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

Thanks you all guys, especially to user “gid” who gave the idea.

My target was to optimize the performance for the get() operation considering the invalidate() operation will be called very rare.

I wrote a testing class that starts 16 threads, each calling get()-Operation one million times. With this class I profiled some implementation on my 2-core maschine.

Testing results

Implementation              Time
no synchronisation          0,6 sec
normal synchronisation      7,5 sec
with MapMaker               26,3 sec
with Suppliers.memoize      8,2 sec
with optimized memoize      1,5 sec

1) “No synchronisation” is not thread-safe, but gives us the best performance that we can compare to.

@Override
public List<String> list() {
    if (cache == null) {
        cache = loadCountryList();
    }
    return cache;
}

@Override
public void invalidateCache() {
    cache = null;
}

2) “Normal synchronisation” – pretty good performace, standard no-brainer implementation

@Override
public synchronized List<String> list() {
    if (cache == null) {
        cache = loadCountryList();
    }
    return cache;
}

@Override
public synchronized void invalidateCache() {
    cache = null;
}

3) “with MapMaker” – very poor performance.

See my question at the top for the code.

4) “with Suppliers.memoize” – good performance. But as the performance the same “Normal synchronisation” we need to optimize it or just use the “Normal synchronisation”.

See the answer of the user “gid” for code.

5) “with optimized memoize” – the performnce comparable to “no sync”-implementation, but thread-safe one. This is the one we need.

The cache-class itself:
(The Supplier interfaces used here is from Google Collections Library and it has just one method get(). see http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/base/Supplier.html)

public class LazyCache<T> implements Supplier<T> {
    private final Supplier<T> supplier;

    private volatile Supplier<T> cache;

    public LazyCache(Supplier<T> supplier) {
        this.supplier = supplier;
        reset();
    }

    private void reset() {
        cache = new MemoizingSupplier<T>(supplier);
    }

    @Override
    public T get() {
        return cache.get();
    }

    public void invalidate() {
        reset();
    }

    private static class MemoizingSupplier<T> implements Supplier<T> {
        final Supplier<T> delegate;
        volatile T value;

        MemoizingSupplier(Supplier<T> delegate) {
            this.delegate = delegate;
        }

        @Override
        public T get() {
            if (value == null) {
                synchronized (this) {
                    if (value == null) {
                        value = delegate.get();
                    }
                }
            }
            return value;
        }
    }
}

Example use:

public class BetterMemoizeCountryList implements ICountryList {

    LazyCache<List<String>> cache = new LazyCache<List<String>>(new Supplier<List<String>>(){
        @Override
        public List<String> get() {
            return loadCountryList();
        }
    });

    @Override
    public List<String> list(){
        return cache.get();
    }

    @Override
    public void invalidateCache(){
        cache.invalidate();
    }

    private List<String> loadCountryList() {
        // this should normally load a full list from the database,
        // but just for this instance we mock it with:
        return Arrays.asList("Germany", "Russia", "China");
    }
}