|Oracle® Fusion Middleware Oracle TopLink Concepts
11g Release 1 (11.1.1)
Part Number E26045-01
|PDF · Mobi · ePub|
This chapter describes the items that can be used in a TopLink project.
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 functionality includes 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 describes many of these features.
The entity architecture is comprised 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
Figure 2-1 Relationships Between Entity Architecture Elements
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 state 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 which 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 on the fly. 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 allows these extended mappings to be stored and managed externally. This external storage allows 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 need to 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 will 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 just 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 may 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 allow 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 defaults 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 allow 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 the 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 should appear 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 in place 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, plus 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:
Note:Using this mapping file will enable 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 will be able to 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 allows you to 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 needed to access the database. For more information and an example of running an application in stand-alone 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 once an object has been 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 will not need to be rebuilt, and its relationships will not need to be re-fetched.
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 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 knows 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 just clear the cache.
For more information, see "EclipseLink/Examples/JPA/Caching" in the EclipseLink documentation.
TopLink provides a
@Cache annotation which allows you to 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 issues, as changes made on one server will not be 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 of invalidate changed objects.
Cache invalidation based on time-to-live or time-of-day.
Optimistic locking will prevent updates to stale objects, and trigger 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 typos 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 query hints the same as named queries. The query criteria API allows you to write any JQPL query—all JPQL keywords are defined in this API. The criteria API uses a set of Java interfaces to allow queries to be dynamically constructed. It also provides compile time checking for correctness to reduce the number of runtime typos.
See "Criteria Query" in the EclipseLink documentation.
A TopLink query hint allows a JPA Query to be customized or optimized 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 raw 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 allow 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 allows you to 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 rich 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 convention
?n 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, allowing you to provide your mapping information through annotations as well as 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% 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 allow you to 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 describes 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 a 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.
Figure 2-2 A Sample Object-XML Architecture
JAXBContextFactory class is the entry point into the TopLink JAXB runtime. It provides the required factory methods and can create new instances of
JAXBContextFactory has the ability to:
JAXBContext from an array of classes and a properties object
JAXBContext 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 your
JAXBContext. You have the option of bootstrapping 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 126.96.36.199, "JAXB Contexts and JAXB Context Factories," TopLink Object-XML provides the concept of a
MetadataSource, which is responsible for serving TopLink metadata. This allows you to store mapping information outside of your application and have it retrieved when the application's
JAXBContext is being created or refreshed. For information on implementing
MetadataSource, see "MetadataSource" in the EclipseLink documentation:
TopLink allows 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, plus 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.
Note:Using this mapping file will enable many TopLink advanced features but it may 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 allows you to access values using an XPath statement. There are special APIs on TopLink's
JAXBContext to allow you to get and set values by XPath. For more information, see "Querying Objects by XPath" in the EclipseLink documentation.