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MultiMap

Hazelcast MultiMap is a specialized map where you can store multiple values under a single key. Just like any other distributed data structure implementation in Hazelcast, MultiMap is distributed and thread-safe.

Hazelcast MultiMap is not an implementation of java.util.Map due to the difference in method signatures. It supports most features of Hazelcast Map except for indexing, predicates and MapLoader/MapStore. Yet, like Hazelcast Map, entries are almost evenly distributed onto all cluster members. When a new member joins the cluster, the same ownership logic used in the distributed map applies.

Getting a MultiMap and Putting an Entry

The following example creates a MultiMap and puts items into it. Use the HazelcastInstance getMultiMap method to get the MultiMap, then use the MultiMap put method to put an entry into the MultiMap.

public class PutMember {
  public static void main( String[] args ) {
    HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
    MultiMap <String , String > map = hazelcastInstance.getMultiMap( "map" );

    map.put( "a", "1" );
    map.put( "a", "2" );
    map.put( "b", "3" ); 
    System.out.println( "PutMember:Done" );
  }
}

Now let's print the entries in this MultiMap.

public class PrintMember {
  public static void main( String[] args ) { 
    HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
    MultiMap <String, String > map = hazelcastInstance.getMultiMap( "map" );
    for ( String key : map.keySet() ){
      Collection <String > values = map.get( key );
      System.out.println( "%s -> %s\n",key, values );
    }
  }
}

After you run the first code sample, run the PrintMember sample. You will see the key a has two values, as shown below.

b -> [3]

a -> [2, 1]

Hazelcast MultiMap uses EntryListener to listen to events which occur when entries are added to, updated in or removed from the MultiMap. Please refer to the Listening for MultiMap Events section for information on how to create an entry listener class and register it.

Configuring MultiMap

When using MultiMap, the collection type of the values can be either Set or List. You configure the collection type with the valueCollectionType parameter. If you choose Set, duplicate and null values are not allowed in your collection and ordering is irrelevant. If you choose List, ordering is relevant and your collection can include duplicate and null values.

You can also enable statistics for your MultiMap with the statisticsEnabled parameter. If you enable statisticsEnabled, statistics can be retrieved with getLocalMultiMapStats() method.

image NOTE: Currently, eviction is not supported for the MultiMap data structure.



RELATED INFORMATION

Please refer to the MultiMap Configuration section for a full description of Hazelcast Distributed MultiMap configuration.