tomcat源码—redirect和forward的实现

全文转载：http://luckaway.iteye.com/blog/557980

作者：luckaway [from javaeye]

 

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)： 
  

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>