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Understanding EJB types
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request from an automated retail system to a supply-chain management system. Don t worry too much about messaging right now; we ll get to the details later in this book. Next we ll explain the concept of persistence and describe how object-relational frameworks help enable automated persistence.
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1.2.3 Entities and the Java Persistence API
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One of the exciting new features of EJB 3 is the way it handles persistence. We briefly mentioned persistence providers and the JPA earlier, but now let s delve into the details. Persistence is the ability to have data contained in Java objects automatically stored into a relational database like Oracle, SQL Server, and DB2. Persistence in EJB 3 is managed by the JPA. It automatically persists the Java objects using a technique called object-relational mapping (ORM). ORM is essentially the process of mapping data held in Java objects to database tables using configuration. It relieves you of the task of writing low-level, boring, and complex JDBC code to persist objects into a database. The frameworks that provide ORM capability to perform automated persistence are known as ORM frameworks. As the name implies, an ORM framework performs transparent persistence by making use of object-relational mapping metadata that defines how objects are mapped to database tables. ORM is not a new concept and has been around for a while. Oracle TopLink is probably the oldest ORM framework in the market; open source framework JBoss Hibernate popularized ORM concepts among the mainstream developer community. In EJB 3 terms, a persistence provider is essentially an ORM framework that supports the EJB 3 Java Persistence API (JPA). The JPA defines a standard for
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The creation of ORM configuration metadata for mapping entities to relational tables The EntityManager API a standard API for performing CRUD (create, read, update, and delete)/persistence operations for entities The Java Persistence Query Language (JPQL), for searching and retrieving persisted application data
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Since JPA standardizes ORM frameworks for the Java platform, you can plug in ORM products like JBoss Hibernate, Oracle TopLink, or BEA Kodo as the underlying JPA persistence provider for your application.
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What s what in EJB 3
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It may occur to you that automated persistence is something you ll find useful for all kinds of applications, not just server-side applications such as those built with EJB. After all, JDBC, the grandfather of JPA, is used in everything from large-scale real-time systems to desktop-based hacked-up prototypes. This is exactly why JPA is completely separate from the rest of EJB 3 and usable in plain Java SE environments. Entities are the session bean and MDB equivalent in the JPA world. Let s take a quick glance at them next, as well as the EntityManager API and the Java Persistence Query Language (JPQL). Entities If you re using JPA to build persistence logic of your applications, then you have to use entities. Entities are the Java objects that are persisted into the database. Just as session beans model processes, entities model lower-level application concepts that high-level business processes manipulate. While session beans are the verbs of a system, entities are the nouns. Examples include an Employee entity, a User entity, an Item entity, and so on. Here s another perfectly valid (and often simplerto-understand) way of looking at entities: they are the OO representations of the application data stored in the database. In this sense, entities survive container crashes and shutdown. You must be wondering how the persistence provider knows where the entity will be stored. The real magic lies in the ORM metadata; an entity contains the data that specifies how it is mapped to the database. You ll see an example of this in the next chapter. JPA entities support a full range of relational and OO capabilities, including relationships between entities, inheritance, and polymorphism. The EntityManager The JPA EntityManager interface manages entities in terms of actually providing persistence services. While entities tell a JPA provider how they map to the database, they do not persist themselves. The EntityManager interface reads the ORM metadata for an entity and performs persistence operations. The EntityManager knows how to add entities to the database, update stored entities, and delete and retrieve entities from the database. In addition, the JPA provides the ability to handle lifecycle management, performance tuning, caching, and transaction management.
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