How to use ReadWriteLock?

Question

I'm the following situation.

At web application startup I need to load a Map which is thereafter used by multiple incoming threads. That is, requests comes in and the Map is used to find out whether it contains a particular key and if so the value (the object) is retrieved and associated to another object.

Now, at times the content of the Map changes. I don't want to restart my application to reload the new situation. Instead I want to do this dynamically.

However, at the time the Map is re-loading (removing all items and replacing them with the new ones), concurrent read requests on that Map still arrive.

What should I do to prevent all read threads from accessing that Map while it's being reloaded ? How can I do this in the most performant way, because I only need this when the Map is reloading which will only occur sporadically (each every x weeks) ?

If the above is not an option (blocking) how can I make sure that while reloading my read request won't suffer from unexpected exceptions (because a key is no longer there, or a value is no longer present or being reloaded) ?

I was given the advice that a ReadWriteLock might help me out. Can you someone provide me an example on how I should use this ReadWriteLock with my readers and my writer ?

Thanks,
E

Answer 1

I suggest to handle this as follow:

1. Have your map accessible at a central place (could be a Spring singleton, a static ...).
2. When starting to reload, let the instance as is, work in a different Map instance.
3. When that new map is filled, replace the old map with this new one (that's an atomic operation).

Sample code:

    static volatile Map<U, V> map = ....;

    // **************************

    Map<U, V> tempMap = new ...;
    load(tempMap);
    map = tempMap;

Concurrency effects :

• volatile helps with visibility of the variable to other threads.
• While reloading the map, all other threads see the old value undisturbed, so they suffer no penalty whatsoever.
• Any thread that retrieves the map the instant before it is changed will work with the old values.
  • It can ask several gets to the same old map instance, which is great for data consistency (not loading the first value from the older map, and others from the newer).
  • It will finish processing its request with the old map, but the next request will ask the map again, and will receive the newer values.

Answer 2

If the client threads do not modify the map, i.e. the contents of the map is solely dependent on the source from where it is loaded, you can simply load a new map and replace the reference to the map your client threads are using once the new map is loaded.

Other then using twice the memory for a short time, no performance penalty is incurred.

In case the map uses too much memory to have 2 of them, you can use the same tactic per object in the map; iterate over the map, construct a new mapped-to object and replace the original mapping once the object is loaded.