Update Javadoc of AsyncHandlerInterceptor

Issue: SPR-12608, SPR-12720

spring-webmvc/src/main/java/org/springframework/web/servlet/AsyncHandlerInterceptor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,20 +22,34 @@ import javax.servlet.http.HttpServletResponse;
 import org.springframework.web.method.HandlerMethod;
 
 /**
- * Extends {@code HandlerInterceptor} with a callback method invoked during
- * asynchronous request handling.
+ * Extends {@code HandlerInterceptor} with a callback method invoked after the
+ * start of asynchronous request handling.
  *
- * <p>When a handler starts asynchronous request handling, the DispatcherServlet
- * exits without invoking {@code postHandle} and {@code afterCompletion}, as it
- * normally does, since the results of request handling (e.g. ModelAndView)
- * will. be produced concurrently in another thread. In such scenarios,
- * {@link #afterConcurrentHandlingStarted(HttpServletRequest, HttpServletResponse, Object)}
- * is invoked instead allowing implementations to perform tasks such as cleaning
- * up thread bound attributes.
+ * <p>When a handler starts an asynchronous request, the DispatcherServlet
+ * exits without invoking {@code postHandle} and {@code afterCompletion} as it
+ * normally does since the results of request handling (e.g. ModelAndView)
+ * is likely not yet ready and will be produced concurrently from another thread.
+ * In such scenarios, {@link #afterConcurrentHandlingStarted} is invoked instead
+ * allowing implementations to perform tasks such as cleaning up thread bound
+ * attributes before releasing the thread to the Servlet container.
  *
  * <p>When asynchronous handling completes, the request is dispatched to the
  * container for further processing. At this stage the DispatcherServlet invokes
- * {@code preHandle}, {@code postHandle} and {@code afterCompletion} as usual.
+ * {@code preHandle}, {@code postHandle} and {@code afterCompletion}.
+ * To distinguish between the initial request and the subsequent dispatch
+ * after asynchronous handling completes, interceptors can check whether the
+ * {@code javax.servlet.DispatcherType} of {@link javax.servlet.ServletRequest}
+ * is "REQUEST" or "ASYNC".
+ *
+ * <p>Note that {@code HandlerInterceptor} implementations may be 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
+ * {@code postHandle} and {@code afterCompletion} methods will not be invoked.
+ * Instead interceptors can register to track an asynchronous request through
+ * the {@code registerCallbackInterceptor} and {@code registerDeferredResultInterceptor}
+ * methods on {@link org.springframework.web.context.request.async.WebAsyncManager
+ * WebAsyncManager}. This can be done proactively on every request from
+ * {@code preHandle} regardless of whether async request processing will start.
  *
  * @author Rossen Stoyanchev
  * @since 3.2

spring-webmvc/src/main/java/org/springframework/web/servlet/HandlerInterceptor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -77,10 +77,16 @@ public interface HandlerInterceptor {
 	/**
 	 * Intercept the execution of a handler. Called after HandlerMapping determined
 	 * an appropriate handler object, but before HandlerAdapter invokes the handler.
+	 *
 	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
 	 * of any number of interceptors, with the handler itself at the end.
 	 * With this method, each interceptor can decide to abort the execution chain,
 	 * typically sending a HTTP error or writing a custom response.
+	 *
+	 * <p><strong>Note:</strong> special considerations apply for asynchronous
+	 * request processing. For more details see
+	 * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
+	 *
 	 * @param request current HTTP request
 	 * @param response current HTTP response
 	 * @param handler chosen handler to execute, for type and/or instance evaluation
@@ -96,10 +102,16 @@ public interface HandlerInterceptor {
 	 * Intercept the execution of a handler. Called after HandlerAdapter actually
 	 * invoked the handler, but before the DispatcherServlet renders the view.
 	 * Can expose additional model objects to the view via the given ModelAndView.
+	 *
 	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
 	 * of any number of interceptors, with the handler itself at the end.
 	 * With this method, each interceptor can post-process an execution,
 	 * getting applied in inverse order of the execution chain.
+	 *
+	 * <p><strong>Note:</strong> special considerations apply for asynchronous
+	 * request processing. For more details see
+	 * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
+	 *
 	 * @param request current HTTP request
 	 * @param response current HTTP response
 	 * @param handler handler (or {@link HandlerMethod}) that started async
@@ -115,11 +127,18 @@ public interface HandlerInterceptor {
 	 * Callback after completion of request processing, that is, after rendering
 	 * the view. Will be called on any outcome of handler execution, thus allows
 	 * for proper resource cleanup.
+	 *
 	 * <p>Note: Will only be called if this interceptor's {@code preHandle}
 	 * method has successfully completed and returned {@code true}!
+	 *
 	 * <p>As with the {@code postHandle} method, the method will be invoked on each
 	 * interceptor in the chain in reverse order, so the first interceptor will be
 	 * the last to be invoked.
+	 *
+	 * <p><strong>Note:</strong> special considerations apply for asynchronous
+	 * request processing. For more details see
+	 * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
+	 *
 	 * @param request current HTTP request
 	 * @param response current HTTP response
 	 * @param handler handler (or {@link HandlerMethod}) that started async