Java Data Objects

Identity

An instance is identified uniquely in the datastore via an identity value. JDO has two durable types of identity: datastore and application. With both types, the identity value is stored in one or more columns of the class’s table. Those columns become the table’s primary key.

For example, we use datastore identity for the Movie class defined in the com.mediamania.content package. The JDO implementation may represent datastore identity as an INTEGER in the relational schema. Line [1] of Example 5-1 illustrates the use of the oid column to store the datastore identity value for the Movie table, which is defined as the table’s primary key on line [2].

Example 5-1. Datastore identity stored in a primary-key column

CREATE TABLE Movie (
    oid         INTEGER     [1]
    title       VARCHAR(24),
    rating      CHAR(4),
    genres      CHAR(16),
    PRIMARY KEY(oid)     [2]
)

Each implementation has its own default name for this column, but you can usually specify the name that should be used.

You may have a table with no primary key defined, but instead have a unique index defined for one or more columns. With either a primary key or a unique index, the associated columns are used for storing the identity value. If you use a unique index for a JDO identity, none of the columns in the index can have a null value.

With datastore identity, either the JDO implementation or the datastore itself provides a unique identity value for each instance. The datastore identity value is separate from the fields you define in your class. The representation ...