/remove. To get concurrency right you must synchronize access to the collection resource in all methods that access it.

To avoid overly long synchronized blocks a common pattern may be to copy the collection in a synchronized block and then iterate over it. If you do it this way, be aware the problem you are talking about in first place (concurrent modification of your object) is again in place – but this time you can lock on another resource.