Question

// in a context listener
public void contextInitialized(ServletContextEvent sce) {
        SessionListener.context = sce.getServletContext();
        HashMap<String, String> messages = new HashMap<>();
        context.setAttribute("messages", messages);
}

Now I want to access the messages map from various servlets – what about synchronization ?

Namely I want to add an element to the map (whose key must be unique) – so I have to try a couple of times maybe – except if there is some method in the java ee api (?)

EDIT : interested also in synchronizing access to a session scoped map

Answer 1

This map is a non-thread-safe object shared by multiple threads. So every access to the map should be synchronized. You have various options:

• have each client of the map synchronize on the map itself: very fragile
• use a thread-safe map like Collections.synchronizedMap() or ConcurrentHashMap. This will never let the map in an inconsistent state, but additional synchronization could still be needed for operations that should be atomic but involve several method calls on the map
• encapsulate the map into an object of your own, and make sure that this object provides all the necessary operations that need to be done on the map. These operations should of course perform the necessary synchronization.

The third solution is probably the best one. The second one might be OK if the operations on the map are very simple, and coveredby methods of the map.

Regarding the generation of unique and random strings, you could combine a UUID (for uniqueness) and a Random (or SecureRandom, depending on your requirements), for the randomness.