10g (9.0.4)
Part Number B10313-01
 Home |
 Solution Area |
 Contents |
 Index |
| Oracle Application Server TopLink Application Developer's Guide 10g (9.0.4) Part Number B10313-01 |
|
Oracle Application Server TopLink is an advanced object-to-relational persistence framework, suitable for a wide range of Java 2 Enterprise Edition (J2EE) and Java application architectures. OracleAS TopLink development tools and runtime capabilities reduce development and maintenance efforts, and increase enterprise application functionality. Use OracleAS TopLink to build high performance applications that store persistent data in a relational database.
This chapter introduces OracleAS TopLink and includes discussions on the following topics:
The remainder of this document provides the necessary details required to build J2EE applications with OracleAS TopLink.
Enterprise applications rely on Java-to-database integration to implement objects and logic. OracleAS TopLink enables developers to efficiently develop and refine enterprise applications. To fully understand OracleAS TopLink, you must understand the problems that enterprise application developers face and how OracleAS TopLink resolves them.
Java-to-database integration is a widely underestimated problem in enterprise Java applications. This complex problem involves more than reading from and writing to a database. The database world includes elements such as tables, rows, columns, and primary and foreign keys; the Java and J2EE world contains entity classes (regular Java classes or Enterprise JavaBeans (EJB) entity beans), business rules, complex relationships, and inheritance. Bridging these two fundamentally different technologies is a challenging and resource-intensive problem.
The process of translating object-oriented data into relational data is referred to as object-relational (O-R) mapping. To enable an O-R solution, developers must resolve the following O-R bridging issues:
Application developers need a product that enables them to integrate Java applications and relational databases, without compromising ideal application design or database integrity. In addition, Java developers need the ability to store (or persist) and retrieve business domain objects using a relational database as a repository.
The OracleAS TopLink solution is a persistence framework that manages O-R mapping in a seamless manner and enables developers to rapidly build applications that combine the best aspects of object technology and relational databases.
OracleAS TopLink provides a mature and powerful solution that addresses the disparity between Java objects and relational databases. OracleAS TopLink enables developers to:
In addition to providing industry leading O-R mapping capabilities, OracleAS TopLink provides flexibility, increases performance and maximizes the productivity of your applications. OracleAS TopLink provides the following features:
At its core, OracleAS TopLink is a runtime engine that provides Java or J2EE applications with access to persistent entities stored in a relational database. In addition to runtime capabilities, the Oracle Application Server TopLink Foundation Library includes the OracleAS TopLink Application Programming Interface (API). This API enables applications to access OracleAS TopLink runtime features, as well as development tools that simplify application development. The tools capture mapping and runtime configuration information in metadata files that OracleAS TopLink passes to the runtime.
OracleAS TopLink application development comprises three elements: the development environment, the OracleAS TopLink runtime, and the metadata that ties them together.
To create an OracleAS TopLink application, map the object and relational models using the OracleAS TopLink Mapping Workbench, and capture the resulting mappings and additional runtime configurations in the OracleAS TopLink project file (the project.xml file). Then build a session configuration file (the sessions.xml file) in the OracleAS TopLink Sessions Editor. These files together represent your entire OracleAS TopLink project.
During development, developers leverage the OracleAS TopLink API to define query and transaction logic. When developers use EJB entity beans, there is generally little or no direct use of the OracleAS TopLink API.
The OracleAS TopLink Foundation Library provides the OracleAS TopLink runtime component. Access the runtime component either directly through the OracleAS TopLink API, or indirectly through a J2EE container when using EJB entity beans. The runtime engine is not a separate or external process; instead, it is embedded within the application. Application calls invoke OracleAS TopLink to provide persistence behavior. This function allows for transactional and thread-safe access to shared database connections and cached objects.
OracleAS TopLink metadata is the bridge between the development of an application and its deployed runtime. Capture the metadata using the OracleAS TopLink Mapping Workbench and OracleAS TopLink Sessions Editor, and pass the metadata to the runtime using deployment project.xml and sessions.xml files. It is also possible to hand-code these files using Java and the OracleAS TopLink API, but this approach is more labor-intensive.
The metadata, encapsulated in the project.xml file and the sessions.xml file, allows developers to pass configuration information into the runtime environment. The runtime uses the information in conjunction with the persistent entities (Java objects or EJB entity beans), and the code written with the OracleAS TopLink API, to complete the application.
The OracleAS TopLink Mapping Workbench is a graphical development tool that enables developers to map between the object and relational models, and configure many of the OracleAS TopLink Foundation Library features. The OracleAS TopLink Mapping Workbench creates an OracleAS TopLink project, the primary object in the OracleAS TopLink metamodel. Export the project as a single deployment XML file (the project.xml file), which OracleAS TopLink uses in conjunction with the OracleAS TopLink runtime to provide the application-specific persistence capabilities.
