RequestContextHolder简析

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：

1. RequestContextHolder为什么能获取到当前的HttpServletRequest
2. 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}.
 *
 * 
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 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 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,
	 * not 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.
	 * 
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.
	 * 
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。