两个线程的同步 ArrayList - 写入和读取

Question

I need to solve this problem. I have console app in which I am inserting some String values. The console app is still running, user can add more and more entries and every 1 minute, there should be another Thread which will print some List statistic (eg size etc.). When I use Main Thread for user's console and different Thread for counting and printing this statistic data to the console (both working with the same List). It is enough to use:

List<String> list = Collections.synchronizedList(new ArrayList<>());

Or I need to user volatile and access the List in the synchronized block as well?

Thank you!

Answer 1

It depends what you want to do with the list.

Collections.synchronizedList(...) simply wraps every method with a synchronized block, synchronizing on itself. The synchronization starts when the invoked method starts executing, and stops when it stops executing.

Notionally, it the list is wrapped like this:

class SynchronizedList<T> implements List<T> {
  private List<T> delegate;

  @Override public int size() {
    synchronized (this) {
      return delegate.size();
    }
  }

  // ...
}

As such, doing simple things like list.size() , list.add(...) , list.get(...) don't require any further synchronization.

However, if you want to do more complex things, for example iterating the list, yes, you need additional synchronization:

synchronized (list) {
  for (String s : list) { ... }
}

This is because the synchronization stops as soon as the hidden call to list.iterator() completes; hence, other threads would be able to modify the list while you are iterating.