Struts2核心工作流程与原理

这是Struts2官方站点提供的Struts 2 的整体结构。

　　一个请求在Struts2框架中的处理大概分为以下几个步骤：
客户端提起一个（HttpServletRequest）请求,如上文在浏览器中输入”http://localhost:8080/TestMvc/add.action”就是提起一个（HttpServletRequest）请求。
请求被提交到一系列（主要是三层）的过滤器（Filter），如（ActionContextCleanUp、其他过滤器（SiteMesh等）、 FilterDispatcher）。注意这里是有顺序的，先ActionContextCleanUp，再其他过滤器（SiteMesh等）、最后到FilterDispatcher。
FilterDispatcher是控制器的核心，就是mvc中c控制层的核心。下面粗略的分析下我理解的FilterDispatcher工作流程和原理：FilterDispatcher进行初始化并启用核心doFilter

其代码如下：

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException ...{ 
        HttpServletRequest request = (HttpServletRequest) req; 
        HttpServletResponse response = (HttpServletResponse) res; 
        ServletContext servletContext = filterConfig.getServletContext(); 
        // 在这里处理了HttpServletRequest和HttpServletResponse。 
        DispatcherUtils du = DispatcherUtils.getInstance(); 
        du.prepare(request, response);//正如这个方法名字一样进行locale、encoding以及特殊request parameters设置 
        try ...{ 
            request = du.wrapRequest(request, servletContext);//对request进行包装 
        } catch (IOException e) ...{ 
            String message = "Could not wrap servlet request with MultipartRequestWrapper!"; 
            LOG.error(message, e); 
            throw new ServletException(message, e); 
        } 
                ActionMapperIF mapper = ActionMapperFactory.getMapper();//得到action的mapper 
        ActionMapping mapping = mapper.getMapping(request);// 得到action 的 mapping 
        if (mapping == null) ...{ 
            // there is no action in this request, should we look for a static resource? 
            String resourcePath = RequestUtils.getServletPath(request); 
            if ("".equals(resourcePath) && null != request.getPathInfo()) ...{ 
                resourcePath = request.getPathInfo(); 
            } 
            if ("true".equals(Configuration.get(WebWorkConstants.WEBWORK_SERVE_STATIC_CONTENT))  
                    && resourcePath.startsWith("/webwork")) ...{ 
                String name = resourcePath.substring("/webwork".length()); 
                findStaticResource(name, response); 
            } else ...{ 
                // this is a normal request, let it pass through 
                chain.doFilter(request, response); 
            } 
            // WW did its job here 
            return; 
        } 
        Object o = null; 
        try ...{ 
            //setupContainer(request); 
            o = beforeActionInvocation(request, servletContext); 
           //整个框架最最核心的方法，下面分析 
            du.serviceAction(request, response, servletContext, mapping); 
        } finally ...{ 
            afterActionInvocation(request, servletContext, o); 
            ActionContext.setContext(null); 
        } 
    } 
du.serviceAction(request, response, servletContext, mapping); 
//这个方法询问ActionMapper是否需要调用某个Action来处理这个（request）请求，如果ActionMapper决定需要调用某个Action，FilterDispatcher把请求的处理交给ActionProxy public void serviceAction(HttpServletRequest request, HttpServletResponse response, String namespace, String actionName, Map requestMap, Map parameterMap, Map sessionMap, Map applicationMap) ...{  
        HashMap extraContext = createContextMap(requestMap, parameterMap, sessionMap, applicationMap, request, response, getServletConfig());  //实例化Map请求 ，询问ActionMapper是否需要调用某个Action来处理这个（request）请求 
        extraContext.put(SERVLET_DISPATCHER, this);  
        OgnlValueStack stack = (OgnlValueStack) request.getAttribute(ServletActionContext.WEBWORK_VALUESTACK_KEY);  
        if (stack != null) ...{  
            extraContext.put(ActionContext.VALUE_STACK,new OgnlValueStack(stack));  
        }  
        try ...{  
            ActionProxy proxy = ActionProxyFactory.getFactory().createActionProxy(namespace, actionName, extraContext);  
//这里actionName是通过两道getActionName解析出来的, FilterDispatcher把请求的处理交给ActionProxy，下面是ServletDispatcher的 TODO:  
            request.setAttribute(ServletActionContext.WEBWORK_VALUESTACK_KEY, proxy.getInvocation().getStack());  
            proxy.execute();  
           //通过代理模式执行ActionProxy 
            if (stack != null)...{  
                request.setAttribute(ServletActionContext.WEBWORK_VALUESTACK_KEY,stack);  
            }  
        } catch (ConfigurationException e) ...{  
            log.error("Could not find action", e);  
            sendError(request, response, HttpServletResponse.SC_NOT_FOUND, e);  
        } catch (Exception e) ...{  
            log.error("Could not execute action", e);  
            sendError(request, response, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, e);  
        }  
}

FilterDispatcher询问ActionMapper是否需要调用某个Action来处理这个（request）请求，如果ActionMapper决定需要调用某个Action，FilterDispatcher把请求的处理交给ActionProxy。
ActionProxy通过Configuration Manager（struts.xml）询问框架的配置文件，找到需要调用的Action类.
如上文的struts.xml配置

