Deployment descriptors still have their place for those who want to override Annotations. No more boilerplate code for looking up EJBs and related resources. By introducing the Inject, EJB, and Resource annotations, these dependencies are now injected transparently by the container into your simple Java Beans.
Developers can now just implement the callbacks they are interested in. Singleton beans offer some support for thread safety via the Lock annotation. The two options are Lock LockType. WRITE , which is the default. Another useful feature of singleton beans is the ability to schedule tasks in a simple way, using the Schedule annotation.
Listing 5 shows how to schedule a task daily at noon. At a high level, CDI offers a general-purpose component framework, while EJB stands out for its richly featured, individual components. Whereas CDI uses dependency injection to define and reference any software component, EJB components are more formally defined, with each offering a specific set of capabilities out of the box. Enterprise JavaBeans was the first specification to offer an easy way of encapsulating and re-using business logic in enterprise Java applications.
Far from the heavyweight behemoth of old, EJB today is a lean, annotations-based framework that lets you access a wide range of enterprise functionality, right out of the box.
Consider EJB the next time you're asked to quickly ramp up a distributed, scalable business application. You might be pleasantly surprised. This story, "What is EJB? He believes in people-first technology. When not playing guitar, Matt explores the backcountry and the philosophical hinterlands.
He has written for JavaWorld since Here are the latest Insider stories. More Insider Sign Out. Sign In Register. Sign Out Sign In Register. Latest Insider. Check out the latest Insider stories here. More from the IDG Network. Josh Long. The Enterprise Java Bean 3. The specification was built very apparently with input from the community. It represented a much more consistent services paradigm, one that was more POJO-friendly and generally less complicated. The level of indirection afforded by Java 5's annotations made the paradigm more powerful while requiring less of the developer.
The willingness to forsake bad legacy decisions for different, new solutions, made the framework interesting to people who might have previously shunned EJB. The handful or more of Java classes and interfaces required for your average bean in EJB 2. Convention-over-configuration based defaults were put into place to make it more straightforward to get up and running. EJB 3. If EJB 3. Its arrival isn't unwelcome, of course. It is also telling that - even with all these new features - the EJB 3.
Some of the biggest changes in EJB 3. Some increase the surface area of the user-facing APIs to introduce more flexibility.
Some simply bring more flexibility. A singleton is a new type of session bean that provides one extra guarantee: the bean will only be created once per running JVM. There are many use cases that are better served with this feature: caching, for example. Another is the ability to guarantee a shared view on a resource that the application server doesn't already provide.
Simple storage for valuable data that needn't be persisted but that might be too expensive to recreate is another option. Let's look at a contrived example. We'll assume that the User entity has already been created elsewhere perhaps using the JPA 2. All clients can update the same mutable state by simple acquiring a reference to the ChatRoom instance. Clients are guaranteed access to the same instance upon acquisition.
As this is a session bean, it offers the same guarantees as any other session bean: this bean is fully thread safe. Singletons are designed for concurrent access. The specification gives the developer sophisticated control over concurrency control. You may use the container to force certain types of access in a declarative way.
This behavior is the default. You may explicityly declare as using container-managed concurrency by annotating it with javax. If you want to exert more control over the bean, use javax. Container-managed concurrency lets you stipulate the type of access at the method-level or class-level. You might by default stipulate that all business methods are to be serialized by using javax. Lock WRITE at the class level and then optimize for the case where a method is effectively "read-only," with no state-modifying side-effects.
All access on a method annotated with Lock WRITE is serialized, and blocks client access until completion, or a timeout occurs. You may exercise control over the length of a timeout using the AccessTimeout annotation, which takes a value of java. Thus, we might be able to rework the code in the first implementation of the ChatRoom to take advantage of this concurrency control.
Naturally, such a ChatRoom will ultimately die a memory starved death as the number of users and posts overwhelms the application server's memory. Some sort of expiration mechanism is in order.
However, it's always worked in terms of millisecond intervals and been fairly tedious to set up. The situation was a bit improved with EJB 3. If you want to set something up at the beginning of the week, for example, then you faced challenges. Let's revisit our ChatRoom. We want to schedule a bean to go through and garbage collect old data. Here, we're using a declarative model to ensure that the method runs every hour. In EJB 3. While indirection via interfaces is a very powerful technique, it sometimes just complicates things.
The client view is then the public methods as exposed by the class. The simple way to handle scaling issues is This approach is most famously characterized by the SEDA staged event driven architecture pattern, wherein bottlenecks are avoided by queuing work.
This lets submissions of work being queued and the client proceed. If a component downstream takes a long time, and the system is under load, this pattern ensures that the slow component doesn't bring the system to its knees.
Another approach to scaling is to not block client invocations on one-way message exchanges. Another approach still is to simply work asynchronously, letting execution on the client proceed until a result comes back. All of these approaches are embodied in the new asynchronous support for services in EJB 3.
A bean class, or individual methods, may be annotated with the javax. Asynchronous annotation to tell the container that clients should not block on the result of the invocation. This lets the client proceed instantaneously, and - in theory - it would enable a container to buffer the work until it's better able to perform it.
If a bean class, or a business interfaces, is annotated with Asynchronous, then all methods on the bean are deferred. Otherwise, only methods that are annotated with Asynchronous are deferred. Asynchronous methods may return void, or an instance of java. This way, if the EJB takes an hour or two, then it doesn't matter - the client is unaffected. Conceptually this is the same as calling a service with the sole job of sending the request to a JMS queue.
A client's transactional context is not propagated to the asynchronous method. Let's take a look at an example: we want to build a service that talks to several other web services and aggregates the results. We want the results, but we can't leave the client request a web page, perhaps? Such a service might look as follows:. WAR file. Beans packaged in a. WAR share a single namespace, and become part of the. WAR 's environment. Packaging a. Another novel feature of the new specification is EJB Lite.
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