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Interceptor Lifecycle
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Interceptor classes have the same lifecycles as the EJBs they intercept. Consider an interceptor class as an extension of the EJB s bean instance. They are created along with bean instances. They are destroyed, passivated, and activated along with their bean instances as well. Also, it is important to note that interceptor classes have the same restrictions as the beans to which they are attached. For instance, you cannot inject an extended persistence context into an interceptor class if that interceptor does not intercept a stateful session bean. Because interceptors have lifecycles and hook into lifecycle events, they can also hold internal state. This might be extremely useful when you want the interceptor class to obtain an open connection to a remote system and then close that connection at destroy time. You may also be interested in maintaining state that is particular to the bean instance on which the interceptor class is intercepted. Maybe you have a custom injection annotation that you have built and it needs special cleanup after the bean instance is destroyed. You can hold internal state within the interceptor class and do the cleanup when the interceptor and bean instance are destroyed.
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Bean Class @AroundInvoke Methods
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This chapter has mostly discussed interceptor classes. @AroundInvoke methods can also exist inside EJB bean classes. When used inside a bean class, the @AroundInvoke method will be the last interceptor to be invoked before the actual bean method:
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@Stateless public class MySessionBean implements MySessionRemote { public void businessMethod( ) { ... } @AroundInvoke public Object beanClassInterceptor(InvocationContext ctx) { try { System.out.println("entering: " + ctx.getMethod( )); return ctx.proceed( );
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} finally { System.out.println("leaving: " + ctx.getMethod( )); }
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This is a simple example of a bean class @AroundInvoke method. For what sorts of things would you want to use it You may want to have a dynamic implementation of your bean class, or you may have an interceptor whose logic is specific to the bean.
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Business systems frequently use scheduling systems to run programs at specified times. Scheduling systems typically run applications that generate reports, reformat data, or do audit work at night. In other cases, scheduling systems provide callback APIs that can alert subsystems of events such as due dates, deadlines, etc. Scheduling systems often run batch jobs (aka scheduled jobs), which perform routine work automatically at a prescribed time. Users in the Unix world frequently run scheduled jobs using cron, a simple but useful scheduling system that runs programs listed in a configuration file. Regardless of the software, scheduling systems are used in many different scenarios: In a credit card processing system, credit card charges are processed in batches so that all the charges made for a block of time are settled together rather than separately. This work may be scheduled to be done in the evening to reduce the impact of processing on the system. In a hospital or clinical system, Electronic Data Interface (EDI) software is used to send medical claims to various HMOs. Each HMO has its own processing requirements, but all of them are routine, so jobs are scheduled to gather claim data, put it in the proper format, and transfer it to the HMO. In just about any company, managers need specific reports run on a regular basis. A scheduling system can be configured to run those reports automatically and deliver them via email to managers. Scheduling systems are also common in workflow applications, which are systems that manage document processing that typically spans days or months and involves many systems and lots of human intervention. In workflow applications, scheduling is employed for auditing tasks that periodically take inventory of the state of an application, invoice, sales order, etc., in order to ensure everything is proceeding as scheduled. The scheduling system maintains timers and delivers events to alert applications and components when a specified date and time are reached, or when some period has expired.
Here are some examples of workflow scheduling: In a mortgage system, a lot of tasks have to be completed (e.g., appraisal, rate lockin, closing appointment, etc.) before the mortgage can be closed. Timers can be set on mortgage applications to perform periodic audits that ensure everything is proceeding on schedule. In a healthcare claims-processing system, claims must be processed within 90 days according to terms negotiated by in-network physicians and clinics. Each claim could have a timer set to go off seven days before the deadline. In a stockbroker system, buy-at-limit orders can be created for a specific number of shares, but only at a specified price or lower. These buy-at-limit orders typically have a time limit. If the stock price falls below the specified price before the time limit, the buy-at-limit order is carried out. If the stock price does not fall below the specified price before the time limit, the timer expires and the buy-at-limit order is canceled. The EJB 2.1 specification introduced a standard Java EE scheduling system called the EJB Timer Service. The 3.1 revision of the spec has overhauled the mechanism in which users define scheduling events, bringing forth a new natural-language syntax that is much friendlier than previous versions and the cron format.
The Java Standard Edition includes the java.util.Timer class, which allows threads to schedule tasks for future execution in a background thread. This facility is useful for a variety of applications, but it s too limited to be used in enterprise computing. Note, however, that the scheduling semantics of java.util.Timer are similar to those of the EJB Timer Service.
The Timer Service is a facility of the EJB container system that provides a timed-event API, which can be used to schedule timers for specified dates, periods, and intervals. A timer is associated with the enterprise bean that set it, and it calls that bean s ejbTimeout() method or a method annotated with @javax.ejb.Timeout when it goes off. The rest of this chapter describes the EJB Timer Service API and its use with stateless session and message-driven beans.
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