qr code java application Exceptions and Transactions in Java

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Exceptions and Transactions
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Exceptions have a large impact on the outcome of transactions.
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Application Exceptions Versus System Exceptions
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System exceptions represent unknown internal errors. The EJB container throws system exceptions when it encounters an internal application server failure. Business logic can throw system exceptions when it wants to abort the business process. Application exceptions are exceptions that are part of your business logic. They denote a strongly typed definition of a specific business problem or failure but do not necessarily abort or roll back the business process.
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System exceptions
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System exceptions include java.lang.RuntimeException and its subclasses. EJBException is a subclass of RuntimeException, so it is considered a system exception. System exceptions also include java.rmi.RemoteException and its subclasses. The Run timeException and RemoteException subclasses differ in that they can be turned into application exceptions using the @javax.ejb.ApplicationException annotation. This annotation is discussed later in this chapter. System exceptions always cause a transaction to roll back when they are thrown from an enterprise bean method. Any RuntimeException not annotated with @Applica tionException that is thrown within a business method (for instance, EJBException, NullPointerException, IndexOutOfBoundsException, and so on) is handled by the container automatically and results in a transaction rollback. In Java, RuntimeException types do not need to be declared in the throws clause of the method signature or handled using try/catch blocks; they are automatically thrown from the method. The container handles system exceptions automatically and it will always do the following: 1. Roll back the transaction. 2. Log the exception to alert the system administrator. 3. Discard the EJB instance. When a system exception is thrown from any callback method (@PostConstruct, @Post Activate, and so on), it is treated the same way as exceptions thrown from any business method. Although EJB requires system exceptions to be logged, it does not specify how they should be logged or the format of the logfile. The exact mechanism for recording exceptions and reporting them to the system administrator is left to the vendor.
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When a system exception occurs, the EJB instance is discarded, which means that it is dereferenced and garbage-collected. The container assumes that the EJB instance may have corrupt variables or otherwise be unstable and is therefore unsafe to use. The impact of discarding an EJB instance depends on the enterprise bean s type. In the case of stateless session beans, the client does not notice that the instance has been discarded. These instance types are not dedicated to a particular client; they are swapped in and out of an instance pool, and so any instance can service a new request. With stateful session beans, however, the impact on the client is severe. Stateful session beans are dedicated to a single client and maintain conversational state. Discarding a stateful bean instance destroys the instance s conversational state and invalidates the client s reference to the EJB. When stateful session instances are discarded, subsequent invocations of the EJB s methods by the client result in a NoSuchEJBException, which is a subclass of RuntimeException. With message-driven beans, a system exception thrown by the onMessage() method or one of the callback methods (@PostConstruct or @PreDestroy) will cause the bean instance to be discarded. If the MDB was a BMT bean, the message it was handling may or may not be redelivered, depending on when the EJB container acknowledges delivery. In the case of container-managed transactions, the container will roll back the transaction, so the message will not be acknowledged and may be redelivered. In session beans, when a system exception occurs and the instance is discarded, a RuntimeException is always thrown, regardless of whether the client is a remote or a local invocation. If the client started the transaction, which was then propagated to the EJB, a system exception (thrown by the enterprise bean method) will be caught by the container and rethrown as a javax.ejb.EJBTransactionRolledbackException. EJBTransactionRolledbackException is a subtype of RuntimeException and gives a more explicit indication to the client that a rollback occurred. If the client did not propagate a transaction to the EJB, the system exception will be caught and rethrown as an EJBException. An EJBException generally should be thrown when a nonbusiness subsystem throws an exception, such as JDBC throwing an SQLException or JMS throwing a JMSException. In some cases, however, the bean developer may attempt to handle the exception and retry an operation instead of throwing an EJBException. This should be done only when the exceptions thrown by the subsystem and their repercussions on the transaction are well understood. As a rule of thumb, rethrow nonbusiness subsystem exceptions as EJBExceptions (or @ApplicationExceptions that cause a rollback) and allow the EJB container to roll back the transaction and discard the bean instance automatically.
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Although the instance is always discarded with a RuntimeException, the impact on the remote reference may vary depending on the vendor.
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