The Java EE 7 Tutorial

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36.1 Asynchronous Method Invocation

Session beans can implement asynchronous methods, business methods where control is returned to the client by the enterprise bean container before the method is invoked on the session bean instance. Clients may then use the Java SE concurrency API to retrieve the result, cancel the invocation, and check for exceptions. Asynchronous methods are typically used for long-running operations, for processor-intensive tasks, for background tasks, to increase application throughput, or to improve application response time if the method invocation result isn't required immediately.

When a session bean client invokes a typical non-asynchronous business method, control is not returned to the client until the method has completed. Clients calling asynchronous methods, however, immediately have control returned to them by the enterprise bean container. This allows the client to perform other tasks while the method invocation completes. If the method returns a result, the result is an implementation of the java.util.concurrent.Future<V> interface, where "V" is the result value type. The Future<V> interface defines methods the client may use to check whether the computation is completed, wait for the invocation to complete, retrieve the final result, and cancel the invocation.

36.1.1 Creating an Asynchronous Business Method

Annotate a business method with javax.ejb.Asynchronous to mark that method as an asynchronous method, or apply @Asynchronous at the class level to mark all the business methods of the session bean as asynchronous methods. Session bean methods that expose web services can't be asynchronous.

Asynchronous methods must return either void or an implementation of the Future<V> interface. Asynchronous methods that return void can't declare application exceptions, but if they return Future<V>, they may declare application exceptions. For example:

@Asynchronous
public Future<String> processPayment(Order order) throws PaymentException { ... }

This method will attempt to process the payment of an order, and return the status as a String. Even if the payment processor takes a long time, the client can continue working, and display the result when the processing finally completes.

The javax.ejb.AsyncResult<V> class is a concrete implementation of the Future<V> interface provided as a helper class for returning asynchronous results. AsyncResult has a constructor with the result as a parameter, making it easy to create Future<V> implementations. For example, the processPayment method would use AsyncResult to return the status as a String:

@Asynchronous
public Future<String> processPayment(Order order) throws PaymentException {
    ...
    String status = ...;
    return new AsyncResult<String>(status);
}

The result is returned to the enterprise bean container, not directly to the client, and the enterprise bean container makes the result available to the client. The session bean can check whether the client requested that the invocation be cancelled by calling the javax.ejb.SessionContext.wasCancelled method. For example:

@Asynchronous
public Future<String> processPayment(Order order) throws PaymentException {
    ...
    if (SessionContext.wasCancelled()) {
        // clean up
    } else {
        // process the payment
    }
    ...
}

36.1.2 Calling Asynchronous Methods from Enterprise Bean Clients

Session bean clients call asynchronous methods just like non-asynchronous business methods. If the asynchronous method returns a result, the client receives a Future<V> instance as soon as the method is invoked. This instance can be used to retrieve the final result, cancel the invocation, check whether the invocation has completed, check whether any exceptions were thrown during processing, and check whether the invocation was cancelled.

36.1.2.1 Retrieving the Final Result from an Asynchronous Method Invocation

The client may retrieve the result using one of the Future<V>.get methods. If processing hasn't been completed by the session bean handling the invocation, calling one of the get methods will result in the client halting execution until the invocation completes. Use the Future<V>.isDone method to determine whether processing has completed before calling one of the get methods.

The get() method returns the result as the type specified in the type value of the Future<V> instance. For example, calling Future<String>.get() will return a String object. If the method invocation was cancelled, calls to get() result in a java.util.concurrent.CancellationException being thrown. If the invocation resulted in an exception during processing by the session bean, calls to get() result in a java.util.concurrent.ExecutionException being thrown. The cause of the ExecutionException may be retrieved by calling the ExecutionException.getCause method.

The get(long timeout, java.util.concurrent.TimeUnit unit) method is similar to the get() method, but allows the client to set a timeout value. If the timeout value is exceeded, a java.util.concurrent.TimeoutException is thrown. See the Javadoc for the TimeUnit class for the available units of time to specify the timeout value.

36.1.2.2 Cancelling an Asynchronous Method Invocation

Call the cancel(boolean mayInterruptIfRunning) method on the Future<V> instance to attempt to cancel the method invocation. The cancel method returns true if the cancellation was successful and false if the method invocation cannot be cancelled.

When the invocation cannot be cancelled, the mayInterruptIfRunning parameter is used to alert the session bean instance on which the method invocation is running that the client attempted to cancel the invocation. If mayInterruptIfRunning is set to true, calls to SessionContext.wasCancelled by the session bean instance will return true. If mayInterruptIfRunning is to set false, calls to SessionContext.wasCancelled by the session bean instance will return false.

The Future<V>.isCancelled method is used to check whether the method invocation was cancelled before the asynchronous method invocation completed by calling Future<V>.cancel. The isCancelled method returns true if the invocation was cancelled.

36.1.2.3 Checking the Status of an Asynchronous Method Invocation

The Future<V>.isDone method returns true if the session bean instance completed processing the method invocation. The isDone method returns true if the asynchronous method invocation completed normally, was cancelled, or resulted in an exception. That is, isDone indicates only whether the session bean has completed processing the invocation.

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