Java Data Objects (JDO): An Object-based Approach to Transparent Persistence

	Figure 1 | A typical JDO-based application scenario

JDO addresses not only enterprise data, but also persistent data in general (even in embedded systems). In a two-tier environment, JDO hides the details of data storage, types, relationships, and retrieval. In the more complex managed multi-tier environment JDO also handles concurrency issues, transactions, scalability, security, and connection management.

Figure 1 depicts a typical JDO-based application scenario.

JDO relies on the J2EE Connector architecture (JCA) for EIS access and uses the Java Transaction API (JTA) for distributed transactions. A JDO instance is a persistence-capable (implements PersistenceCapable interface) class, each instance of which represents some form of persistent data. You can make a Java class persistence-capable by either explicitly implementing the PersistenceCapable interface or using a JDO enhancer during or after compile time. (A JDO enhancer is a byte code enhancer program that modifies the byte codes of Java class files to enable transparent loading and storing of the persistent instances' fields.) You can make almost any user-defined class PersistenceCapable; however, some system classes, such as Thread, Socket, and File, among others, can never be persistence capable.

Instances of a persistence-capable class can be either transient or persistent. Java defines two concepts for determining whether two instances are the same instance (identity) or whether they represent the same data (equality). Java object identity is entirely managed by the Instances are identical if and only if they occupy the same storage location within the Java Virtual Machine (JVM). JDO extends these concepts to determine if two instances represent the same data store object. Hence, a JDO identity differs from that of a Java object identity.