56.6 The taskcreator Concurrency Example

The taskcreator example demonstrates how to use Concurrency Utilities for Java EE to run tasks immediately, periodically, or after a fixed delay. This example provides a JavaServer Faces interface that enables users to submit tasks to be executed and displays information messages for each task. The example uses the Managed Executor Service to run tasks immediately and the Managed Scheduled Executor Service to run tasks periodically or after a fixed delay. (See Main Components of the Concurrency Utilities for information about these services.)

The taskcreator example consists of the following components.

• A JavaServer Faces page (index.xhtml) that contains three elements: a form to submit tasks, a task execution log, and a form to cancel periodic tasks. This page submits Ajax requests to create and cancel tasks. This page also receives WebSocket messages, using JavaScript code to update the task execution log.

• A CDI managed bean (TaskCreatorBean) that processes the requests from the JavaServer Faces page. This bean invokes the methods in TaskEJB to submit new tasks and to cancel periodic tasks.

• An enterprise bean (TaskEJB) that obtains executor service instances using resource injection and submits tasks for execution. This bean is also a JAX-RS web service endpoint. The tasks send information messages to this endpoint.

• A WebSocket endpoint (InfoEndpoint) that the enterprise bean uses to send information messages to the clients.

• A task class (Task) that implements the Runnable interface. The run method in this class sends information messages to the web service endpoint in TaskEJB and sleeps for 1.5 seconds.

Figure 56-1 shows the architecture of the taskcreator example.

Figure 56-1 Architecture of the taskcreator Example

Description of "Figure 56-1 Architecture of the taskcreator Example"

The TaskEJB class obtains the default executor service objects from the application server as follows:

@Resource(name="java:comp/DefaultManagedExecutorService")
ManagedExecutorService mExecService;

@Resource(name="java:comp/DefaultManagedScheduledExecutorService")
ManagedScheduledExecutorService sExecService;

The submitTask method in TaskEJB uses these objects to submit tasks for execution as follows:

public void submitTask(Task task, String type) {
    /* Use the managed executor objects from the app server */
    switch (type) {
        case "IMMEDIATE":
            mExecService.submit(task);
            break;
        case "DELAYED":
            sExecService.schedule(task, 3, TimeUnit.SECONDS);
            break;
        case "PERIODIC":
            ScheduledFuture<?> fut;
            fut = sExecService.scheduleAtFixedRate(task, 0, 8, 
                    TimeUnit.SECONDS);
            periodicTasks.put(task.getName(), fut);
            break;
    }
}

For periodic tasks, TaskEJB keeps a reference to the ScheduledFuture object, so that the user can cancel the task at any time.