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Understanding EclipseLink, 2.4
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About Object-Relational Mapping

EclipseLink provides a complete, JPA-compliant JPA implementation. It provides complete compliance for all of the mandatory features and many of the optional features. It also supports EclipseLink features not described in the JPA specification, such as object-level cache, distributed cache coordination, extensive performance tuning options, enhanced Oracle Database support, advanced mappings, optimistic and pessimistic locking options, extended annotations, and query hints.

For more information, see Java Persistence API (JPA) Extensions Reference for EclipseLink.

The following sections describe many of these features.

Understanding Object-Relational Entity Architecture

The entity architecture is composed of entities, persistence units, persistence contexts, entity manager factories, and entity managers. Figure 2-1 illustrates the relationships between these elements:

  • Persistence creates one or more EntityManagerFactory objects.

  • Each EntityManagerFactory is configured by one persistence unit.

  • EntityManagerFactory creates one or more EntityManager objects.

  • One or more EntityManagers manage one PersistenceContext.

Figure 2-1 Relationships Between Entity Architecture Elements

Relationships between entity architectue elements
Description of "Figure 2-1 Relationships Between Entity Architecture Elements"

Entities

An entity is any application-defined object with the following characteristics:

  • It can be made persistent.

  • It has a persistent identity (a key that uniquely identifies an entity instance and distinguishes it from other instances of the same entity type. An entity has a persistent identity when there is a representation of it in a data store).

  • It is transactional in a sense that a persistence view of an entity is transactional (an entity is created, updated, and deleted within a transaction, and a transaction is required for the changes to be committed in the database). However, in-memory entities can be changed without the changes being persisted.

  • It is not a primitive, a primitive wrapper, or built-in object. An entity is a fine-grained object that has a set of aggregated states that is typically stored in a single place (such as a row in a table) and have relationships to other entities.

The entity also contains entity metadata that describes the entity. Entity metadata is not persisted to the database. It is used by the persistence layer to manage the entity from when it is loaded until it is invoked at runtime. Metadata can be expressed as annotations on the Java programming elements or in XML files (descriptors). For more information, see Chapter 4, "Understanding Entities."

Beginning with the current release, you can define and use extensible entities where mappings can be added spontaneously. In this case, the entity stores extended attributes within a map instead of static attributes. The entity then defines how values from this map are mapped to the database using an eclipselink-orm.xml mapping file. In addition to being able to dynamically define mappings, EclipseLink also enables these extended mappings to be stored and managed externally. This external storage enables your extended mappings to be defined while the application is running. For more information on making entities extensible, see "Providing Software as a Service" in Solutions Guide for EclipseLink.

Persistence and Persistence Units

Persistence is a characteristic of an entity. This means that the entity can be represented in a data store, and it can be accessed at a later time.

A persistence unit identifies a persistable unit and defines the properties associated with it. It also defines the objects that must be persisted. The objects can be entity classes, embeddable classes, or mapped superclasses. The persistence unit provides the configuration for the entity manager factory. Entity managers created by the entity manager factory inherit the properties defined in the persistence unit.

Entity Managers

An entity manager enables API calls to perform operations on an entity. Until an entity manager is used to create, read, or write an entity, the entity is a nonpersistent Java object. When an entity manager obtains a reference to an entity, that entity becomes managed by the entity manager. The set of managed entity instances within an entity manager at any given time is called its persistence context; only one Java instance with the same persistent identity can exist in a persistence context at any time.

You can configure an entity manager to read or write to a particular database, to persist or manage certain types of objects, and to be implemented by a specific persistence provider. The persistence provider supplies the implementation for JPA, including the EntityManager interface implementation, the Query implementation, and the SQL generation.Entity managers are provided by an EntityManagerFactory. The configuration for an entity manager is bound to the EntityManagerFactory, but it is defined separately as a persistence unit. You name persistence units to enable differentiation between EntityManagerFactory objects. This way, your application obtains control over which configuration to use for operations on a specific entity. The configuration that describes the persistence unit is defined in a persistence.xml file. You name persistence units to be able to request a specific configuration to be bound to an EntityManagerFactory.

Adding Metadata Using Annotations

An annotation is a simple, expressive means of decorating Java source code with metadata that is compiled into the corresponding Java class files for interpretation at run time by a JPA persistence provider to manage persistent behavior.

A metadata annotation represents a Java language feature that lets you attach structured and typed metadata to the source code. Annotations alone are sufficient for the metadata specification—you do not need to use XML. Standard JPA annotations are in the javax.persistence package.

