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1.3.2 Accessing JPA services: the persistence provider
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In section 1.2.3, we mentioned that the persistence provider s job is to provide standardized JPA services. Let s explore how it does that. Instead of following the JVM-like container model, JPA follows a model similar to APIs, like JDBC. JPA provides persistence services such as retrieving, adding, modifying, and deleting JPA entities when you explicitly ask for them by invoking EntityManager API methods. The provider terminology comes from APIs such as JDBC and JNDI too. If you ve worked with JDBC, you know that a provider is essentially the vendor implementation that the JDBC API uses under the covers. Products that provide JPA implementation are persistence providers or persistence engines. JBoss Hibernate and Oracle TopLink are two popular JPA providers.
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Getting inside EJB
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Since JPA is completely pluggable and separable, the persistence provider and container in an EJB 3 solution need not come from the same vendor. For example, you could use Hibernate inside a BEA WebLogic container if it suits you better, instead of the Kodo implementation WebLogic ships with. But without services, what good are containers In the next section, we explore the services concept critical to EJB.
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1.3.3 Gaining functionality with EJB services
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The first thing that should cross your mind while evaluating any technology is what it really gives you. What s so special about EJB Beyond a presentation-layer technology like JSP, JSF, or Struts, couldn t you create your web application using just the Java language and maybe some APIs like JDBC for database access The plain answer is that you could if deadlines and cutthroat competition were not realities. Indeed, before anyone dreamed up EJB this is exactly what people did. What the resulting long hours proved is that you tend to spend a lot of time solving very common system-level problems instead of focusing on the real business solution. These bitter experiences emphasized the fact that there are common solutions that can be reused to solve common development problems. This is exactly what EJB brings to the table. EJB is a collection of canned solutions to common server application development problems as well as a roadmap to common server component patterns. These canned solutions, or services, are provided by either the EJB container or the persistence provider. To access those services, you build the application components and deploy them into the container. Most of this book will be spent explaining how you can exploit EJB services. In this section, we briefly introduce some of the services EJB offers. Obviously, we can t explain the implementation details of each service in this section. Neither is it necessary to cover every service EJB offers right now. Instead, we briefly list the major EJB 3 services in table 1.1 and explain what they mean to you from a practical perspective. This book shows you how to use each of the services shown in table 1.1 in your application. Despite its robust features, one of the biggest beefs people had with EJB 2 was that it was too complex. It was clear that EJB 3 had to make development as simple as possible instead of just continuing to add additional features or services. If you have worked with EJB 2 or have simply heard or read that it is complex, you should be curious as to what makes EJB 3 different. Let s take a closer look.
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Table 1.1 Major EJB 3 component services and why they are important to you. The persistence services are provided by the JPA provider. Service Integration Applies To Session beans and MDBs Stateless session beans, MDBs What It Means for You Helps glue together components, ideally through simple configuration instead of code. In EJB 3, this is done through dependency injection (DI) as well as lookup. For each EJB component, the EJB platform creates a pool of component instances that are shared by clients. At any point in time, each pooled instance is only allowed to be used by a single client. As soon as an instance is finished servicing a client, it is returned to the pool for reuse instead of being frivolously discarded for the garbage collector to reclaim. EJB makes all components thread-safe and highly performant in ways that are completely invisible. This means that you can write your server components as if you were developing a single-threaded desktop application. It doesn t matter how complex the component itself is; EJB will make sure it is thread-safe. The EJB container manages state transparently for stateful components instead of having you write verbose and error-prone code for state management. This means that you can maintain state in instance variables as if you were developing a desktop application. EJB takes care of all the details of session maintenance behind the scenes. EJB 3 allows you to write messaging-aware components without having to deal with a lot of the mechanical details of the Java Messaging Service (JMS) API. EJB supports declarative transaction management that helps you add transactional behavior to components using simple configuration instead of code. In effect, you can designate any component method to be transactional. If the method completes normally, EJB commits the transaction and makes the data changes made by the method permanent. Otherwise the transaction is rolled back. EJB supports integration with the Java Authentication and Authorization Service (JAAS) API, so it is very easy to completely externalize security and secure an application using simple configuration instead of cluttering up your application with security code. EJB 3 introduces AOP in a very lightweight, accessible manner using interceptors. This allows you to easily separate out crosscutting concerns such as logging, auditing, and so on in a configurable way. In EJB 3, you can make components remotely accessible without writing any code. In addition, EJB 3 enables client code to access remote components as if they were local components using DI. continued on next page
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