While creating my app. architecture I faced the need for one structure, that will be described below.
I'm pretty sure, that there is a well known design pattern with the same functionality, because I think that problem, for which I develop it is really common.
I write my own implementation of this, but I always try to use "build in language" implementations of patterns, so - please help me to name this construction.
The idea is close to reader-writer pattern. We have a "container" in which we can add Objects by the key (). And also we can get this objects by keys, removing it from container.
So, the implemented class should have two methods:
void putObject(Key key, Object object);
Object getObject(Key key); // remove <Key,Object> from container.
The next is most interesting. This container should work in multi-threading environment as follows:
I think that it is common structure, does it have "official" name?
My Java implementation of this pattern:
private static interface BlackBox {
public void addObject(IdObject object);
public IdObject getObject(ObjectId id);
}
private static class BlackBoxImpl implements BlackBox {
private final Lock conditionLock = new ReentrantLock();
private final Map<ObjectId, IdObject> savedObjects;
private final Map<ObjectId, Condition> waitingConditions;
public BlackBoxImpl() {
this.savedObjects = new ConcurrentHashMap<ObjectId, IdObject>(20);
this.waitingConditions = new ConcurrentHashMap<ObjectId, Condition>(20);
}
@Override
public void addObject(IdObject object) {
savedObjects.put(object.getId(), object);
if (waitingConditions.containsKey(object.getId())) {
Condition waitCondition = waitingConditions.get(object.getId());
conditionLock.lock();
waitCondition.signal();
conditionLock.unlock();
}
}
@Override
public IdObject getObject(ObjectId id) {
if (savedObjects.containsKey(id)) {
return savedObjects.get(id);
} else {
conditionLock.lock();
Condition waitCondition = conditionLock.newCondition();
waitingConditions.put(id, waitCondition);
waitCondition.awaitUninterruptibly();
conditionLock.unlock();
return savedObjects.get(id);
}
}
}
private static interface IdObject {
public ObjectId getId();
}
private static class IdObjectImpl implements IdObject {
protected final ObjectId id;
public IdObjectImpl(ObjectId id) {
this.id = id;
}
@Override
public ObjectId getId() {
return id;
}
}
private static interface ObjectId {
}
private static class ObjectIdImpl implements ObjectId {
}
I would probably use something like a
ConcurrentMap<K,BlockingQue<V>>.
Use the concurrent methods of the Map to add the pair. Take from your queue for the value. Use an ArrayBlockingQue(1).
Something like this perhaps:
static class MultiQueue<K, V> {
// The base structure.
final ConcurrentMap<K, BlockingQueue<V>> queues = new ConcurrentHashMap<>();
/**
* Put an item in the structure.
*
* The entry in the map will be created if no entry is currently there.
*
* The value will then be posted to the queue.
*/
public void put(K k, V v) throws InterruptedException {
// Make it if not present.
ensurePresence(k).put(v);
}
/**
* Get an item from the structure.
*
* The entry in the map will be created if no entry is currently there.
*
* The value will then be taken from the queue.
*/
public void get(K k) throws InterruptedException {
// Make it if not present - and wait for it.
ensurePresence(k).take();
}
private BlockingQueue<V> ensurePresence(K k) {
// Make it if not present.
return queues.computeIfAbsent(k, v -> new ArrayBlockingQueue(1));
}
}