RequestDispater

网上已经有很多关于redirect和forward区别的文章，更多的都是只是一些概念上的描述，虽然在大多情况下，知道这些就已经足够了。但也有例外：forward not working for struts2，why？我也是在工作中碰到了这个问题，才特意看了下tomcat有关这部分的源代码。深刻的了解下也无妨。 
redirect和forward都是属于servlet规范的，不同的servlet容器的实现可能会有一些区别，但原理都是类似的。 

redirect和forward的定义： 
1. redirect（重定向）：服务端发送给客户端一个重定向的临时响应头，这个响应头包含重定向之后的URL，客户端用新的URL重新向服务器发送一个请求。 
2. forward（请求转向）：服务器程序内部请求转向，这个特性允许前一个程序用于处理请求，而后一个程序用来返回响应。 

Redirect的原理比较简单，它的定义也已经描述的很清楚了，我也不想多讲什么，就贴一段简单的代码吧！ 

org.apache.catalina.connector.Response#sendRedirect(String)： 
  

 public void sendRedirect(String location)
        throws IOException {

        if (isCommitted())
            throw new IllegalStateException
                (sm.getString("coyoteResponse.sendRedirect.ise"));

        // Ignore any call from an included servlet
        if (included)
            return;

        // Clear any data content that has been buffered
        resetBuffer();

        // Generate a temporary redirect to the specified location
        try {
            String absolute = toAbsolute(location);
            setStatus(SC_FOUND);
            setHeader("Location", absolute);
        } catch (IllegalArgumentException e) {
            setStatus(SC_NOT_FOUND);
        }

        // Cause the response to be finished (from the application perspective)
        setSuspended(true);
}

方法行为：先把相对路径转换成绝对路径，再包装一个包含有新的URL的临时响应头，“SC_FOUND”的值是302，就是重定向临时响应头的状态码。如果传入的“location”值不合法，就包装一个404的响应头。 

下面就来看看tomcat是如何实现forward的，forward为什么在struts2下会无效(注解：其实是可以设置的)。 

先看下程序是如何调用forward的： 

req.getRequestDispatcher("testForward").forward(req, resp);

整个过程分两个步骤来执行 
1. 得到一个请求调度器 
2. 通过调度器把请求转发过去。 


第一步骤，获取请求调度器。 
org.apache.catalina.connector.Request#getRequestDispatcher(String) 

public RequestDispatcher getRequestDispatcher(String path) {

        if (request == null) {
            throw new IllegalStateException(
                            sm.getString("requestFacade.nullRequest"));
        }

        if (Globals.IS_SECURITY_ENABLED){
            return (RequestDispatcher)AccessController.doPrivileged(
                new GetRequestDispatcherPrivilegedAction(path));
        } else {
             return request.getRequestDispatcher(path);
        }

方法行为：把获取RequestDispatcher的任务交个内部的request。它们之间的关系如下所示 

 


org.apache.catalina.connector.RequestFacade和类org.apache.catalina.connector.Request都是实现了javax.servlet.http.HttpServletRequest接口，而RequestFacade内部有包装了个Request，对Request的访问做了些控制，应该是代理模式 

org.apache.catalina.connector.Request#getRequestDispatcher(String) 

public RequestDispatcher getRequestDispatcher(String path) {
         if (path.startsWith("/"))
           return (context.getServletContext().getRequestDispatcher(path));

       //省略了部分代码
       return (context.getServletContext().getRequestDispatcher(relative));

   }

方法行为：把绝对路径转换成相对路径，最终的格式如“/testForward”。若已经是这种格式的相对路径，就无需再转换了。 
接下来就转交给ServletContext来处理，ServletContext是web项目的一个上下文，包含所有的Servlet集合，还定义了一些Servlet与容器之间交互的接口。 
org.apache.catalina.core.ApplicationContext#getRequestDispatcher(String) 

public RequestDispatcher getRequestDispatcher(String path) {
          //省去部分代码
          context.getMapper().map(uriMB, mappingData);
          //省去部分代码
      Wrapper wrapper = (Wrapper) mappingData.wrapper;
      String wrapperPath = mappingData.wrapperPath.toString();
      String pathInfo = mappingData.pathInfo.toString();

      mappingData.recycle();

      // Construct a RequestDispatcher to process this request
      return new ApplicationDispatcher
          (wrapper, uriCC.toString(), wrapperPath, pathInfo,
           queryString, null);
  }

