This chapter describes the items that can be used in Object-Relational and Object-XML mapping projects.
This chapter contains the following sections:
TopLink provides a complete, JPA 2.0-compliant JPA implementation. It provides complete compliance for all of the mandatory features, many of the optional features, and some additional features. The additional nonmandatory features include 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 "The EclipseLink JPA User's Guide" in the EclipseLink documentation:
The following sections describe many of these features.
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 is configured by one persistence unit.
EntityManagerFactory creates one or more
One or more
EntityManagers manage one
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).
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, TopLink 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, see "EclipseLink/Examples/JPA/Extensibility" in the EclipseLink documentation:
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.
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
TopLink 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.
TopLink annotations expose some features of TopLink that are currently not available through the use of JPA metadata:
Basic properties—By default, TopLink 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—TopLink has defaults for some relationships, such as OneToOne and OneToMany. 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, TopLink 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.
The following sections describe some of the key configuration files in a TopLink Object Relational Mapping project.
Oracle TopLink is compliant with the JPA 2.0 specification. The configuration files enable you to change the default values of properties that are defined in the specification. The defaults are extensive and specified in Chapter 10, "Metadata Annotations" in the JPA specification.
The configuration is done by exception: if a value is not specified in one of the configuration files, then a default value is used.
Use the JPA persistence file,
persistence.xml, to configure a persistence unit. A persistence unit defines the details that are required when you acquire an entity manager. You can specify any vendor-specific extensions in the file by using a
This file appears in the
META-INF/ directory of your persistence unit JAR file or in the classpath.
For more information, see "Configuring Persistence Units Using persistence.xml" in the EclipseLink documentation:
The standard JPA
orm.xml file is used to apply 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
eclipselink-orm.xml file supports the mappings defined by the
orm.xml file and the full set of EclipseLink extensions beyond JPA 2.0.
For more information on the
eclipselink-orm.xml file, see "Specifying EclipseLink Object-Relational Mappings Using eclipselink-orm.xml" in the EclipseLink documentation:
See also "EclipseLink/Examples/JPA/EclipseLink-ORM.XML" in the EclipseLink documentation:
Using this mapping file enables many TopLink advanced features, but it may prevent the persistence unit from being portable to other JPA implementations.
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. In TopLink, the data source is often 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 TopLink 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
TopLink also lets you specify a non-relational database data source, such as an XML schema.
Applications that use TopLink 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 "EclipseLink/Examples/JPA/OutsideContainer" in the EclipseLink documentation:
See also "EclipseLink/Examples/JPA/Tomcat Web Tutorial" in the EclipseLink documentation:
By default, TopLink uses a shared object cache that caches a subset of all objects read and persisted for the persistence unit. The TopLink 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.
TopLink offers several mechanism to deal with stale data including:
The shared cache can also be disabled, or it can be selectively enabled and disabled by using the
@Cacheable annotations.TopLink 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 object is 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 "EclipseLink/Examples/JPA/Caching" in the EclipseLink documentation.
TopLink provides an
@Cache annotation which lets you define cache properties. The properties include cache type, size, and refresh rules, among others. See "Using the
@Cache Annotation" in the EclipseLink documentation.
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.
TopLink 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.
The object-relational component of TopLink supports a variety of queries.
JPQL is a query language that is similar to SQL, but differs because it presents queries from an object model perspective and includes path expressions that enable navigation over the relationships defined for entities and dependent objects. TopLink enables you to use JPQL with regular Java objects. In TopLink, JPQL enables you to declare queries, using the attributes of each abstract entity in the object model.
The disadvantage of JPQL is that dynamic queries require performing string concatenations to build queries dynamically from web forms or dynamic content. JPQL is also not checked until runtime, making typographical errors more common. These disadvantages are reduced by using the query criteria API, described in the next section.
See "JPQL" in the EclipseLink documentation.
See also "EclipseLink/Release/2.1.0/JPAQueryEnhancements" in the EclipseLink documentation.
JPA 2.0 defines a query criteria API to simplify dynamic query creation. Criteria queries can use parameters, and can use query hints the same as named queries. The query criteria API enables you to write any JQPL query—all JPQL keywords are defined in this API. The criteria API uses a set of Java interfaces to enable queries to be dynamically constructed. It also provides compile time checking for correctness to reduce the number of runtime typographical errors.
See "Criteria Query" in the EclipseLink documentation.
A TopLink query hint lets you customize or optimize a JPA Query beyond what is available in the JPA specification. Use TopLink JPA query hints to:
Construct a JPA query
Specify a JPA query using the
See "Query Hints" in the EclipseLink documentation.
TopLink provides an expression framework (also known as TopLink Native Query Support) with which you can express queries in a database-neutral fashion as an alternative to SQL when writing queries not supported by JPQL. TopLink expressions offer the following advantages over SQL when you access a database:
Expressions are easier to maintain because the database is abstracted.
Changes to descriptors or database tables do not affect the querying structures in the application.
Expressions enhance readability by standardizing the
Query interface so that it looks similar to traditional Java calling conventions.
Expressions enable read queries to transparently query between two classes that share a relationship. If these classes are stored in multiple tables in the database, TopLink automatically generates the appropriate join statements to return information from both tables.
