Table of Contents
All entity classes stored in JE using the DPL must have a primary index, or key, identified for them. All such classes may also have one or more secondary keys declared for them. This chapter describes primary and secondary indexes in detail, and shows how to access the indexes created for a given entity class.
One way to organize access to your primary and secondary indexes is to create a data accessor class. We show an implementation of a data accessor class in SimpleDA.class.
In order to retrieve any object from an entity store, you must access at least the primary index for that object. Different entity classes stored in an entity store can have different primary indexes, but all entity classes must have a primary index declared for it. The primary index is just the default index used for the class. (That is, it is the data's primary key for the underlying database.)
Entity classes can optionally have secondary indexes declared for them. In order to access these secondary indexes, you must first access the primary index.
You retrieve a primary index using the
EntityStore.getPrimaryIndex()
method. To do this, you indicate the index key type
(that is, whether it is a String, Integer, and
so forth) and the class of the entities stored
in the index.
For example, the following retrieves the
primary index for an Inventory
class (we provide an implementation of this class in
Inventory.java).
These index keys are of type String.
PrimaryIndex<String,Inventory> inventoryBySku =
store.getPrimaryIndex(String.class, Inventory.class);
You retrieve a secondary index using the
EntityStore.getSecondaryIndex()
method. Because secondary indices actually
refer to a primary index somewhere in your data
store, to access a secondary index you:
Provide the primary index as
returned by
EntityStore.getPrimaryIndex().
Identify the key data type used by
the secondary index
(String,
Long,
and so forth).
Identify the name of the
secondary key field.
When you declare the
SecondaryIndex
object, you identify the entity class
to which the secondary index
must refer.
For example, the following first retrieves the
primary index, and then uses that to retrieve a secondary
index. The secondary key is held by the
itemName field of the
Inventory class.
PrimaryIndex<String,Inventory> inventoryBySku =
store.getPrimaryIndex(String.class, Inventory.class);
SecondaryIndex<String,String,Inventory> inventoryByName =
store.getSecondaryIndex(inventoryBySku, String.class, "itemName");