方法行为：根据路径名“path”找到一个包含有Servlet的Wrapper，最后实例化一个ApplicationDispatcher，并且返回该ApplicationDispatcher。 

该方法里非常关键的一行：context.getMapper().map(uriMB, mappingData)。 
Mapper的类定义我不知道如何描述，就贴上原文吧：Mapper, which implements the servlet API mapping rules (which are derived from the HTTP rules)。 
不过只想了解forward的原理，熟悉map函数就够了。 

org.apache.tomcat.util.http.mapper.Mapper#map(org.apache.tomcat.util.buf.MessageBytes, org.apache.tomcat.util.http.mapper.MappingData)： 

public void map(MessageBytes uri, MappingData mappingData)
    throws Exception {

    uri.toChars();
    CharChunk uricc = uri.getCharChunk();
    uricc.setLimit(-1);
    internalMapWrapper(context, uricc, mappingData);

}

方法行为：。。。。。。。就介绍下参数吧，uri可以理解是path(“/testforward”)的一个变形,而mappingData用于存储当前线程用到的部分数据。该函数是没有返回值的，处理之后的结果就是存放到mappingData里的。 

org.apache.tomcat.util.http.mapper.Mapper#internalMapWrapper(Mapper$Context,org.apache.tomcat.util.buf.CharChunk, org.apache.tomcat.util.http.mapper.MappingData)： 

private final void internalMapWrapper(Context context, CharChunk path,
                                         MappingData mappingData)
       throws Exception {

       int pathOffset = path.getOffset();
       int pathEnd = path.getEnd();
       int servletPath = pathOffset;
       boolean noServletPath = false;

       int length = context.name.length();
       if (length != (pathEnd - pathOffset)) {
           servletPath = pathOffset + length;
       } else {
           noServletPath = true;
           path.append('/');
           pathOffset = path.getOffset();
           pathEnd = path.getEnd();
           servletPath = pathOffset+length;
       }

       path.setOffset(servletPath);

       // Rule 1 -- Exact Match
       Wrapper[] exactWrappers = context.exactWrappers;
       internalMapExactWrapper(exactWrappers, path, mappingData);

       // Rule 2 -- Prefix Match
       boolean checkJspWelcomeFiles = false;
       Wrapper[] wildcardWrappers = context.wildcardWrappers;
       if (mappingData.wrapper == null) {
           internalMapWildcardWrapper(wildcardWrappers, context.nesting,
                                      path, mappingData);
           if (mappingData.wrapper != null && mappingData.jspWildCard) {
               char[] buf = path.getBuffer();
               if (buf[pathEnd - 1] == '/') {
                   /*
                    * Path ending in '/' was mapped to JSP servlet based on
                    * wildcard match (e.g., as specified in url-pattern of a
                    * jsp-property-group.
                    * Force the context's welcome files, which are interpreted
                    * as JSP files (since they match the url-pattern), to be
                    * considered. See Bugzilla 27664.
                    */
                   mappingData.wrapper = null;
                   checkJspWelcomeFiles = true;
               } else {
                   // See Bugzilla 27704
                   mappingData.wrapperPath.setChars(buf, path.getStart(),
                                                    path.getLength());
                   mappingData.pathInfo.recycle();
               }
           }
       }

       if(mappingData.wrapper == null && noServletPath) {
           // The path is empty, redirect to "/"
           mappingData.redirectPath.setChars
               (path.getBuffer(), pathOffset, pathEnd);
           path.setEnd(pathEnd - 1);
           return;
       }

       // Rule 3 -- Extension Match
       Wrapper[] extensionWrappers = context.extensionWrappers;
       if (mappingData.wrapper == null && !checkJspWelcomeFiles) {
           internalMapExtensionWrapper(extensionWrappers, path, mappingData);
       }

       // Rule 4 -- Welcome resources processing for servlets
       if (mappingData.wrapper == null) {
           boolean checkWelcomeFiles = checkJspWelcomeFiles;
           if (!checkWelcomeFiles) {
               char[] buf = path.getBuffer();
               checkWelcomeFiles = (buf[pathEnd - 1] == '/');
           }
           if (checkWelcomeFiles) {
               for (int i = 0; (i < context.welcomeResources.length)
                        && (mappingData.wrapper == null); i++) {
                   path.setOffset(pathOffset);
                   path.setEnd(pathEnd);
                   path.append(context.welcomeResources[i], 0,
                               context.welcomeResources[i].length());
                   path.setOffset(servletPath);