<?xml version="1.0" encoding="GBK"?> 
 <!DOCTYPE struts PUBLIC "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN" "http://struts.apache.org/dtds/struts-2.0.dtd"> 
 <struts> 
     <include file="struts-default.xml"/> 
     <package name="struts2" extends="struts-default"> 
         <action name="add"  
             class="edisundong.AddAction" > 
             <result>add.jsp</result> 
         </action>     
     </package> 
 </struts>

如果提交请求的是add.action，那么找到的Action类就是edisundong.AddAction。
ActionProxy创建一个ActionInvocation的实例，同时ActionInvocation通过代理模式调用Action。但在调用之前ActionInvocation会根据配置加载Action相关的所有Interceptor。（Interceptor是struts2另一个核心级的概念）

下面我们来看看ActionInvocation是如何工作的：

ActionInvocation 是Xworks 中Action 调度的核心。而对Interceptor 的调度，也正是由ActionInvocation负责。ActionInvocation 是一个接口， 而DefaultActionInvocation 则是Webwork 对ActionInvocation的默认实现。

Interceptor 的调度流程大致如下：
1. ActionInvocation初始化时，根据配置，加载Action相关的所有Interceptor。
2. 通过ActionInvocation.invoke方法调用Action实现时，执行Interceptor。

Interceptor将很多功能从我们的Action中独立出来，大量减少了我们Action的代码，独立出来的行为具有很好的重用性。XWork、WebWork的许多功能都是有Interceptor实现，可以在配置文件中组装Action用到的Interceptor，它会按照你指定的顺序，在Action执行前后运行。
那么什么是拦截器。
拦截器就是AOP（Aspect-Oriented Programming）的一种实现。（AOP是指用于在某个方法或字段被访问之前，进行拦截然后在之前或之后加入某些操作。）
拦截器的例子这里就不展开了。
struts-default.xml文件摘取的内容：

< interceptor name ="alias" class ="com.opensymphony.xwork2.interceptor.AliasInterceptor" />  
< interceptor name ="autowiring" class ="com.opensymphony.xwork2.spring.interceptor.ActionAutowiringInterceptor" />  
< interceptor name ="chain" class ="com.opensymphony.xwork2.interceptor.ChainingInterceptor" />  
< interceptor name ="conversionError" class ="org.apache.struts2.interceptor.StrutsConversionErrorInterceptor" />  
< interceptor name ="createSession" class ="org.apache.struts2.interceptor.CreateSessionInterceptor" />  
< interceptor name ="debugging" class ="org.apache.struts2.interceptor.debugging.DebuggingInterceptor" />  
< interceptor name ="external-ref" class ="com.opensymphony.xwork2.interceptor.ExternalReferencesInterceptor" />  
< interceptor name ="execAndWait" class ="org.apache.struts2.interceptor.ExecuteAndWaitInterceptor" />  
< interceptor name ="exception" class ="com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor" />  
< interceptor name ="fileUpload" class ="org.apache.struts2.interceptor.FileUploadInterceptor" />  
< interceptor name ="i18n" class ="com.opensymphony.xwork2.interceptor.I18nInterceptor" />  
< interceptor name ="logger" class ="com.opensymphony.xwork2.interceptor.LoggingInterceptor" />  
< interceptor name ="model-driven" class ="com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor" />  
< interceptor name ="scoped-model-driven" class ="com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor" />  
< interceptor name ="params" class ="com.opensymphony.xwork2.interceptor.ParametersInterceptor" />  
< interceptor name ="prepare" class ="com.opensymphony.xwork2.interceptor.PrepareInterceptor" />  
< interceptor name ="static-params" class ="com.opensymphony.xwork2.interceptor.StaticParametersInterceptor" />  
< interceptor name ="scope" class ="org.apache.struts2.interceptor.ScopeInterceptor" />  
< interceptor name ="servlet-config" class ="org.apache.struts2.interceptor.ServletConfigInterceptor" />  
< interceptor name ="sessionAutowiring" class ="org.apache.struts2.spring.interceptor.SessionContextAutowiringInterceptor" />  
< interceptor name ="timer" class ="com.opensymphony.xwork2.interceptor.TimerInterceptor" />  
< interceptor name ="token" class ="org.apache.struts2.interceptor.TokenInterceptor" />  
< interceptor name ="token-session" class ="org.apache.struts2.interceptor.TokenSessionStoreInterceptor" />  
< interceptor name ="validation" class ="com.opensymphony.xwork2.validator.ValidationInterceptor" />  
< interceptor name ="workflow" class ="com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor" />  
< interceptor name ="store" class ="org.apache.struts2.interceptor.MessageStoreInterceptor" />  
< interceptor name ="checkbox" class ="org.apache.struts2.interceptor.CheckboxInterceptor" />  
< interceptor name ="profiling" class ="org.apache.struts2.interceptor.ProfilingActivationInterceptor" />

一旦Action执行完毕，ActionInvocation负责根据struts.xml中的配置找到对应的返回结果。如上文中将结构返回“add.jsp”，但大部分时候都是返回另外一个action，那么流程又得走一遍………