How to prevent nested synchronized blocks when iterating over a collection

Question

In a multithreaded Java application I need to iterate over a collection of objects. Since both the collection and the objects could be modified by another thread while I iterate over them, I need to use synchronization.

However nested synchronized blocks are not recommended since they could lead to deadlocks. How would I solve this problem?

Collection<Data> dataCollection = something.getDataCollection();

synchronized ( dataCollection ) {
  for ( final Data data : dataCollection ) {
    synchronized ( data ) {
      data.doSomething();  // doSomething() changes object state
    }
  }
}

Answer 1

I think you can use CopyOnWriteArrayList instead of the outer synchronization.

A thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array. This is ordinarily too costly, but may be more efficient than alternatives when traversal operations vastly outnumber mutations, and is useful when you cannot or don't want to synchronize traversals, yet need to preclude interference among concurrent threads

Answer 2

You can take a copy of the collection and only lock one object at a time.

Collection<Data> dataCollection = something.getDataCollection();
Collection<Data> copy;
synchronized ( dataCollection ) {
  copy = new ArrayList<Data>(dataCollection);
}

for (Data data : copy) {
    synchronized ( data ) {
      data.doSomething();  // doSomething() changes object state
    }
}