                   // Rule 4a -- Welcome resources processing for exact macth
                   internalMapExactWrapper(exactWrappers, path, mappingData);

                   // Rule 4b -- Welcome resources processing for prefix match
                   if (mappingData.wrapper == null) {
                       internalMapWildcardWrapper
                           (wildcardWrappers, context.nesting,
                            path, mappingData);
                   }

                   // Rule 4c -- Welcome resources processing
                   //            for physical folder
                   if (mappingData.wrapper == null
                       && context.resources != null) {
                       Object file = null;
                       String pathStr = path.toString();
                       try {
                           file = context.resources.lookup(pathStr);
                       } catch(NamingException nex) {
                           // Swallow not found, since this is normal
                       }
                       if (file != null && !(file instanceof DirContext) ) {
                           internalMapExtensionWrapper(extensionWrappers,
                                                       path, mappingData);
                           if (mappingData.wrapper == null
                               && context.defaultWrapper != null) {
                               mappingData.wrapper =
                                   context.defaultWrapper.object;
                               mappingData.requestPath.setChars
                                   (path.getBuffer(), path.getStart(),
                                    path.getLength());
                               mappingData.wrapperPath.setChars
                                   (path.getBuffer(), path.getStart(),
                                    path.getLength());
                               mappingData.requestPath.setString(pathStr);
                               mappingData.wrapperPath.setString(pathStr);
                           }
                       }
                   }
               }

               path.setOffset(servletPath);
               path.setEnd(pathEnd);
           }

       }

方法行为：通过“path”从“context”里找到对应的Servlet，存放到“mappingData”里。 
可以看到这里有7个匹配Servlet规则： 
1. Rule 1 -- Exact Match:精确匹配，匹配web.xml配置的格式如“<url-pattern>/testQiu</url-pattern>”的Servlet 
2. Rule 2 -- Prefix Matcha:前缀匹配，匹配的Servlet格式如“<url-pattern>/testQiu/*</url-pattern>” 
3. Rule 3 -- Extension Match:扩展匹配，匹配jsp或者jspx 
4. ---Rule 4a -- Welcome resources processing for exact macth: 
5. ---Rule 4b -- Welcome resources processing for prefix match: 
6. ---Rule 4c -- Welcome resources processing for physical folder: 
7. Rule 7 --如果前面6条都没匹配到，那就返回org.apache.catalina.servlets.DefaultServlet。 

其实这里真正的匹配的是Wapper，而不是Servlet，因为Wapper最重要的一个属性就是Servlet，说成“匹配Servlet”是为了更容易的表达。 

至此返回RequestDispatcher就结束了。 



接下来就是讲解RequestDispatcher.forward了。Forward的就不贴出全部的源代码，只贴一些重要的片段，绝大部分的逻辑都在org.apache.catalina.core.ApplicationDispatcher类里。 
先描述下过程： 
1. 设置request里的部分属性值，如：请求的路径、参数等。 
2. 组装一个FilterChain链，调用doFilter方法。 
3. 最后根据实际情况调用Filter的doFilter函数或者Servlet的service函数。 

注：FilterChain和Filter是两个不同的接口，两个接口的UML 

 

org.apache.catalina.core.ApplicationDispatcher#doForward(ServletRequest,ServletResponse)： 

private void doForward(ServletRequest request, ServletResponse response)
        throws ServletException, IOException
         //省略了部分代码
        // Handle an HTTP named dispatcher forward
        if ((servletPath == null) && (pathInfo == null)) {
//省略了部分代码
        } else {// Handle an HTTP path-based forward
            ApplicationHttpRequest wrequest =
                (ApplicationHttpRequest) wrapRequest(state);
            String contextPath = context.getPath();
            HttpServletRequest hrequest = state.hrequest;
            if (hrequest.getAttribute(Globals.FORWARD_REQUEST_URI_ATTR) == null) {
                wrequest.setAttribute(Globals.FORWARD_REQUEST_URI_ATTR,
                                      hrequest.getRequestURI());
                wrequest.setAttribute(Globals.FORWARD_CONTEXT_PATH_ATTR,
                                      hrequest.getContextPath());
                wrequest.setAttribute(Globals.FORWARD_SERVLET_PATH_ATTR,
                                      hrequest.getServletPath());
                wrequest.setAttribute(Globals.FORWARD_PATH_INFO_ATTR,
                                      hrequest.getPathInfo());
                wrequest.setAttribute(Globals.FORWARD_QUERY_STRING_ATTR,
                                      hrequest.getQueryString());
            }

