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 收藏代码
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 收藏代码
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 收藏代码
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 收藏代码
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 收藏代码
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 收藏代码
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 收藏代码
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
[点击查看原始大小图片]
org.apache.catalina.core.ApplicationDispatcher#doForward(ServletRequest,ServletResponse)：
Java代码 收藏代码
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)) {
//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：
Java代码 收藏代码
wrequest.setContextPath(contextPath);
wrequest.setRequestURI(requestURI);
wrequest.setServletPath(servletPath);
wrequest.setPathInfo(pathInfo);
if (queryString != null) {
wrequest.setQueryString(queryString);
wrequest.setQueryParams(queryString);
}
第2步：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 收藏代码
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3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 收藏代码
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);
}
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 收藏代码
<filter-mapping>
<filter-name>struts2</filter-name>
<url-pattern>/*</url-pattern>
<dispatcher>REQUEST</dispatcher>
<dispatcher>FORWARD</dispatcher>
</filter-mapping>
forward not working for struts2，why？
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转载自Qingjune.iteye.com/blog/1612981
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