Persistence Layer Services

When developing Java applications that interface with Oracle, the integration between Java s object-oriented features such as object references, business rules, complex relationships and inheritance, and Oracle s relational database features such as stored procedures, tables, rows, columns , and foreign keys (referred to as object-relational[O-R] mapping) requires an effort that is commonly underestimated by both developers and project managers. The problem of bridging object-oriented and relational technologies is referred to as object-relational impedance mismatch and the fundamental differences in modeling, design, and skill sets can prolong development work. This can lead to deployed applications that are, at best, difficult to maintain and, at worst, unreliable and impossible to enhance. In addition to these issues, developers also need the ability to translate object-oriented data into relational data, referred to as O-R mapping. A product that easily enables the integration of relational database and Java technologies, while allowing database designers and Java developers to maintain database and application design principles, would be invaluable. Oracle recognized the developer s need for these features and has created a product that not only addresses all of the complex issues mentioned above, but also adds features such as:

• A query framework that allows developers to define queries using various technologies

• A transaction framework that provides object-level transaction support

• Performance enhancements that includes a fully-configurable cache that ensures object-identity

Oracle Application Server TopLink integrates the object and relational worlds . It allows applications and application developers to manage Java objects using relational databases. TopLink is robust enough to work with any Java-supporting database and any application server that supports Java and J2EE containers. Some of the major features of Oracle Application Server TopLink include:

• The Oracle Application Server Mapping Workbench The Oracle Application Server Mapping Workbench is a visual tool that creates and maps metadata describing the relationship between Java classes and relational tables.

• Advanced mapping support Advanced mapping support includes support for the following mappings:

  • Direct-to-field

  • One-to-one

  • Variable one-to-one

  • One-to-many

  • Many-to-many

  • Aggregate Object

  • Aggregate-collection

  • Transformation

  • Object type

  • Type conversion

  • Serialized object

  • Direct collection

  • Nested tables

  • VArrays and OREFS

  • Multitable

  • Inheritance

• Object caching Object caching stores data returned as objects from the database for future use, minimizing database and network access. The following cache types are supported:

  • Soft Objects are maintained through garbage collection sweeps until memory is at a premium.

  • Hard Objects are not garbage collected.

  • Weak Objects exist in the cache as long as they are being used.

  • Full Objects are cached but not removed.

• Query Flexibility Developers can define queries using:

  • An object-oriented expression framework

  • Query By Example (QBE)

  • Enterprise Java Beans Query Language (EJB QL)

  • SQL

  • Stored procedures

• Object-level transaction support Transactions are supported through the Unit of Work, a Java-based transaction. During a Unit of Work, the application modifies business objects; when the Unit of Work is committed, Oracle Application Server TopLink updates the database, based on what has changed, thereby executing the minimally required SQL.

Oracle Application Server TopLink consists of three main components :

• Development tools Development tools consist of the Oracle Application Server Mapping Workbench (which creates and manages the mapping metadata that describes the relationship between Java classes and relational tables) and the Oracle Application Server Sessions Editor (which describes how TopLink communicates with the datasource at run time by creating the session.xml file).

• Run-time tools Development tools consist of the web client, an interface to interact with any Oracle Application Server TopLink server session, and the Performance Profiler, providing performance tuning and run-time diagnostics.

• Class libraries Class libraries consist of the TopLink API, which is called at run time to retrieve and store Java objects.

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