            wrequest.setContextPath(contextPath);
            wrequest.setRequestURI(requestURI);
            wrequest.setServletPath(servletPath);
            wrequest.setPathInfo(pathInfo);
            if (queryString != null) {
                wrequest.setQueryString(queryString);
                wrequest.setQueryParams(queryString);
            }

            processRequest(request,response,state);
        }
        }
//省略了部分代码
    }

第1步：设置新的request的属性： 
         

wrequest.setContextPath(contextPath);
          wrequest.setRequestURI(requestURI);
          wrequest.setServletPath(servletPath);
          wrequest.setPathInfo(pathInfo);
          if (queryString != null) {
              wrequest.setQueryString(queryString);
              wrequest.setQueryParams(queryString);
          }

第2步：组装FitlerChain链，根据web.xml配置信息，是否决定添加Filter---- 
<filter-mapping> 
<filter-name>struts2</filter-name> 
<url-pattern>/*</url-pattern> 
<dispatcher>REQUEST</dispatcher> 
</filter-mapping> 


org.apache.catalina.core.ApplicationFilterFactory#createFilterChain(ServletRequest, Wrapper, Servlet)： 

public ApplicationFilterChain createFilterChain(ServletRequest request, Wrapper wrapper, Servlet servlet) {
        //省略部分代码
            filterChain = new ApplicationFilterChain();
        }

        filterChain.setServlet(servlet);

        filterChain.setSupport
            (((StandardWrapper)wrapper).getInstanceSupport());

        // Acquire the filter mappings for this Context
        StandardContext context = (StandardContext) wrapper.getParent();
        FilterMap filterMaps[] = context.findFilterMaps();

        // If there are no filter mappings, we are done
        if ((filterMaps == null) || (filterMaps.length == 0))
            return (filterChain);

        // Acquire the information we will need to match filter mappings
        String servletName = wrapper.getName();

        // Add the relevant path-mapped filters to this filter chain
        for (int i = 0; i < filterMaps.length; i++) {
            if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
                continue;
            }
            if (!matchFiltersURL(filterMaps[i], requestPath))
                continue;
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                context.findFilterConfig(filterMaps[i].getFilterName());
            if (filterConfig == null) {
                ;       // FIXME - log configuration problem
                continue;
            }
            boolean isCometFilter = false;
            if (comet) {
                try {
                    isCometFilter = filterConfig.getFilter() instanceof CometFilter;
                } catch (Exception e) {
                    // Note: The try catch is there because getFilter has a lot of
                    // declared exceptions. However, the filter is allocated much
                    // earlier
                }
                if (isCometFilter) {
                    filterChain.addFilter(filterConfig);
                }
            } else {
                filterChain.addFilter(filterConfig);
            }
        }

       //省略部分代码

        // Return the completed filter chain
        return (filterChain);

}

如果是<dispatcher>REQUEST</dispatcher>，那就不添加Filter，默认设置是REQUEST 
如果是<dispatcher>FORWARD</dispatcher>，添加Filter到FilterChain。 

第3步：调用doFilter或者service,代码删减了很多。 

org.apache.catalina.core.ApplicationFilterChain#doFilter(ServletRequest, ServletResponse)： 

  public void doFilter(ServletRequest request, ServletResponse response)throws IOException, ServletException {
            internalDoFilter(request,response);
  }


org.apache.catalina.core.ApplicationFilterChain#internalDoFilter(ServletRequest, ServletResponse)
private void internalDoFilter(ServletRequest request,
                                  ServletResponse response)
        throws IOException, ServletException {

        // Call the next filter if there is one
        if (pos < n) {
                    filter.doFilter(request, response, this);
            return;
        }
       servlet.service((HttpServletRequest) request,(HttpServletResponse) response);
}

如果我对Filter非常了解的，根本就不需要花那么多时间去查看tomcat源代码。只要在web.xml增加一点配置就OK了。 

<filter-mapping>
        <filter-name>struts2</filter-name>
        <url-pattern>/*</url-pattern>
        <dispatcher>REQUEST</dispatcher>
        <dispatcher>FORWARD</dispatcher>
</filter-mapping>