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tomcat源码—redirect和forward的实现

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tomcat源码—redirect和forward的实现

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)：

Java代码

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：

Java代码

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)

Java代码

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)

Java代码

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)

Java代码

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)：

Java代码

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)：

Java代码

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)：

Java代码

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)) {
ApplicationHttpRequest wrequest =
                    (ApplicationHttpRequest) wrapRequest(state);
                HttpServletRequest hrequest = state.hrequest;
                wrequest.setRequestURI(hrequest.getRequestURI());
                wrequest.setContextPath(hrequest.getContextPath());
                   wrequest.setServletPath(hrequest.getServletPath());
                wrequest.setPathInfo(hrequest.getPathInfo());
                wrequest.setQueryString(hrequest.getQueryString());
            processRequest(request,response,state);
//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);
}
}
if (wrapper.getLogger().isDebugEnabled() )
                wrapper.getLogger().debug(" Disabling the response for futher output");

                 if (response instanceof ResponseFacade) {
                ((ResponseFacade) response).finish();
             } else {
                // Servlet SRV.6.2.2. The Request/Response may have been wrapped
                // and may no longer be instance of RequestFacade
                 。。。

                // Close anyway
                try {
                    PrintWriter writer = response.getWriter();
                    writer.close();
            } catch (IllegalStateException e) {
            。。
                }
            }
}

   private ServletRequest wrapRequest(State state) {
	        // Locate the request we should insert in front of
	        ServletRequest previous = null;
	        ServletRequest current = state.outerRequest;
                ServletRequest wrapper = null;
                     //省略
	        if ((current instanceof ApplicationHttpRequest) ||
	            (current instanceof Request) ||
	            (current instanceof HttpServletRequest)) {
		    //省略
                    wrapper = new ApplicationHttpRequest(hcurrent, context, crossContext);
        } else {
	            wrapper = new ApplicationRequest(current);
	        }
         }


  下面是 ApplicationRequest类class ApplicationRequest extends ServletRequestWrapper
 public ApplicationRequest(ServletRequest request) { 
         super(request);
	 setRequest(request);
	    }

private void processRequest(ServletRequest request,
	                                ServletResponse response,
	                                State state)
	        throws IOException, ServletException {
	                 
	        DispatcherType disInt = (DispatcherType) request.getAttribute(ApplicationFilterFactory.DISPATCHER_TYPE_ATTR);
	        if (disInt != null) {
	         ....
	                invoke(state.outerRequest, response, state);
	            } else {
	                invoke(state.outerRequest, response, state);
	            }
	        }
	    }

 private void invoke(ServletRequest request, ServletResponse response,
	            State state) throws IOException, ServletException {
	 
	        // Checking to see if the context classloader is the current context
	        // classloader. If it's not, we're saving it, and setting the context
	        // classloader to the Context classloader
	        ClassLoader oldCCL = Thread.currentThread().getContextClassLoader();
	        ClassLoader contextClassLoader = context.getLoader().getClassLoader();
	 
	        if (oldCCL != contextClassLoader) {
	            Thread.currentThread().setContextClassLoader(contextClassLoader);
	        } else {
	            oldCCL = null;
	        }
	 
	        // Initialize local variables we may need
	        HttpServletResponse hresponse = state.hresponse;
	        Servlet servlet = null;
	        IOException ioException = null;
	        ServletException servletException = null;
	        RuntimeException runtimeException = null;
	        boolean unavailable = false;
	 
	        // Check for the servlet being marked unavailable
	        if (wrapper.isUnavailable()) {
	            wrapper.getLogger().warn(
	                    sm.getString("applicationDispatcher.isUnavailable",
	                    wrapper.getName()));
	            long available = wrapper.getAvailable();
	            if ((available > 0L) && (available < Long.MAX_VALUE))
	                hresponse.setDateHeader("Retry-After", available);
	            hresponse.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE, sm
	                    .getString("applicationDispatcher.isUnavailable", wrapper
	                            .getName()));
	            unavailable = true;
	        }
	 
	        // Allocate a servlet instance to process this request
	        try {
	            if (!unavailable) {
	                servlet = wrapper.allocate();
	            }
	        } catch (ServletException e) {
	            wrapper.getLogger().error(sm.getString("applicationDispatcher.allocateException",
	                             wrapper.getName()), StandardWrapper.getRootCause(e));
	            servletException = e;
	        } catch (Throwable e) {
	            ExceptionUtils.handleThrowable(e);
	            wrapper.getLogger().error(sm.getString("applicationDispatcher.allocateException",
	                             wrapper.getName()), e);
	            servletException = new ServletException
	                (sm.getString("applicationDispatcher.allocateException",
	                              wrapper.getName()), e);
	            servlet = null;
	        }
	                 
