These examples illustrate Struts build in support for async request processing.
When you have a process that takes a long time, it can make your app not scalable under heavy load conditions.
Scalability limitations include running out of memory or exhausting the pool of container threads.
To create scalable web applications, you must ensure that no threads associated with a request
are sitting idle, so the container can use them to process new requests.
Asynchronous processing refers to assigning these blocking operations to a new thread and returning
the thread associated with the request immediately to the container.
Reference: Asynchronous Processing
An interesting and vital use case for the async request processing is server push.
A good solution is to use the Servlet 3.0+ asynchronous feature.