This blog post explores the usage of CDI along with Java EE Concurrency Utilities – specifically using CDI beans as managed tasks. Here is the sample application on GitHub.
Let's Begin With a Quick Overview
Java EE Concurrency Utilities provides APIs and constructs to manage concurrency within Java EE applications. Many of the Java EE components have specific concurrency semantics e.g. EJBs, JAX-RS resources, WebSocket endpoints, etc. Writing components with custom concurrency properties was traditionally difficult, since starting unmanaged threads in a Java EE container was forbidden, i.e. one was not able to leverage Java SE concurrency libraries. With Concurrency Utilities, Java EE applications have access to managed versions of the Java SE counterparts, namely,
ManagedExecutorSeviceManagedScheduledExecutorSeviceManagedThreadFactory
...and a bunch of other APIs as well, but the above ones are the Java EE equivalent of the Java SE concurrency APIs.
Tasks as CDI Beans
Both ManagedExecutorSevice and ManagedScheduledExecutorService can accept tasks to execute (in a container-managed thread pool) in the form of Runnable and Callable instances. The good thing is that these tasks can be CDI beans as well. Points worth noting are:
- These CDI beans can be injected into other components as well as inject other beans.
- The scope of the CDI beans, which can be used as tasks as restricted to
@ApplicationScopedand@Dependent(for details, read section2.3.2.1of the specification).
Here Is a Summary of...
...what’s going on in the application. For more details, refer to the README and explore the code:
- Tasks are
POSTEDvia a REST interface and the client gets back aHTTP 202 (Accepted)in response along with a task id - Yhe
BackgroundTask(CDI bean) is executed in a background thread by theManagedExecutorService– it is injected (@Inject) and a different instance is created on every invocation since the CDI bean is marked@Dependentand the JAX-RS resource is created on each request by the client - The status is store in a
@SingletonEJB (TaskStore) – this is injected in theBackgroundTaskCDI bean - Status of each task (in progress, failed, completed) can be tracked via a REST interface by querying against a
task ID - One can also get the status of all tasks
If you want to read more, take a look at this Concurrency Utilities specification.
Cheers!
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