RequestContextListener#requestInitializedのチェックの意味が理解できない。

Springのリスナークラスの処理が分かりません。

Java
1/*
2 * Copyright 2002-2015 the original author or authors.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package org.springframework.web.context.request;
18
19import javax.servlet.ServletRequestEvent;
20import javax.servlet.ServletRequestListener;
21import javax.servlet.http.HttpServletRequest;
22
23import org.springframework.context.i18n.LocaleContextHolder;
24
25/**
26 * Servlet listener that exposes the request to the current thread,
27 * through both {@link org.springframework.context.i18n.LocaleContextHolder} and
28 * {@link RequestContextHolder}. To be registered as listener in {@code web.xml}.
29 *
30 * <p>Alternatively, Spring's {@link org.springframework.web.filter.RequestContextFilter}
31 * and Spring's {@link org.springframework.web.servlet.DispatcherServlet} also expose
32 * the same request context to the current thread. In contrast to this listener,
33 * advanced options are available there (e.g. "threadContextInheritable").
34 *
35 * <p>This listener is mainly for use with third-party servlets, e.g. the JSF FacesServlet.
36 * Within Spring's own web support, DispatcherServlet's processing is perfectly sufficient.
37 *
38 * @author Juergen Hoeller
39 * @since 2.0
40 * @see javax.servlet.ServletRequestListener
41 * @see org.springframework.context.i18n.LocaleContextHolder
42 * @see RequestContextHolder
43 * @see org.springframework.web.filter.RequestContextFilter
44 * @see org.springframework.web.servlet.DispatcherServlet
45 */
46public class RequestContextListener implements ServletRequestListener {
47
48	private static final String REQUEST_ATTRIBUTES_ATTRIBUTE =
49			RequestContextListener.class.getName() + ".REQUEST_ATTRIBUTES";
50
51
52	@Override
53	public void requestInitialized(ServletRequestEvent requestEvent) {
54		if (!(requestEvent.getServletRequest() instanceof HttpServletRequest)) {
55			throw new IllegalArgumentException(
56					"Request is not an HttpServletRequest: " + requestEvent.getServletRequest());
57		}
58		HttpServletRequest request = (HttpServletRequest) requestEvent.getServletRequest();
59		ServletRequestAttributes attributes = new ServletRequestAttributes(request);
60		request.setAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE, attributes);
61		LocaleContextHolder.setLocale(request.getLocale());
62		RequestContextHolder.setRequestAttributes(attributes);
63	}
64
65	@Override
66	public void requestDestroyed(ServletRequestEvent requestEvent) {
67		ServletRequestAttributes attributes = null;
68		Object reqAttr = requestEvent.getServletRequest().getAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE);
69		if (reqAttr instanceof ServletRequestAttributes) {
70			attributes = (ServletRequestAttributes) reqAttr;
71		}
72		RequestAttributes threadAttributes = RequestContextHolder.getRequestAttributes();
73		if (threadAttributes != null) {
74			// We're assumably within the original request thread...
75			LocaleContextHolder.resetLocaleContext();
76			RequestContextHolder.resetRequestAttributes();
77			if (attributes == null && threadAttributes instanceof ServletRequestAttributes) {
78				attributes = (ServletRequestAttributes) threadAttributes;
79			}
80		}
81		if (attributes != null) {
82			attributes.requestCompleted();
83		}
84	}
85
86}
87

このコードの

if (!(requestEvent.getServletRequest() instanceof HttpServletRequest)) {
			throw new IllegalArgumentException(
					"Request is not an HttpServletRequest: " + requestEvent.getServletRequest());
		}

の部分は、HttpServletRequest以外のリクエストが来るということだと思うのですが、具体的にどのようなリクエストがあるのでしょうか？
また、それをどうしてチェックしなければならないのでしょうか？

Spring理解してる人、教えてください。