The OracleAS TopLink Mapping Workbench can import compiled entity classes (Java objects or EJB entity beans), as well as relational schema through a JDBC driver that the developer configures. Because OracleAS TopLink imports the object and relational models for mapping, developers can develop the two models relatively independently from the O-R mapping phase of a project development.
Most OracleAS TopLink applications include a session configuration file, the sessions.xml file, to simplify the application deployment process. The OracleAS TopLink Sessions Editor provides a graphical environment to configure the sessions.xml file.
Use the sessions.xml file to configure one or more sessions for the OracleAS TopLink project, and associate the sessions with the project. This approach allows developers to specify individual configurations for each session and to add or modify:
The Oracle Application Server TopLink Foundation Library includes a Java library that forms the runtime component of the product. It provides support and the API for the components that make up an OracleAS TopLink application. The API enables developers to interact with OracleAS TopLink to retrieve and modify their application persistent entities.
|
Note: Although this chapter describes how these components fit into J2EE architectures, note that OracleAS TopLink also supports non-J2EE solutions. Chapter 2, "OracleAS TopLink Architectures" describes theses solutions in more detail. |
A session is the primary interface between the client application and OracleAS TopLink, and represents the connection to the underlying relational database. OracleAS TopLink offers several different session types, each optimized for different design requirements and architectures. The session manager configures and manages the session as a singleton within the application.
The most commonly-used session is the server session, a singleton session that clients access on the server through a client session. The server session provides a shared cache and shared JDBC connection resources. OracleAS TopLink supports sessions for two-tier architectures, distributed applications, and multiple databases.
The OracleAS TopLink data access component provides access to JDBC connections through connection pooling, provided either by OracleAS TopLink or a host application server. This component manages the SQL generation required by the various query operations and reconciles any differences between JDBC drivers and SQL dialects. OracleAS TopLink offers many performance tuning options that optimize its data access capabilities.
OracleAS TopLink supplies an object level cache that guarantees object identity and enhances performance. Developers can configure the OracleAS TopLink cache and maximize application efficiency by reducing the number of times the application accesses the database. In a clustered environment, developers can configure OracleAS TopLink to synchronize changes with other instances of the deployed application.
The OracleAS TopLink query framework provides developers with the flexibility necessary to manage the complex persistence requirements of enterprise applications. The key features of this query framework include:
Developers can define OracleAS TopLink queries using the OracleAS TopLink Mapping Workbench, in Java code using the OracleAS TopLink API, or, in the case of EJB entity beans, through EJB Finders.
OracleAS TopLink provides the ability to write transactional code isolated from the underlying database and schema. OracleAS TopLink achieves this functionality through the Unit of Work.
The Unit of Work isolates changes in a transaction from other threads until it successfully commits the changes to the database. Unlike other transaction mechanisms, the Unit of Work automatically manages changes to the objects in the transaction, the order of the changes, and changes that might invalidate other OracleAS TopLink caches. The Unit of Work manages these issues by calculating a minimal change set, ordering the database calls to comply with referential integrity rules and deadlock avoidance, and merging changed objects into the shared cache. In a clustered environment, the Unit of Work also synchronizes changes with the other servers in the cluster.
If an application uses EJB entity beans, developers do not access the Unit of Work API directly, but they still benefit from its features: the integration between the OracleAS TopLink runtime and the J2EE container leverages the Unit of Work automatically.
By default, OracleAS TopLink allows the application to create transaction boundaries for all object-level changes. OracleAS TopLink explicitly manages the database transaction, and if it encounters problems, safely rolls back both the database changes and the object-level changes.
In the case of a J2EE application, developers can configure OracleAS TopLink to synchronize with the JTA/JTS subsystem of the host application server. This feature allows an application to use container-managed transactions, rather than the default user-managed transactions.
Note that this functionality is not limited to EJB architectures. Developers can configure any OracleAS TopLink architecture to use container-managed transactions.
The OracleAS TopLink approach to persistence is based on metadata that defines the class structure (objects) and relational schema, along with other configuration information used by OracleAS TopLink at runtime. Developers can use the OracleAS TopLink Mapping Workbench to define this metadata, and the OracleAS TopLink runtime component uses the metadata to provide the necessary persistence capabilities, using Java's reflective and introspective capabilities.
The OracleAS TopLink application metadata model is based around the OracleAS TopLink project. The project includes descriptors, mappings, and various policies that customize the runtime capabilities.
Use the sessions.xml file to configure sessions for the project. Developers can build and edit these files with the OracleAS TopLink Sessions Editor. The session manager uses the sessions.xml configuration file during application initialization.
