在Web開發中，service層或者某個工具類中需要獲取到HttpServletRequest對象還是比較常見的。一種方式建一個basiControl ，
是將HttpServletRequest作爲方法的參數從controller層一直放下傳遞, 後續所有control繼承此basiControl，，不過這種有點費勁，且做起來不是優雅；
還有另一種則是RequestContextHolder，直接在需要用的地方使用如下方式取HttpServletRequest即可，使用代碼如下：
HttpServletRequest request = ((ServletRequestAttributes) RequestContextHolder
.getRequestAttributes()).getRequest();
要理解上面的爲何可以這麼使用，需要理解兩個問題：
- RequestContextHolder爲什麼能獲取到當前的HttpServletRequest
- HttpServletRequest是在什麼時候設置到RequestContextHolder
對於第1個問題，熟悉ThreadLocal的人應該很容易看出來這個是ThreadLocal的應用，對於ThreadLocal 不懂的童鞋請自行補課，
有講到，其實很類似上篇博文文末提到的UserContextHolder。
第2個問題應該屬於spring-mvc的問題，這個是在spring-mvc執行時設置進去的
源碼分析
首先我們先來看下RequestContextHolder的源碼，源碼如下：
/*
* 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標準的一種用戶界面框架 過時的技術，所以此處不再做討論
private static final boolean jsfPresent =
ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());
//現成和request綁定的容器
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<>("Request attributes");
// 和上面比較，它是被子線程繼承的request Inheritable:可繼承的
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)
*/
// 把傳入的RequestAttributes和當前線程綁定。 注意這裏傳入false：表示不能被繼承
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})
*/
//將RequestAttributes對象放入到ThreadLocal中，而HttpServletRequest和HttpServletResponse等則封裝在RequestAttributes對象中，在此處就不對RequestAttributes這個類展開。反正我們需要知道的就是要獲取RequestAttributes對象，然後再從RequestAttributes對象中獲取到我們所需要的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()
*/
//在沒有jsf的時候，效果完全同getRequestAttributes() 因爲jsf幾乎廢棄了，所以效果可以說一致
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);
}
}
}
那麼在spring-mvc中是怎麼實現的呢，我們來簡單分析的，想了解具體機制的可以去看看spring-mvc的源碼。
我們看下spring-mvc 中非常重要的一個類 DispatcherServlet 應用的入口類 ，它的父類是 FrameworkServlet 裏面有個processRequest方法，根據方法名稱我們也可以大概瞭解到這個是方法用於處理請求的。
/**
* 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());
//將RequestAttributes設置到RequestContextHolder
initContextHolders(request, localeContext, requestAttributes);
try {
//具體的業務邏輯處理
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之前設置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);
}
}
簡單看下源碼，我們可以知道HttpServletRequest是在執行doService方法之前，也就是具體的業務邏輯前進行設置的，然後在執行完業務邏輯或者拋出異常時重置RequestContextHolder移除當前的HttpServletRequest。
DispatcherServlet在處理請求的時候，父類FrameworkServlet#processRequest就有向RequestContextHolder初始化綁定一些通用參數的操作，這樣子使用者可以在任意地方，拿到這些公用參數了，可謂特別的方便。
發散閱讀：
LocaleContextHolder 是用來處理Local的上下文容器。