Extends HandlerInterceptor with a callback method invoked after the start of asynchronous request handling.

When a handler starts an asynchronous request, the DispatcherServlet exits without invoking postHandle and afterCompletion as it normally does for a synchronous request, since the result of request handling (e.g. ModelAndView) is likely not yet ready and will be produced concurrently from another thread. In such scenarios, afterConcurrentHandlingStarted is invoked instead, allowing implementations to perform tasks such as cleaning up thread-bound attributes before releasing the thread to the Servlet container.

When asynchronous handling completes, the request is dispatched to the container for further processing. At this stage the DispatcherServlet invokes preHandle, postHandle, and afterCompletion. To distinguish between the initial request and the subsequent dispatch after asynchronous handling completes, interceptors can check whether the jakarta.servlet.DispatcherType of jakarta.servlet.ServletRequest is "REQUEST" or "ASYNC".

Note that HandlerInterceptor implementations may need to do work when an async request times out or completes with a network error. For such cases the Servlet container does not dispatch and therefore the postHandle and afterCompletion methods will not be invoked. Instead, interceptors can register to track an asynchronous request through the registerCallbackInterceptor and registerDeferredResultInterceptor methods on WebAsyncManager. This can be done proactively on every request from preHandle regardless of whether async request processing will start.