Interface MapLoader<K,V>
- Type Parameters:
K
- type of the MapLoader keyV
- type of the MapLoader value
- All Known Subinterfaces:
EntryLoader<K,
,V> EntryStore<K,
,V> MapStore<K,
V>
- All Known Implementing Classes:
GenericMapLoader
,GenericMapStore
,MapStoreAdapter
IMap.get(Object)
normally returns the value that
is available in memory. If the entry doesn't exist in memory, Hazelcast
returns null
. If a MapLoader implementation is provided, then instead
of returning null
, Hazelcast will attempt to load the unknown entry
by calling the implementation's load(Object)
or
loadAll(Collection)
methods. Loaded entries will be placed into
the distributed map, and they will stay in memory until they are explicitly
removed or implicitly evicted (if eviction is configured).
MapLoader implementations are executed by a partition thread, therefore care should be taken not to block the thread with an expensive operation or an operation that may potentially never return, the partition thread does not time out the operation. While the partition thread is executing the MapLoader it is unable to respond to requests for data on any other structure that may reside in the same partition, or to respond to other partitions mapped to the same partition thread. For example a very slow MapLoader for one map could block a request for data on another map, or even a queue. It is therefore strongly recommended not to use MapLoader to call across a WAN or to a system which will take on average longer than a few milliseconds to respond.
MapLoaders should not be used to perform cascading operations on other data
structures via a HazelcastInstance
, the MapLoader should only
concern itself with the operation on the assigned map. If the MapLoader
attempts to access another data structure on a different partition to the
key used in the MapLoader, a IllegalThreadStateException
is thrown. A MapLoader can only interact with other data structures that
reside on the same partition.
If a blocked partition thread is called from a Hazelcast Client the caller
will also block indefinitely, for example
IMap.get(Object)
. If the same call is made from
another cluster member the operation will eventually time out with an
OperationTimeoutException
.
-
Method Summary
-
Method Details
-
load
Loads the value of a given key. If distributed map doesn't contain the value for the given key then Hazelcast will call implementation's load (key) method to obtain the value. Implementation can use any means of loading the given key; such as an O/R mapping tool, simple SQL or reading a file etc.- Parameters:
key
- , cannot benull
- Returns:
- value of the key; returning
null
value signals value missing in the underlying store
-
loadAll
Loads given keys. This is batch load operation so that implementation can optimize the multiple loads.For any key in the input keys, there should be a single mapping in the resulting map. Also the resulting map should not have any keys that are not part of the input keys.
The given collection should not contain any
null
keys. The returned Map should not contain anynull
keys or values.Loading other items than what provided in
keys
prevents the map from being filled from the map store.- Parameters:
keys
- keys of the values entries to load- Returns:
- map of loaded key-value pairs.
-
loadAllKeys
Loads all of the keys from the store. The returnedIterable
may return the keys lazily by loading them in batches. TheIterator
of thisIterable
may implement theCloseable
interface in which case it will be closed once iteration is over. This is intended for releasing resources such as closing a JDBC result set.The returned Iterable should not contain any
null
keys.- Returns:
- all the keys. Keys inside the Iterable cannot be
null
.
-