The OracleAS TopLink deployment project is the primary container for the metadata. A project generally represents an application and contains the mapping information for all persistent classes and their relationships. Each session (excluding the session broker) in the deployed application references a single project. Although developers can build a project by coding it using the OracleAS TopLink API, we recommend that developers create and manage the project in the OracleAS TopLink Mapping Workbench, and use the OracleAS TopLink Mapping Workbench to generate either an XML or Java source version of the project for use at runtime.
A descriptor represents the association between a persistent Java class and a relational table(s). The descriptor contains configuration information for the class level within a project, as well as a set of mappings for each of its persistent attributes. Many of the more advanced configuration options are set at the descriptor level. The OracleAS TopLink Mapping Workbench supports most of these options, but there are a few that developers must set using the OracleAS TopLink API.
Mappings describe how the attributes of a mapped class are associated with columns in the database. OracleAS TopLink provides a sophisticated set of flexible and customizable mappings that allow for complex mapping scenarios between the object and relational models.
There are two types of mappings: direct mappings, and relationship mappings.
Direct mappings relate an attribute or attributes to a column or columns in the relational schema. OracleAS TopLink provides several direct mappings that allow for conversions between the types from the database and the object model's attribute types. Here are the direct mappings and their function:
OracleAS TopLink offers sophisticated relationship mapping, which enables developers to represent object relationships based on the database table columns and foreign keys. Here are the relationship mappings and their function:
Using OracleAS TopLink to build an application does not affect the choice of development tools or the creative process. However, OracleAS TopLink does influence how developers approach development. This section highlights some of the key areas in which using OracleAS TopLink affects application development. These areas exist, regardless of whether developers are building an application to support Java objects, EJB entity beans, or both.
OracleAS TopLink maps the application's persistent entities to the database, using the descriptors and mappings developers build with the OracleAS TopLink Mapping Workbench. The OracleAS TopLink Mapping Workbench supports several approaches to project development, including:
The OracleAS TopLink Mapping Workbench supports all these options; however, the most common solution is to develop the persistent entities using a development tool, such as an integrated development environment (IDE) or modeling tool, and to develop the relational model through appropriate relational design tools. Developers then use the OracleAS TopLink Mapping Workbench to construct mappings that relate these two models.
The OracleAS TopLink Mapping Workbench does offer some facilities for generating persistent entities or the relational model components for an application; however, these utilities are intended only to assist in rapid initial development strategies, rather than complete round-trip application development.
For more information about mapping, see Chapter 3, "Mapping" and the Oracle Application Server TopLink Mapping Workbench User's Guide.
Sessions are the primary interface between the application and OracleAS TopLink persistence capabilities. When developing an OracleAS TopLink application, developers must ensure that they properly initialize and manage the sessions.
When using EJB entity beans with container-managed persistence (CMP) or bean-managed persistence (BMP), the client code that modifies the entity beans does not access the OracleAS TopLink session directly. Instead, changes occur transparently, through integration with the container or through EJB callbacks.
Well-designed applications that employ Java objects as persistent entities use the session manager provided in the OracleAS TopLink API. This class initializes and manages the singleton session. Developers configure the session manager in the sessions.xml file, which allows for easy configuration and customization of the deployed application.
For more information about session management, see Chapter 4, "Sessions".
OracleAS TopLink provides several object and data query types, and offers flexible options for query selection criteria, including:
With these options, developers can build any type of query. We recommend that developers use predefined queries to define application queries. Predefined queries are held in the project metadata and referenced by name. This simplifies application development and encapsulates the queries to reduce maintenance costs.
The OracleAS TopLink Mapping Workbench provides the simplest way to define queries. Developers can also build queries in code, using the OracleAS TopLink API.
If the application includes EJB entity beans, developers can code finders completely using EJB QL, which enables the application to comply with the J2EE specification. Alternatively, developers can use any of the other OracleAS TopLink query options. All querying options are available, regardless of the architecture or persistent entity type.
For more information about querying, see Chapter 6, "Queries".
In an OracleAS TopLink application, the Unit of Work ensures that OracleAS TopLink transactions comply with the transactional requirements of the application.
The Unit of Work is one of the most sophisticated and powerful components of the OracleAS TopLink Foundation Library. Although developers that use CMP or BMP entity beans do not use the OracleAS TopLink API to apply transactional changes to their persistent entities, the Unit of Work is used behind the scenes. Understanding how the Unit of Work behaves, and developing simple coding patterns to use it, are the keys to building efficient, maintainable applications.
For more information about transaction, see Chapter 7, "Transactions".
Application packaging (for deployment in the host Java or J2EE environment) influences OracleAS TopLink use and configuration. For example, developers package a J2EE enterprise application in an Enterprise Archive (EAR) file. Within the EAR file, there are several ways to package persistent entities within Web Application (WAR) and Java libraries (JAR). How developers configure OracleAS TopLink depends, in part, on how they package the application and how they use the host application server class loader.
