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Asynchronous Methods
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New in the EJB 3.1 Specification is the feature of fire-and-forget invocations upon session beans. Often we ll have requirements where a request may take some time to be fully processed, and the client may not need to wait for the result or confirmation of completion. In these cases, we can take advantage of the new @javax.ejb.Asynchro nous annotation to return control to the client before EJB is invoked. When the client does need access to the return value, it can do so via the facilities provided by java.util.concurrent.Future. In the case of our EncryptionEJB, perhaps we have a very intensive hashing function that takes some time to complete:
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/** * Returns a one-way hash of the specified argument, calculated asynchronously. * Useful for safely storing passwords. * * @param input * @return * @throws IllegalArgumentException * @throws EncryptionException */ Future<String> hashAsync(String input) throws IllegalArgumentException, EncryptionException;
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During implementation, the bean provider may mark this method (either on the business interface or the bean class) as @Asynchronous, wrapping the real return value in a convenience implementation of java.util.concurrent.Future called javax.ejb.AsyncResult:
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@Asynchronous @Override public Future<String> hashAsync(final String input) throws IllegalArgumentException, EncryptionException { // Get the real hash final String hash = this.hash(input); // Wrap and return return new AsyncResult<String>(hash);
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Client usage therefore may look something like this:
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// Declare the input final String input = "Async Hashing Input"; // Hash final Future<String> hashFuture = myEjbProxyReference.hashAsync(input); // Now we're free to do whatever work we want here while the // EJB invocation runs concurrently
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// ...[work] // At this point we need the hash, so get the result, // and block for up to 10 seconds to get it final String hash = hashFuture.get(10,TimeUnit.SECONDS); log.info("Hash of \"" + input + "\": " + hash);
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Here control is returned to the client immediately after invoking upon hashAsync, and other work may be performed while the potentially expensive hashing function chugs along elsewhere concurrently. When we need the asynchronous result, we can use Future.get() or its derivatives to block until the result is available. Methods with no return value (i.e., void) may also be marked as @Asynchronous. Any session bean business method (in Stateless, Stateful, or Singleton) is eligible to be made asynchronous, and @Asynchronous may be applied upon a business interface or bean class to denote that all methods should be handled async.
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The Stateful Session Bean
While the strengths of the stateless session bean lie in its speed and efficiency, stateful session beans are built as a server-side extension of the client. Each SFSB is dedicated to one client for the life of the bean instance; it acts on behalf of that client as its agent (see Figure 6-1). Stateful session beans are not swapped among EJB objects, nor are they kept in an instance pool like their stateless session counterparts. Once a stateful session bean is instantiated and assigned to an EJB object, it is dedicated to that EJB object for its entire lifecycle.*
Stateful session beans maintain conversational state, which means that the instance variables of the bean class can maintain data specific to the client between method
* This is a conceptual model. Some EJB containers may actually use instance swapping with stateful session beans but make it appear as if the same instance is servicing all requests. Conceptually, however, the same stateful session bean instance services all requests.
invocations. This makes it possible for methods to be interdependent such that changes made to the bean s state in one method call can affect the results of subsequent method invocations. Therefore, every method call from a client must be serviced by the same instance (at least conceptually), so the bean instance s state can be predicted from one method invocation to the next. In contrast, stateless session beans don t maintain client-specific data from one method call to the next, so any instance can be used to service any method call from any client. Although stateful session beans maintain conversational state, they are not themselves persistent; the state of a SFSB is lost when the session is removed, the session times out, or the server restarts. Persistent state in EJB is modeled by the entity bean, which we cover starting in 9. Because SFSBs are often considered extensions of the client, we may think of a client as being composed from a combination of operations and state. Each task may rely on some information gathered or changed by a previous operation. A GUI client is a perfect example: when you fill in the fields on a GUI client, you are creating conversational state. Pressing a button executes an operation that might fill in more fields, based on the information you entered previously. The information in the fields is conversational state. Stateful session beans allow you to encapsulate some of the business logic and conversational state of a client and move it to the server. Moving this logic to the server thins the client application and makes the system as a whole easier to manage. The stateful session bean acts as an agent for the client, managing processes or taskflow to accomplish a set of tasks; it manages the interactions of other beans in addition to direct data access over several operations to accomplish a complex set of tasks. By encapsulating and managing taskflow on behalf of the client, stateful beans present a simplified interface that hides the details of many interdependent operations on the database and other beans from the client.
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