在Web开发中，service层或者某个工具类中需要获取到HttpServletRequest对象还是比较常见的。一种方式建一个basiControl ，
是将HttpServletRequest作为方法的参数从controller层一直放下传递, 后续所有control继承此basiControl，，不过这种有点费劲，且做起来不是优雅；
还有另一种则是RequestContextHolder，直接在需要用的地方使用如下方式取HttpServletRequest即可，使用代码如下：
HttpServletRequest request = ((ServletRequestAttributes) RequestContextHolder
.getRequestAttributes()).getRequest();
要理解上面的为何可以这么使用，需要理解两个问题：
- RequestContextHolder为什么能获取到当前的HttpServletRequest
- HttpServletRequest是在什么时候设置到RequestContextHolder
对于第1个问题，熟悉ThreadLocal的人应该很容易看出来这个是ThreadLocal的应用，对于ThreadLocal 不懂的童鞋请自行补课，
有讲到，其实很类似上篇博文文末提到的UserContextHolder。
第2个问题应该属于spring-mvc的问题，这个是在spring-mvc执行时设置进去的
源码分析
首先我们先来看下RequestContextHolder的源码，源码如下：
/*
* Copyright 2002-2016 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.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.web.context.request;
import javax.faces.context.FacesContext;
import org.springframework.core.NamedInheritableThreadLocal;
import org.springframework.core.NamedThreadLocal;
import org.springframework.lang.Nullable;
import org.springframework.util.ClassUtils;
/**
* Holder class to expose the web request in the form of a thread-bound
* {@link RequestAttributes} object. The request will be inherited
* by any child threads spawned by the current thread if the
* {@code inheritable} flag is set to {@code true}.
*
* <p>Use {@link RequestContextListener} or
* {@link org.springframework.web.filter.RequestContextFilter} to expose
* the current web request. Note that
* {@link org.springframework.web.servlet.DispatcherServlet}
* already exposes the current request by default.
*
* @author Juergen Hoeller
* @author Rod Johnson
* @since 2.0
* @see RequestContextListener
* @see org.springframework.web.filter.RequestContextFilter
* @see org.springframework.web.servlet.DispatcherServlet
*/
public abstract class RequestContextHolder {
// jsf是JSR-127标准的一种用户界面框架 过时的技术，所以此处不再做讨论
private static final boolean jsfPresent =
ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());
//现成和request绑定的容器
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<>("Request attributes");
// 和上面比较，它是被子线程继承的request Inheritable:可继承的
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal<>("Request context");
/**
* Reset the RequestAttributes for the current thread.
*/
public static void resetRequestAttributes() {
requestAttributesHolder.remove();
inheritableRequestAttributesHolder.remove();
}
/**
* Bind the given RequestAttributes to the current thread,
* <i>not</i> exposing it as inheritable for child threads.
* @param attributes the RequestAttributes to expose
* @see #setRequestAttributes(RequestAttributes, boolean)
*/
// 把传入的RequestAttributes和当前线程绑定。 注意这里传入false：表示不能被继承
public static void setRequestAttributes(@Nullable RequestAttributes attributes) {
setRequestAttributes(attributes, false);
}
/**
* Bind the given RequestAttributes to the current thread.
* @param attributes the RequestAttributes to expose,
* or {@code null} to reset the thread-bound context
* @param inheritable whether to expose the RequestAttributes as inheritable
* for child threads (using an {@link InheritableThreadLocal})
*/
//将RequestAttributes对象放入到ThreadLocal中，而HttpServletRequest和HttpServletResponse等则封装在RequestAttributes对象中，在此处就不对RequestAttributes这个类展开。反正我们需要知道的就是要获取RequestAttributes对象，然后再从RequestAttributes对象中获取到我们所需要的HttpServletRequest即可
public static void setRequestAttributes(@Nullable RequestAttributes attributes, boolean inheritable) {
if (attributes == null) {
resetRequestAttributes();
}
else {
if (inheritable) {
inheritableRequestAttributesHolder.set(attributes);
requestAttributesHolder.remove();
}
else {
requestAttributesHolder.set(attributes);
inheritableRequestAttributesHolder.remove();
}
}
}
/**
* Return the RequestAttributes currently bound to the thread.
