Synchronized and local copies of variables

Question

I\'m looking at some legacy code which has the following idiom:

Map myMap = someGlobalInstance.getMap();
synchronized (myMap) {
    item =         


        

Answer 1

Alex is correct in that adding a synchronized wrapper by calling Collections.synchronizedMap(Map) is a typical approach here. However, if you take this approach there may still be situations where you need to synchronized on the Map's lock; e.g. when iterating over the map.

Map syncMap = Collections.synchronizedMap(new HashMap());

// Synchronized on map to prevent ConcurrentModificationException whilst iterating.
synchronized (syncMap) {
  for (Map.Entry entry : syncMap.entrySet()) {
    // Do work
  }
}

In your example, the warning from IDEA can be ignored, as it's evident that your local variable: map is retrieved from somewhere else (someGlobalInstance) rather than being created within the method, and can therefore potentially be accessed from other threads.