Thread-safe cache of one object in java

Question

let's say we have a CountryList object in our application that should return the list of countries. The loading of countries is a heavy operation, so the list should be cached.

Additional requirements:

• CountryList should be thread-safe
• CountryList should load lazy (only on demand)
• CountryList should support the invalidation of the cache
• CountryList should be optimized considering that the cache will be invalidated very rarely

I came up with the following solution:

public class CountryList {     private static final Object ONE = new Integer(1);      // MapMaker is from Google Collections Library         private Map<Object, List<String>> cache = new MapMaker()         .initialCapacity(1)         .makeComputingMap(             new Function<Object, List<String>>() {                 @Override                 public List<String> apply(Object from) {                     return loadCountryList();                 }             });      private List<String> loadCountryList() {         // HEAVY OPERATION TO LOAD DATA     }      public List<String> list() {         return cache.get(ONE);     }      public void invalidateCache() {         cache.remove(ONE);     } } 

What do you think about it? Do you see something bad about it? Is there other way to do it? How can i make it better? Should i look for totally another solution in this cases?

Thanks.

Answer 1

google collections actually supplies just the thing for just this sort of thing: Supplier

Your code would be something like:

private Supplier<List<String>> supplier = new Supplier<List<String>>(){     public List<String> get(){         return loadCountryList();     } };   // volatile reference so that changes are published correctly see invalidate() private volatile Supplier<List<String>> memorized = Suppliers.memoize(supplier);   public List<String> list(){     return memorized.get(); }  public void invalidate(){     memorized = Suppliers.memoize(supplier); }