* @return the RequestAttributes currently bound to the thread,
* or {@code null} if none bound
*/
@Nullable
public static RequestAttributes getRequestAttributes() {
RequestAttributes attributes = requestAttributesHolder.get();
if (attributes == null) {
attributes = inheritableRequestAttributesHolder.get();
}
return attributes;
}
/**
* Return the RequestAttributes currently bound to the thread.
* <p>Exposes the previously bound RequestAttributes instance, if any.
* Falls back to the current JSF FacesContext, if any.
* @return the RequestAttributes currently bound to the thread
* @throws IllegalStateException if no RequestAttributes object
* is bound to the current thread
* @see #setRequestAttributes
* @see ServletRequestAttributes
* @see FacesRequestAttributes
* @see javax.faces.context.FacesContext#getCurrentInstance()
*/
//在没有jsf的时候，效果完全同getRequestAttributes() 因为jsf几乎废弃了，所以效果可以说一致
public static RequestAttributes currentRequestAttributes() throws IllegalStateException {
RequestAttributes attributes = getRequestAttributes();
if (attributes == null) {
if (jsfPresent) {
attributes = FacesRequestAttributesFactory.getFacesRequestAttributes();
}
if (attributes == null) {
throw new IllegalStateException("No thread-bound request found: " +
"Are you referring to request attributes outside of an actual web request, " +
"or processing a request outside of the originally receiving thread? " +
"If you are actually operating within a web request and still receive this message, " +
"your code is probably running outside of DispatcherServlet/DispatcherPortlet: " +
"In this case, use RequestContextListener or RequestContextFilter to expose the current request.");
}
}
return attributes;
}
/**
* Inner class to avoid hard-coded JSF dependency.
*/
private static class FacesRequestAttributesFactory {
@Nullable
public static RequestAttributes getFacesRequestAttributes() {
FacesContext facesContext = FacesContext.getCurrentInstance();
return (facesContext != null ? new FacesRequestAttributes(facesContext) : null);
}
}
}
那么在spring-mvc中是怎么实现的呢，我们来简单分析的，想了解具体机制的可以去看看spring-mvc的源码。
我们看下spring-mvc 中非常重要的一个类 DispatcherServlet 应用的入口类 ，它的父类是 FrameworkServlet 里面有个processRequest方法，根据方法名称我们也可以大概了解到这个是方法用于处理请求的。
/**
* Process this request, publishing an event regardless of the outcome.
* <p>The actual event handling is performed by the abstract
* {@link #doService} template method.
*/
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
//将RequestAttributes设置到RequestContextHolder
initContextHolders(request, localeContext, requestAttributes);
try {
//具体的业务逻辑处理
doService(request, response);
}
catch (ServletException | IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
//重置RequestContextHolder之前设置RequestAttributes
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
if (logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", failureCause);
}
else {
if (asyncManager.isConcurrentHandlingStarted()) {
logger.debug("Leaving response open for concurrent processing");
}
else {
this.logger.debug("Successfully completed request");
}
}
}
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
private void initContextHolders(HttpServletRequest request,
@Nullable LocaleContext localeContext, @Nullable RequestAttributes requestAttributes) {
if (localeContext != null) {
LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
}
if (requestAttributes != null) {
RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
}
if (logger.isTraceEnabled()) {
logger.trace("Bound request context to thread: " + request);
}
}
//重置上下文信息
private void resetContextHolders(HttpServletRequest request,
@Nullable LocaleContext prevLocaleContext, @Nullable RequestAttributes previousAttributes) {
LocaleContextHolder.setLocaleContext(prevLocaleContext, this.threadContextInheritable);
RequestContextHolder.setRequestAttributes(previousAttributes, this.threadContextInheritable);
if (logger.isTraceEnabled()) {
logger.trace("Cleared thread-bound request context: " + request);
}
}
简单看下源码，我们可以知道HttpServletRequest是在执行doService方法之前，也就是具体的业务逻辑前进行设置的，然后在执行完业务逻辑或者抛出异常时重置RequestContextHolder移除当前的HttpServletRequest。
DispatcherServlet在处理请求的时候，父类FrameworkServlet#processRequest就有向RequestContextHolder初始化绑定一些通用参数的操作，这样子使用者可以在任意地方，拿到这些公用参数了，可谓特别的方便。
发散阅读：
LocaleContextHolder 是用来处理Local的上下文容器。