Building and Using the Persistence Layer

Using EclipseLink JPA Metatdata, Annotations, and XML

When using JPA, you can specify persistence layer components using any combination of standard JPA annotations and persistence.xml, EclipseLink JPA annotation extensions, and EclipseLink JPA persistence.xml extensions.

For more information, see About Configuration Basics.

Using EclipseLink Metadata Java API

Persistence layer components can be coded or generated as Java. To use Java code, you must manually write code for each element of the project including: project, login, platform, descriptors, and mappings. This may be more efficient if your application is model-based and relies heavily on code generation.

Using Method and Direct Field Access

You can access the fields (data members) of a class by using a getter/setter method (also known as property access) or by accessing the field itself directly.

When to use method or direct field access depends on your application design. Consider the following guidelines:

• Use method access outside of a class.

  This is the natural public API of the class. The getter/setter methods handle any necessary side-effects and the client need not know anything about those details.

• Use direct field access within a class to improve performance.

  In this case, you are responsible for taking into consideration any side-effects not invoked by bypassing the getter/setter methods.

When considering using method or direct field access, consider the following limitations.

If you enable change tracking on a getter/setter method (for example, you decorate method setPhone with @ChangeTracking), then EclipseLink tracks changes accordingly when a client modifies the field (phone) using the getter/setter methods.

Similarly, if you enable change tracking on a field (for example, you decorate field phone with @ChangeTracking), then EclipseLink tracks changes accordingly when a client modifies the field (phone) directly.

However, if you enable change tracking on a getter/setter method (for example, you decorate method setPhone with @ChangeTracking) and a client accesses the field (phone) directly, EclipseLink does not detect the change. If you choose to code in this style of field access within a class for performance and method access outside of a class, then be aware of this limitation.

For more information, see the description of the @ChangeTracking annotation in Java Persistence API (JPA) Extensions Reference for EclipseLink.

Using Java Byte-code Weaving

Weaving is a technique of manipulating the byte-code of compiled Java classes.

Weaving is used to enhance both JPA entities and Plain Old Java Object (POJO) classes for such things as lazy loading, change tracking, fetch groups, and internal optimizations.

For more information, see About Weaving.

Mapping Metadata

The EclipseLink application metadata model is based on the project. The project includes descriptors, mappings, and various policies that customize the run-time capabilities. You associate this mapping and configuration information with a particular data source and application by referencing the project from a session.

For more information, see the following:

• Creating Project Metadata

• Chapter 5, "Understanding Descriptors"

• Chapter 6, "Understanding Mappings"

Cache

By default, EclipseLink sessions provide an object-level cache that guarantees object identity and enhances performance by reducing the number of times the application needs to access the data source. EclipseLink provides a variety of cache options, including locking, refresh, invalidation, isolation, and coordination. Using cache coordination, you can configure EclipseLink to synchronize changes with other instances of the deployed application. You configure most cache options at the persistence unit or entity level. You can also configure cache options on a per-query basis or on a descriptor to apply to all queries on the reference class.

For more information, see Chapter 8, "Understanding Caching."

Queries and Expressions

For Object-relational architectures, EclipseLink provides several object and data query types, and offers flexible options for query selection criteria, including the following:

• EclipseLink expressions

• JPQL (Java Persistence Query Language)

• SQL

• Stored procedures

• Query by example

With these options, you can build any type of query. Oracle recommends using named queries to define application queries. Named queries are held in the project metadata and referenced by name. This simplifies application development and encapsulates the queries to reduce maintenance costs.

For Object-relational architectures, you are free to use any of the query options regardless of the persistent entity type. Alternatively, you can build queries in code, using the EclipseLink API.

For more information, see Chapter 9, "Understanding Queries" and Chapter 10, "Understanding EclipseLink Expressions."