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目录

HttpServletRequest对象被获取一次之后再次去操作与流相关的操作会抛异常，原因是HttpServletRequest不允许直接对其流的读取进行二次访问，如果要访问我们必须获取到原始的HttpServletRequest或者是由Spring托管的本地化线程的HttpServletRequest对象。RequestContextHolder就可以帮我们办到。

通过上下文过滤器拿走HttpServletRequest请求

以下是过滤器反编译代码：

/*     */ package com.xxx.core.filter;/*     */ /*     */ import java.io.IOException;/*     */ import javax.servlet.Filter;/*     */ import javax.servlet.FilterChain;/*     */ import javax.servlet.FilterConfig;/*     */ import javax.servlet.ServletContext;/*     */ import javax.servlet.ServletException;/*     */ import javax.servlet.ServletRequest;/*     */ import javax.servlet.ServletResponse;/*     */ import javax.servlet.annotation.WebFilter;/*     */ import javax.servlet.http.HttpServletRequest;/*     */ import javax.servlet.http.HttpServletResponse;/*     */ import javax.servlet.http.HttpSession;/*     */ import org.apache.log4j.Logger;/*     */ @WebFilter(filterName="context_Filter", urlPatterns={"/*"})/*     */ public class ContextFilter/*     */   implements Filter/*     */ {/*  34 */   static Logger logger = Logger.getLogger(ContextFilter.class);/*     */   /*     */ /*  37 */   private static ThreadLocal
   
     threadLocalRequest = new ThreadLocal();/*  38 */   private static ThreadLocal
    
      threadLocalResponse = new ThreadLocal();/*     */   /*  40 */   private static ThreadLocal
     
       threadLocalObject = new ThreadLocal();/*     */   /*     */ /*     */ /*     */   public void destroy() {}/*     */   /*     */ /*     */   public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)/*     */     throws IOException, ServletException/*     */   {/*  50 */     threadLocalRequest.set((HttpServletRequest)arg0);/*  51 */     threadLocalResponse.set((HttpServletResponse)arg1);/*  52 */     arg2.doFilter(arg0, arg1);/*     */   }/*     */   /*     */ /*     */ /*     */ /*     */ /*     */   public void init(FilterConfig arg0)/*     */     throws ServletException/*     */   {}/*     */   /*     */ /*     */ /*     */ /*     */   public static HttpServletRequest getRequest()/*     */   {/*  68 */     HttpServletRequest req = (HttpServletRequest)threadLocalRequest.get();/*  69 */     return req;/*     */   }/*     */   /*     */ /*     */ /*     */ /*     */ /*     */ /*     */ /*     */ /*     */   public static HttpSession getSession()/*     */   {/*  81 */     HttpServletRequest req = (HttpServletRequest)threadLocalRequest.get();/*  82 */     if (req == null) {/*  83 */       return null;/*     */     }/*  85 */     return req.getSession();/*     */   }/*     */   /*     */ /*     */ /*     */ /*     */ /*     */ /*     */ /*     */ /*     */   public ServletContext getServletContext()/*     */   {/*  97 */     if (null == getSession()) {/*  98 */       return null;/*     */     }/* 100 */     return getSession().getServletContext();/*     */   }/*     */   /*     */   public static void setSchema(String value)/*     */   {/* 105 */     if (null == getRequest()) {/* 106 */       threadLocalObject.set(value);/*     */     } else {/* 108 */       getRequest().setAttribute("_schema_", value);/*     */     }/* 110 */     logger.debug(" setSchema >> id=" + Thread.currentThread().getId() + ",name " + Thread.currentThread().getName() + "," + value);/*     */   }/*     */   /*     */   public static String getSchema() {/* 114 */     if (null == getRequest()) {/* 115 */       return (String)threadLocalObject.get();/*     */     }/* 117 */     return (String)getRequest().getAttribute("_schema_");/*     */   }/*     */ }/* Location:           D:\DEVELOPEWORKS\dataissue\WebRoot\WEB-INF\lib\forestar-core-3.6.4-SNAPSHOT.jar * Qualified Name:     com.forestar.core.filter.ContextFilter * Java Class Version: 7 (51.0) * JD-Core Version:    0.7.1 */
     
    
   

上面表现的处理中我们已经取走了HttpServletRequest对象作为本地化线程的请求保存起来了。我们在其他地方可以这样使用而不用将HttpServletRequest作为参数传递：

HttpServletRequest  request = ContextFilter.getRequest();

但是当需要读取流的时候，我们就会遇到问题。此时我们需要获得持有被Spring管理的HttpServletRequest。

RequestContextHolder获取HttpServletRequest

获取HttpServletRequest：

HttpServletRequest request = ((ServletRequestAttributes) RequestContextHolder        .getRequestAttributes()).getRequest();

RequestContextHolder源码：

public abstract class RequestContextHolder  {    private static final ThreadLocal
   
     requestAttributesHolder =            new NamedThreadLocal
    
     ("Request attributes");    private static final ThreadLocal
     
       inheritableRequestAttributesHolder =            new NamedInheritableThreadLocal
      
       ("Request context");    public static void resetRequestAttributes() {        requestAttributesHolder.remove();        inheritableRequestAttributesHolder.remove();    }    public static void setRequestAttributes(RequestAttributes attributes) {        setRequestAttributes(attributes, false);    }    //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(RequestAttributes attributes, boolean inheritable) {        if (attributes == null) {            resetRequestAttributes();        }        else {            if (inheritable) {                inheritableRequestAttributesHolder.set(attributes);                requestAttributesHolder.remove();            }            else {                requestAttributesHolder.set(attributes);                inheritableRequestAttributesHolder.remove();            }        }    }    public static RequestAttributes getRequestAttributes() {        RequestAttributes attributes = requestAttributesHolder.get();        if (attributes == null) {            attributes = inheritableRequestAttributesHolder.get();        }        return attributes;    }}
      
     
    
   

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FrameworkServlet这个类，也就是DispatcherServlet的父类，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。

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            doService(request, response);        }        catch (ServletException ex) {            failureCause = ex;            throw ex;        }        catch (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, LocaleContext localeContext, 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);        }    }

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。

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