Java Data Objects

Chapter 5. Datastore Mappings

JDO supports the storage of your object model in a variety of datastore architectures. The primary datastore architectures envisioned for use with JDO are:

Relational database

Organized as a set of tables, each containing a set of rows and columns. A column can store values of a particular atomic type. Each table cell in a particular row and column stores a value of the column’s type. The value stored can be a null value. Instances are identified uniquely by the value of primary-key columns. Relationships are defined, and may be enforced, by annotating specific columns as foreign keys that reference columns in a table.

Pure object database

An extension of the JVM object model. Domain objects are stored with their primitive fields, just as instances are stored in the JVM. Instances are identified by a system-generated unique identifier. References are stored as objects, including instances of system-defined classes. Unreferenced instances are garbage collected. An extent is not an intrinsic construct in a pure object database; it is implemented as a class containing a set of objects. In this model, any reference type can be shared among multiple objects, and changes made to the instance of the reference type are visible to all objects that reference it.

Hybrid object database

Organized as a set of class extents, each containing a set of instances in which primitive and complex fields are stored. Domain objects are stored with their primitive fields; some ...