By Robin M. Roos

Java information gadgets is a standardized Java API for item patience. It allows the garage and retrieval of complicated item versions with a number of garage mechanisms, together with either item and relational databases. Analysts agree that JDO's obvious patience will speed up software program improvement and yield extra versatile functions and item models.According to Infoworld, "The JDO common provides to be a winner for company leaders and company builders alike. Unifying information access-that is, utilizing wide-spread Java constructs to entry details housed in relational databases, item databases, dossier structures, or maybe flash RAM-means that company functions will event a rise in usability, portability and integration".This booklet is for Java builders, designers and designers with an outstanding knowing of the Java language who desire to write purposes that use JDO for patience. past wisdom of JDBC and J2EE isn't really particularly required, yet may support to speed up the reader's realizing of JDO's courting with those technologies.The spouse CD-ROM comprises the next JDO implementations:FastObjects, Kodo JDO, LiDO and OpenFusion JDO.

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The identity type can be specified explicitly with the identity-type attribute of the tag, as shown below. 2 Application identity With application identity the application is responsible for the identity of an instance, which is derived from the values of a subset of its persistent fields. The persistence descriptor is used to specify one or more persistent fields that will make up a primary key for the instance, alongside a name for the class that will act as the Object ID class. As such, application identity is often referred to as primary key identity, and the Object ID class for such an instance is often referred to as the primary key class.

This is in contrast to non-transient instances, the methods of which might throw runtime exceptions from the JDO exception hierarchy. These exceptions are covered in detail in Chapter 9. 2 Persistent-New This is the state of instances that have been made persistent during the current transaction. During the transition from transient to persistent, the associated persistence manager: ● becomes responsible for implementing state interrogation and further state transitions; ● saves persistent and transactional non-persistent field values for use during rollback; ● assigns a JDO identity to the instance.

A no-argument constructor and a string constructor must both exist. The toString() method must have been overridden so that the string it returns can be used as an argument to the string constructor, in order to create an equivalent instance of the primary key. ) All non-static fields must be primitives, or references to Serializable classes, and must be public. For every field identified in the persistence descriptor as a primary key field there must be a corresponding and identically named field in the primary key class; all of these (and only these) fields must be utilized by the primary key class’s equals() and hashcode() methods for the determination of equivalence.

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