For more information, see Chapter 10 "Metadata Annotations" in the JPA Specification http://jcp.org/en/jsr/detail?id=317

EclipseLink provides a set of proprietary annotations as an easy way to add metadata to the Java source code. The metadata is compiled into the corresponding Java class files for interpretation at run time by a JPA persistence provider to manage persistent behavior. You can apply annotations at the class, method, and field levels.

EclipseLink annotations expose some features that are currently not available through the use of JPA metadata:

  • Basic properties—By default, the EclipseLink persistence provider automatically configures a basic mapping for simple types. Use these annotations to fine-tune the immediate state of an entity in its fields or properties.

  • Relationships—EclipseLink has defaults for some relationships, such as One-To-One and One-To-Many. Other relationships must be mapped explicitly. Use the annotations to specify the type and characteristics of entity relationships and to fine-tune how your database implements these relationships.

  • Embedded objects—An embedded object does not have its own persistent identity; it is dependent upon an entity for its identity. By default, the persistence provider assumes that every entity is mapped to its own table. Use the following annotations to override this behavior for entities that are owned by other entities.

Advantages and Disadvantages of Using Annotations

Using annotations provides several advantages:

  • They are relatively simple to use and understand.

  • They provide in-line metadata within with the code that it describes; you do not need to replicate the source code context of where the metadata applies.

The primary disadvantage of annotations is that the metatdata becomes unnecessarily coupled to the code; changes to metadata require changing and recompiling the source code.

About Configuration Basics

The following sections describe some of the key configuration files in an Object Relational Mapping project.

Default Configuration Values

Each annotation has a default value (consult the JPA specification for defaults). A persistence engine defines defaults that apply to the majority of applications. You only need to supply values when you want to override the default value. Therefore, having to supply a configuration value is not a requirement, but the exception to the rule. This is known as configuration by exception.


NoteNote:

You should be familiar with the defaults to be able to change the behavior when necessary.


The default values are described in Java Persistence API (JPA) Extensions Reference for EclipseLink. See also Chapter 10, "Metadata Annotations" in the JPA specification.

http://jcp.org/en/jsr/detail?id=317

The configuration is done by exception: if a value is not specified in one of the configuration files, then a default value is used.

Configuring Persistence Units Using persistence.xml

A persistence unit defines the details that are required when you acquire an entity manager. You specify a persistence unit by name when you acquire an entity manager factory. Use the JPA persistence file, persistence.xml, to configure a persistence unit. You can specify any vendor-specific extensions in the file by using a <properties> element.

This file appears in the META-INF/ directory of your persistence unit JAR file or in the classpath.

For more information, see About the Persistence Unit. See also "Persistence Property Extensions Reference" in Java Persistence API (JPA) Extensions Reference for EclipseLink.

Object-Relational Data Type Mappings

Object-relational data type mappings transform certain object data member types to structured data source representations optimized for storage in specialized object-relational databases such as Oracle Database. Object-relational data type mappings let you map an object model into an object-relational model. You can use only object-relational data type mappings with specialized object-relational databases optimized to support object-relational data type data source representations.

For more information, see Java Persistence API (JPA) Extensions Reference for EclipseLink.

Specifying Object-Relational Mappings Using orm.xml

Use the orm.xml file to apply the metadata to the persistence unit. This metadata is a union of all the mapping files and the annotations (if there is no xml-mapping-metadata-complete element). If you use one mapping orm.xml file for your metadata and place this file in a META-INF directory on the classpath, then you do not need to explicitly list it. The persistence provider will automatically search for this file (orm.xml) and use it.

The schema for the JPA 2.0 orm.xml is orm_2_0.xsd. (http://java.sun.com/xml/ns/persistence/orm_2_0.xsd)

If you use a different name for your mapping files or place them in a different location, you must list them in the mapping-file element of the persistence.xml file.

Specifying EclipseLink Object-Relational Mappings Using eclipselink-orm.xml

The standard JPA orm.xml file applies metadata to the persistence unit. It provides support for all of the JPA 2.0 mappings. You can use this file instead of annotations or to override JPA annotations in the source code. The eclipselink-orm.xml file supports the mappings defined by the orm.xml file and the full set of EclipseLink extensions beyond JPA 2.0. Any settings that you make in the eclipselink-orm.xml file will override settings in the orm.xml file.

For more information on the eclipselink-orm.xml file, see "eclipselink-orm.xml Schema Reference" in Java Persistence API (JPA) Extensions Reference for EclipseLink.


NoteNote:

Using this mapping file enables many EclipseLink advanced features, but it may prevent the persistence unit from being portable to other JPA implementations.


For more information, on overriding values, see:

Overriding and Merging Mapping Information

To override the orm.xml file's mapping, you must define the META-INF/eclipselink-orm.xml file in the project. When both orm.xml and eclipselink-orm.xml are specified, the contents of eclipselink-orm.xml override orm.xml and any other JPA mapping file specified in the persistence unit. If there are overlapping specifications in multiple ORM files, the files are merged if there are no conflicting entities.

