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Virtual Trainer uses a generic layer of data access objects to communicate to a persistence tier Figure 9-23 illustrates the architecture strategy that Virtual Trainer employs for accessing/updating training event data The TrainingEventRegistry interface provides a consistent set of methods to interact with the persisted data As you can see in the diagram in Figure 9-23, the Virtual Trainer application relies entirely on the data access layer accessed from page backing beans to interact with persistent data
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In a general sense, using a persistence framework involves mapping the database objects as Java objects and creating data access objects The process of creating both a set of mapped Java objects and a set of data access objects can be done either by hand or by using a development tool Fortunately, all of the popular persistence frameworks, including Hibernate and TopLink, have support for auto-mapping and data access code generation capabilities Even Enterprise JavaBeans have numerous tools that can automatically generate mapping and data access code
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The following discussion shows how to create a Hibernate-based data access layer, but the methods employed are virtually the same whether working with Hibernate, TopLink, or EJB version 3 The discussion also centers on building just the master-detail portion of the application, which is the set of training events and their associated sets of workout sessions Building Hibernate persistence of the user registry is not shown but it s trivial since it is a simple, single table/Java class mapping
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FIGURE 9-23 The Virtual Trainer application using a data access layer
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The process of object relational mapping involves matching a Java class with a similar database table in structure The mapping information is usually stored in a separate file that literally maps each database column and data type to a respective Java class s property There are also different approaches to mapping One approach is to map existing database tables to new Java classes This is a common approach when the database structure has already been defined and is referred to as reverse-engineering the database tables into a Java persistence layer The other approach is to forward-engineer your Java classes into database objects This is a logical approach if your application begins as a pure Java prototype To get a feel for how database objects and Java classes can be mapped together, consider the Virtual Trainer Java class, TrainingEvent, which contains information about a single athletic event with properties such as ename (event name), etype (event type), and completionDate (date of the completion of the event) A corresponding database object could be a table, TRAININGEVENT (or TRAININGEVENTS), with respective columns ENAME, ETYPE, and COMPLETIONDATE As you can guess, the challenge to object relational mapping lies in mapping equivalent or similar data types from the Java world to the relational database world Mapping simple data types such as strings or integers is easily done as databases and Java handle these simple types in a similar manner There are, however, more complicated mappings such as mapping the Java boolean type Unfortunately,
9:
A p p l y i n g J S F : I n t r o d u c i n g t h e Vi r t u a l Tr a i n e r A p p l i c a t i o n
databases do not handle boolean data types in the same manner, so persistence technologies have to be aware of the database dialect and provide a solution on the database that emulates the behavior of the Java boolean data type Another area that can be tricky in object relational mappings deals with handling masterdetail relationships, which is the case with the Virtual Trainer application On the Java side, Virtual Trainer s TrainingEvent class has a property, workoutsessions, of type List A database equivalent of this requires another table (representing workoutsessions data) with a foreign key relationship referring back to the master TRAININGEVENT table The challenge here lies in providing the correct mapping information detailing the master-detail relationship between the two tables and their respective Java class counterparts This is exactly what was needed for implementing a persistence layer for Virtual Trainer The remaining discussion focuses on how the Java classes TrainingEvent and TrainingSession are mapped to respective master-detail database tables Since the Java-only version of Virtual Trainer has existing Java objects but doesn t yet have actual database entities representing them, it is possible to have Hibernate generate the necessary DDL to create the database entities based on the Java objects TopLink also offers this capability Conversely, had we started with an existing database schema, we could have used Hibernate or TopLink to reverse-engineer the database objects into appropriate Java objects However, since the Virtual Trainer prototype started from a pure Java prototype, we will use the persistence technology to auto-generate the database entities In order to generate a corresponding set of database entities that map to the Java objects, we first need to create mappings of the Java objects so Hibernate will know how to map them to database entities Again, creating Hibernate mappings can be done automatically using tools, but it is very useful to build your mappings by hand, especially if you are new to Hibernate, so that you fully understand the relationships between your Java objects and the database objects they are mapped to Recall the Java class, TrainingEventjava, which has the following properties (Setters and getters are not shown)
private private private private private private private private private Long id; String ename; String etype; Date completionDate; String skill; String status; String trainer; List workoutsessions; String userid;
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