For more information about packaging and deployment, see Chapter 9, "Packaging for Deployment".
OracleAS TopLink enables developers to monitor functionality and performance throughout application development, testing, and quality assurance cycles. OracleAS TopLink offers many textual logging, as well as the API required to implement custom logging strategies. Developers can use these features to ensure that the application behaves and performs as they expect.
OracleAS TopLink includes a performance profiler feature, available through the OracleAS TopLink Foundation Library API. This runtime feature tracks query execution time, which developers can use for performance analysis. This tool provides the information necessary to identify bottlenecks that hinder application performance.
OracleAS TopLink also offers a rich set of performance enhancement features. Understanding how to configure these features can have a strong influence on application performance, especially in the later phases of application development.
For more information about monitoring and performance tuning, see Chapter 10, "Tuning for Performance".
OracleAS TopLink is designed to work in both Java and J2EE applications. Since it was first introduced, the flexibility OracleAS TopLink provides has led to its use in many architectural styles. This section introduces the five most common architectures associated with OracleAS TopLink. Although this section describes the architectures in relation to J2EE, OracleAS TopLink continues to fully support non-J2EE and Java applications as well.
For more information about OracleAS TopLink's flexible architecture support, see Chapter 2, "OracleAS TopLink Architectures".
The three-tier (or J2EE Web) application is one of the most common OracleAS TopLink architectures. This architecture is characterized by a server-hosted environment in which the business logic, persistent entities, and the OracleAS TopLink Foundation Library all exist in a single Java virtual machine (JVM).
The most common example of this architecture is a simple three-tier application in which the client browser accesses the application through servlets, Java Server Pages (JSPs) and HTML. The presentation layer communicates with OracleAS TopLink through other Java classes in the same JVM, to provide the necessary persistence logic. This architecture supports multiple servers in a clustered environment, but there is no separation across JVMs from the presentation layer and the code that invokes the persistence logic against the persistent entities using OracleAS TopLink.
A popular variation on the three-tier application involves wrapping the business logic, including the OracleAS TopLink access, in EJB session beans. This architecture provides a scalable deployment and includes integration with transaction services from the host application server. Communication from the presentation layer occurs through calls to the EJB session beans. This architecture separates the application into different tiers for the deployment.
The session bean architecture can persist either Java objects or EJB entity beans.
OracleAS TopLink provides CMP support for applications that require the use of EJB entity beans. This support is available on the leading application servers. OracleAS TopLink CMP support provides the developer with an EJB 1.1 and 2.1 CMP solution transparent to the application code, but still offers all the OracleAS TopLink runtime benefits.
Applications can access OracleAS TopLink-enabled EJB entity beans using CMP directly from the client, or from within a session bean layer. OracleAS TopLink also offers the ability to use regular Java objects in relationships with EJB entity beans.
Another option for using EJB entity beans is to leverage OracleAS TopLink BMP in the application. This architecture enables developers to access the persistent data through the EJB API, but is platform independent.
The BMP approach is portable--that is, after a developer creates an application, you can move it from one application server platform to another.
A two-tier (or client-server) application is one in which the OracleAS TopLink application accesses the database directly. Although less common than the others architectures discussed here, OracleAS TopLink supports this architecture for smaller or embedded data processing applications.
In addition to the OracleAS TopLink specific concepts, familiarity with several industry standard concepts helps you understand and implement OracleAS TopLink applications more effectively.
The Java 2 Platform, Standard Edition (J2SE) is the core Java technology platform. It provides software compilers, tools, runtimes, and APIs for writing, deploying, and running applets and applications in Java.
The Java 2 Platform, Enterprise Edition (J2EE) is an environment for developing and deploying enterprise applications. J2EE includes a set of services, APIs, and protocols for developing multi-tiered Web-based applications.
A J2EE container is a runtime environment for EJBs that includes such basic functions as security, life cycle management, transaction management, and deployment services. J2EE containers are usually provided by a J2EE server, such as Oracle Application Server Containers for J2EE.
The Java Transaction API (JTA) specifies the interfaces between a transaction manager, a resource manager, an application server, and transactional applications involved in a distributed transaction system.
Java Data Objects (JDO) represent a standard Java model for persistence that enables programmers to create code in Java that transparently accesses the underlying data store without using database-specific code. OracleAS TopLink provides support for most of the JDO specification, but, because OracleAS TopLink is a persistence framework, developers may find it easier and more effective to build your applications using OracleAS TopLink functionality rather than JDO.
|
 Copyright © 2000, 2003 Oracle Corporation. All Rights Reserved. |
|
