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Understanding object/relational persistence
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Hibernate EntityManager is a small wrapper around Hibernate Core that provides JPA compatibility.) Working with standardized interfaces and using a standardized query language has the benefit that you can execute your JPA-compatible persistence layer with any EJB 3.0 compliant application server. Or, you can use JPA outside of any particular standardized runtime environment in plain Java (which really means everywhere Hibernate Core can be used). Hibernate Annotations should be considered in combination with Hibernate EntityManager. It s unusual that you d write your application code against JPA interfaces and with JPA queries, and not create most of your mappings with JPA annotations. Java EE 5.0 application servers We don t cover all of EJB 3.0 in this book; our focus is naturally on persistence, and therefore on the JPA part of the specification. (We will, of course, show you many techniques with managed EJB components when we talk about application architecture and design.) Hibernate is also part of the JBoss Application Server (JBoss AS), an implementation of J2EE 1.4 and (soon) Java EE 5.0. A combination of Hibernate Core, Hibernate Annotations, and Hibernate EntityManager forms the persistence engine of this application server. Hence, everything you can use stand-alone, you can also use inside the application server with all the EJB 3.0 benefits, such as session beans, message-driven beans, and other Java EE services. To complete the picture, you also have to understand that Java EE 5.0 application servers are no longer the monolithic beasts of the J2EE 1.4 era. In fact, the JBoss EJB 3.0 container also comes in an embeddable version, which runs inside other application servers, and even in Tomcat, or in a unit test, or a Swing application. In the next chapter, you ll prepare a project that utilizes EJB 3.0 components, and you ll install the JBoss server for easy integration testing. As you can see, native Hibernate features implement significant parts of the specification or are natural vendor extensions, offering additional functionality if required. Here is a simple trick to see immediately what code you re looking at, whether JPA or native Hibernate. If only the javax.persistence.* import is visible, you re working inside the specification; if you also import org.hibernate.*, you re using native Hibernate functionality. We ll later show you a few more tricks that will help you cleanly separate portable from vendor-specific code.
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What is the future of Hibernate Hibernate Core will be developed independently from and faster than the EJB 3.0 or Java Persistence specifications. It will be the testing ground for new ideas, as it has always been. Any new feature developed for Hibernate Core is immediately and automatically available as an extension for all users of Java Persistence with Hibernate Annotations and Hibernate EntityManager. Over time, if a particular concept has proven its usefulness, Hibernate developers will work with other expert group members on future standardization in an updated EJB or Java Persistence specification. Hence, if you re interested in a quickly evolving standard, we encourage you to use native Hibernate functionality, and to send feedback to the respective expert group. The desire for total portability and the rejection of vendor extensions were major reasons for the stagnation we saw in EJB 1.x and 2.x.
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After so much praise of ORM and Hibernate, it s time to look at some actual code. It s time to wrap up the theory and to set up a first project.
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In this chapter, we ve discussed the concept of object persistence and the importance of ORM as an implementation technique. Object persistence means that individual objects can outlive the application process; they can be saved to a data store and be re-created at a later point in time. The object/relational mismatch comes into play when the data store is an SQL-based relational database management system. For instance, a network of objects can t be saved to a database table; it must be disassembled and persisted to columns of portable SQL datatypes. A good solution for this problem is object/relational mapping (ORM), which is especially helpful if we consider richly typed Java domain models. A domain model represents the business entities used in a Java application. In a layered system architecture, the domain model is used to execute business logic in the business layer (in Java, not in the database). This business layer communicates with the persistence layer beneath in order to load and store the persistent objects of the domain model. ORM is the middleware in the persistence layer that manages the persistence. ORM isn t a silver bullet for all persistence tasks; its job is to relieve the developer of 95 percent of object persistence work, such as writing complex SQL statements with many table joins, and copying values from JDBC result sets to objects or graphs of objects. A full-featured ORM middleware solution may provide database portability, certain optimization techniques like caching, and other viable functions that aren t easy to hand-code in a limited time with SQL and JDBC.
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