Question

Using Guava, I would like to have a map that has the following properties:

• A lot of reads, but very few writes.
• the data doesn’t expire.
• must be synchronized, so a write is “atomic” and multiple reads don’t interfere with each other.
• the map should use the MapConstraint API and a few of these MapConstraint are against the content of the map itself (typically if the records or another exists, do not overwrite it: throw an IllegalStateException instead). I see that the MapConstraint interface doesn’t give the Map being constrained.
• the MapConstraint‘s check must be done inside the synchronization part.

I’ve well thought about using a ReadWriteLock, but I’m wondering if the MapMaker can help me here since I’m not very familiar with that API.

So what are my options?

---

Edit: My goal is not a simple putIfAbsent: I need to perform several checks against the map before inserting the value, always in the synchronized write.

Answer 1

I’m not sure if you can do this easily with the MapConstraint semantics. You could make the MapConstraint aware of the underlying map, by passing it a reference to the map during construction:

MapConstraints.constrainedMap(map, new MyCustomMapConstraint(map));

But it would be ugly / risky. Somebody could erroneously do:

MapConstraint constraint = new MyCustomMapConstraint(firstMap);
Map constrainedMap = MapConstraints.constrainedMap(secondMap, constraint);

Plus, it wouldn’t solve the synchronization problem.

---

If I were to do this, I would use the “putIfAbsent” method provided by ConcurrentMap. I would create a ConcurrentMap wrapper using ForwardingConcurrentMap:

public class ProtectionistMap<K, V> extends ForwardingConcurrentMap<K, V> {

    private final ConcurrentMap<K, V> delegate;

    public ProtectionistMap(ConcurrentMap<K, V> delegate) {
        this.delegate = checkNotNull(delegate);
    }

    @Override
    protected ConcurrentMap<K, V> delegate() {
        return delegate;
    }

    @Override
    public V put(K key, V value) {
        V result = putIfAbsent(key, value);

        // The second part of the test is necessary when a map may contain null values...
        if (result != null || value == null && containsKey(key)) {
            throw new IllegalArgumentException("Map already had an entry for key " + key);
        }
        return result;
    }

    @Override
    public void putAll(Map<? extends K, ? extends V> map) {
        standardPutAll(map);
    }
}