For more information, see "Overriding and Merging" in Java Persistence API (JPA) Extensions Reference for EclipseLink.

Validating the XML Schema

By default the content of your .orm XML file is not validated against the JPA .orm XML schema.

During development it is a good idea to validate your .orm XML file against the schema to ensure it is valid. In EclipseLink, validating the .orm XML schema can be enabled using the persistence unit property "eclipselink.orm.validate.schema" in your persistence.xml file.

Advantages and Disadvantages of Using XML

Some advantages of using XML instead of annotations include:

  • No coupling between the metadata and the source code

  • Compliance with the existing, pre-EJB 3.0 development process

  • Support in IDEs and source control systems

The main disadvantages of mapping with XML include:

  • It is inherently complex (when compared to annotations)

  • The need for replication of the code context (that is, defining the structure in both the XML and the source code)

For more information, see Chapter 10 "Metadata Annotations" in the JPA Specification:

http://jcp.org/en/jsr/detail?id=317

About Data Sources

An important part of the definition of the persistence unit is the location where the provider can find data to read and write. This is called the data source. The data source is typically a database. The database location is specified in the form of a JDBC data source in the JNDI namespace of the server.

Typically, applications that use EclipseLink are run in the context of a JTA transaction. Specify the name of the data source in the jta-data-source element in the persistence.xml file. If the application is not run in the context of a transaction, then it is considered to be resource-local. In this case, specify the name of the data source in the non-jta-data-source element.

You can also specify a non-relational database data source, such as an XML schema.

For more information, see Chapter 8, "Understanding Data Access."

Applications can be run in standalone, or Java SE, mode. In this mode, the application runs outside the server, with a non-JTA compliant data source, and in a non-Oracle stack. In this case, you must provide driver-specific information, such as the JDBC driver class, the URL that the client uses to connect to the database, and the user name and password to access the database. For more information and an example of running an application in standalone mode, see "Testing EclipseLink JPA Outside a Container" in Solutions Guide for EclipseLink.

About EclipseLink Caches

By default, EclipseLink uses a shared object cache that caches a subset of all objects read and persisted for the persistence unit. The shared cache differs from the local EntityManager cache. The shared cache exists for the duration of the persistence unit (EntityManagerFactory or server) and is shared by all EntityManagers and users of the persistence unit. The local EntityManager cache is not shared and only exists for the duration of the EntityManager or transaction.

The benefit of the shared cache is that after an object is read, the database does not need to be accessed if the object is read again. Also, if the object is read by using a query, it does not need to be rebuilt, and its relationships do not need to be fetched again.

The limitation of the shared cache is that if the database is changed directly through JDBC, or by another application or server, the objects in the shared cache will be stale.

EclipseLink offers several mechanism to deal with stale data including:

  • Refreshing

  • Invalidation

  • Optimistic locking

  • Cache coordination

  • Database Change Notification (DCN)

The shared cache can also be disabled, or it can be selectively enabled and disabled by using the @Cache or @Cacheable annotations.EclipseLink also offers several different caching strategies, to configure how many objects are cached and how much memory is used.

If the application detects that the cache is out of date, it can clear, refresh, or invalidate it programmatically. Clearing the cache can cause object identity issues if any of the cached objects are in use, so invalidating is safer. If you know that none of the cached objects are in use, then you can clear the cache.

For more information, see Chapter 9, "Understanding Caching."

Defining Cache Behavior

EclipseLink provides an @Cache annotation which lets you define cache properties. The properties include cache type, size, and refresh rules, among others. See Java Persistence API (JPA) Extensions Reference for EclipseLink.

Caching in Clustered Environments

Caching in a clustered environment can have problems because changes made on one server are not reflected on objects cached in other servers. This is not a problem for read-only objects, but it is for objects that are frequently updated.

EclipseLink offers several solutions to this problem.

  • The cache can be disabled for the classes that frequently change.

  • Cache coordination can be used to broadcast changes between the servers in the cluster to update or invalidate changed objects.

  • Cache invalidation based on time-to-live or time-of-day.

  • Optimistic locking prevents updates to stale objects and triggers the objects to be invalidated in the cache.

For more information, see Coordinated Cache and Clustering.

About Database Queries

The object-relational component of EclipseLink supports a variety of queries.

  • JPQL queries

  • SQL queries

  • Criteria API queries

  • Native SQL queries

  • EclipseLink JPA query hints

  • Query casting

  • Oracle Extensions for queries

  • Advanced EclipseLink native queries

For information on these queries, see Chapter 10, "Understanding Queries."

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