Expressions simplify complex operations.
TopLink automatically generates the appropriate SQL from the specified expression.
The expression framework lets you work with expressions, database queries, call objects, and native queries. For more information on the queries described in the following list, see "Native SQL Queries" in the EclipseLink documentation.
JPA Query Using an EclipseLink
DatabaseQuery is a query object that provides a robust API for handling a variety of database query requirements, including reading and writing at the object level and at the data level.
JPA Query Using a TopLink Call Object
setCall, you can define your own TopLink
Call to accommodate a variety of data source options, such as SQL stored procedures and stored functions, EJB QL queries, and EIS interactions.
Named Parameters in a Native Query
Using TopLink, you can specify a named parameter in a native query using the TopLink # convention.
JPQL Positional Parameters in a Native Query
Using TopLink, you can specify positional parameters in a native query using the Java Persistence Query Language (JPQL) positional parameter
?n convention to specify a parameter by number.
JDBC-Style Positional Parameters in a Native Query
Using TopLink, you can specify positional parameters in a native query using the JDBC-style positional parameter
The TopLink Object-XML component enables you to efficiently bind Java classes to XML schemas. Object-XML implements JAXB, enabling you to provide your mapping information through annotations and providing support for storing the mappings in XML format.
JAXB (Java Architecture for XML Binding—JSR 222) is the standard for XML Binding in Java. JAXB covers 100 percent of XML Schema concepts. TopLink provides a JAXB implementation with many extensions.
When using TopLink Object-XML as the JAXB provider, no metadata is required to convert your existing object model to XML. You can supply metadata (using annotations or XML) if you want to fine-tune the XML representation.
TopLink Object-XML includes many advanced mappings that let you handle complex XML structures without having to mirror the schema in your Java class model.
For more information, see "The EclipseLink MOXy (JAXB) User's Guide" in the EclipseLink documentation:
The following sections describe many of these features.
To use TopLink Object-XML as your JAXB provider, you must identify the entry point to the TopLink JAXB runtime. This entry point is the
Create a text file called
jaxb.properties and enter the path to the
JAXBContextFactory class as the value of the
javax.xml.bind.context.factory context parameter, for example:
jaxb.properties file must appear in the same package as the domain classes.
In the sample Object-XML architecture illustrated in Figure 2-2, the starting point is an XML schema. A binding compiler binds the source schema to a set of schema-derived program classes and interfaces. JAXB-annotated classes within the application are generated either by a schema compiler or the result of a developer adding JAXB annotations to existing Java classes. The application can either marshal data to an XML document or unmarshal the data to a tree of content objects. Each content object is an instance of either a schema derived or an existing program element mapped by the schema generator and corresponds to an instance in the XML.
JAXBContextFactory class is the entry point into the TopLink JAXB runtime. It provides the required factory methods and can create new instances of
JAXBContextFactory class has the ability to:
JAXBContext object from an array of classes and a properties object
JAXBContext object from a context path and a classloader
JAXBContext class provides the client's entry point to the JAXB API. The
JAXBContext class is responsible for interpreting the metadata, generating schema files, and for creating instances of these JAXB objects:
TopLink Object-XML offers several options when creating the
JAXBContext object. You have the option of booting from:
A list of one or more JAXB-annotated classes
A list of one or more TopLink XML Bindings documents defining the mappings for your Java classes
A combination of classes and XML Bindings
A list of context paths
A list of session names, referring to TopLink sessions defined in
In addition to the input options described in Section 188.8.131.52, "JAXB Contexts and JAXB Context Factories," TopLink Object-XML provides the concept of a
MetadataSource object. This object lets you to store mapping information outside of your application and retrieve it when the application's
JAXBContext object is being created or refreshed. For information on implementing
MetadataSource, see "MetadataSource" in the EclipseLink documentation:
TopLink enables you to use all of the standard JAXB annotations. In addition to the standard annotations, TopLink offers another way of expressing your metadata—the TopLink XML Bindings document. Not only can XML Bindings separate your mapping information from your actual Java class, it can also be used for more advanced metadata tasks such as:
Augmenting or overriding existing annotations with additional mapping information.
Specifying all mappings information externally, without using Java annotations.
Defining your mappings across multiple Bindings documents.
Specifying virtual mappings that do not correspond to concrete Java fields.
For more information, see "XML Bindings" in the EclipseLink documentation:
You can use Java annotations to specify JAXB features in your TopLink projects. In addition to Java annotations, TopLink provides an XML mapping configuration file called
eclipselink-oxm.xml. This mapping file contains the standard JAXB mappings and configuration options for advanced mapping types. You can use the
eclipselink-oxm.xml file in place of or to override JAXB annotations in source code.
Using this mapping file will enable many TopLink advanced features but it can prevent the model from being portable to other JAXB implementations.
In addition to using conventional Java access methods to get and set your object's values, TopLink Object-XML also lets you access values using an XPath statement. There are special APIs on TopLink's
JAXBContext that enable you to get and set values by XPath. For more information, see "Querying Objects by XPath" in the EclipseLink documentation.