	        // Get the FilterChain Here
	        ApplicationFilterFactory factory = ApplicationFilterFactory.getInstance();
	        ApplicationFilterChain filterChain = factory.createFilterChain(request, wrapper,servlet);
	         
	        // Call the service() method for the allocated servlet instance
	        try {
	            support.fireInstanceEvent(InstanceEvent.BEFORE_DISPATCH_EVENT,
	                                      servlet, request, response);
	            // for includes/forwards
	            if ((servlet != null) && (filterChain != null)) {
	               filterChain.doFilter(request, response);
	             }
	            // Servlet Service Method is called by the FilterChain
	            support.fireInstanceEvent(InstanceEvent.AFTER_DISPATCH_EVENT,
	                                      servlet, request, response);
	        } catch (ClientAbortException e) {
                       。。。
	        }
	        // Release the filter chain (if any) for this request
	        try {
	            if (filterChain != null)
	                filterChain.release();
	        } catch (Throwable e) {
	           。。
               }
	            // FIXME: Exception handling needs to be similar to what is in the StandardWrapperValue
	 
	        // Deallocate the allocated servlet instance
	        try {
	            if (servlet != null) {
	                wrapper.deallocate(servlet);
	            }
	        } catch (ServletException e) {
	           。。
	        }
	 
        // Reset the old context class loader
	        if (oldCCL != null)
            Thread.currentThread().setContextClassLoader(oldCCL);
         
        // Unwrap request/response if needed
        // See Bugzilla 30949
	        unwrapRequest(state);
	        unwrapResponse(state);
	        // Recycle request if necessary (also BZ 30949)
        recycleRequestWrapper(state);
	         
	        // Rethrow an exception if one was thrown by the invoked servlet
	        if (ioException != null)
	            throw ioException;
	        if (servletException != null)
	            throw servletException;
	        if (runtimeException != null)
	            throw runtimeException;
    }

第1步：设置新的request的属性：

Java代码

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

第2步：分配一个Servelt实例

                   servlet = wrapper.allocate();
                  // 如果用户请求的是用户自己写的Servlet，就会返回直接由开发人员自己编写的servlet实例，
                  //如果请求是一个.JSP文件时，Servlet是org.apache.jasper.servlet.JspServlet的实例，
                 // 如果请求是一个静态资源时，Servlet用的是org.apache.catalina.servlets.DefaultServlet实例。
                    servlet = wrapper.allocate();
                    //在获取这个servlet实例时都调用了 servlet.init()方法了对资源进行初始化。

第3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)：

Java代码

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4步：调用doFilter或者service,代码删减了很多。

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

Java代码

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);
// //最终
Filter全部调用完毕后，就会把请求真正的传递给Servlet了，调用它的#service()方法。
servlet分别对应着对Servlet，JSP，和静态资源的处理。
如果是真正Servlet比较简单，因为这里的"servlet"就是真实的Servlet实例了，
直接调用开发人员自己编写的#service()方法了
(#service()内部是会调用#doGet()，#doPost()等方法的)。
对于静态资源，是调用org.apache.catalina.servlets.DefaultServlet的#service()方法，由于DefaultServlet并没有重写这个方法，所以直接使用HttpServlet的#service()方法。但是DefaultServlet重写了#doGet()，#doPost()等方法(#doPost()内部又调用了#doGet())，
所以请求就又到了#doGet()这个方法中，DefaultServlet的#doGet()只调用了#serveResource()这个方法来提取资源，代码太长，就不再仔细的看了。
    如果是JSP资源，要调用org.apache.jasper.servlet.JspServlet的#service()方法进行
   // 获得请求的JSP文件的路径
// 获取JspServletWrapper实例然后调用org.apache.jasper.servlet.JspServletWrapper的service()方法
             进行 // (1) 编译   // (2) 载入由jsp变成的Servlet类文件 // (3) 调用Servlet.service(req,resp)处理请求
}

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。

Java代码

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

分享到：

“/”代表的是什么 | include与jsp:include区别

2013-04-16 09:48
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