Web Component Development Using Servlet & JSP Technologies (EE6) - Chapter 11 - Asynchronous Servlets and Clients

Asynchronous Servlets in Java: An excessive number of threads running simultaneously in a Java application may consume a lot of resources. This is no exception for a web based Java application. Incoming requests are handled by dedicated HTTP worker threads which will then process those incoming requests until a response is finally assembled and sent back to the client. Considering a web application scenario where the number of simultaneous users is expected to be very high, problems regarding resource consumption is what matters to active threads may become a real problem. HTTP worker threads belong to a dedicated pool, which may become exhausted. Additionally, the threads themselves have to consume their own resources, so incrementing the HTTP thread pool size to a (very) large number will also lead to system resources exhaustion: it simply does not scale. This is where Asynchronous Servlets may be helpful. In short, an asynchronous servlet enables an application to process incoming requests in an asynchronous fashion: A given HTTP worker thread handles an incoming request and then passes the request to another background thread which in turn will be responsible for processing the request and send the response back to the client. The initial HTTP worker thread will return to the HTTP thread pool as soon as it passes the request to the background thread, so it becomes available to process another request. This approach by itself may solve the problem of HTTP thread pool exhaustion, but will not solve the problem of system resources consumption. After all, another background thread was created for processing the request, so the number of simultaneous active threads will not decrease and the system resource consumption will not be improved.