Struts2核心原理

 

        请求首先通过Filter chain，Filter主要包括ActionContextCleanUp，它主要清理当前线程的ActionContext和Dispatcher；FilterDispatcher主要通过AcionMapper来决定需要调用哪个Action。ActionMapper取得了ActionMapping后，在Dispatcher的serviceAction方法里创建ActionProxy，ActionProxy创建ActionInvocation，然后ActionInvocation调用Interceptors，执行Action本身，创建Result并返回，当然，如果要在返回之前做些什么，可以实现PreResultListener。 

2. Struts2部分类介绍 
这部分从Struts2参考文档中翻译就可以了。 
ActionMapper 
        ActionMapper其实是HttpServletRequest和Action调用请求的一个映射，它屏蔽了Action对于Request等java Servlet类的依赖。Struts2中它的默认实现类是DefaultActionMapper，ActionMapper很大的用处可以根据自己的需要来设计url格式，它自己也有Restful的实现，具体可以参考文档的docs\actionmapper.html。 
ActionProxy&ActionInvocation 
        Action的一个代理，由ActionProxyFactory创建，它本身不包括Action实例，默认实现DefaultActionProxy是由ActionInvocation持有Action实例。ActionProxy作用是如何取得Action，无论是本地还是远程。而ActionInvocation的作用是如何执行Action，拦截器的功能就是在ActionInvocation中实现的。 
ConfigurationProvider&Configuration 
        ConfigurationProvider就是Struts2中配置文件的解析器，Struts2中的配置文件主要是尤其实现类XmlConfigurationProvider及其子类StrutsXmlConfigurationProvider来解析。 

3. Struts2请求流程 
1、客户端发送请求 
2、请求先通过ActionContextCleanUp-->FilterDispatcher 
3、FilterDispatcher通过ActionMapper来决定这个Request需要调用哪个Action 
4、如果ActionMapper决定调用某个Action，FilterDispatcher把请求的处理交给ActionProxy，这儿已经转到它的Delegate--Dispatcher来执行 
5、ActionProxy根据ActionMapping和ConfigurationManager找到需要调用的Action类 
6、ActionProxy创建一个ActionInvocation的实例 
7、ActionInvocation调用真正的Action，当然这涉及到相关拦截器的调用 
8、Action执行完毕，ActionInvocation创建Result并返回，当然，如果要在返回之前做些什么，可以实现PreResultListener。添加PreResultListener可以在Interceptor中实现，不知道其它还有什么方式？

ActionProxy，ActionInvocation。ActionProxy是Action的一个代理类，也就是说Action的调用是通过ActionProxy实现的，其实就是调用了ActionProxy.execute()方法，而该方法又调用了ActionInvocation.invoke()方法。归根到底，最后调用的是DefaultActionInvocation.invokeAction()方法。

另一个版本：大同小异~
一个请求在Struts2框架中的处理大概分为以下几个步骤：
1.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。
2.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。
3.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);
        }
}

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);  
        }  
}  

4.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。
5.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>

<?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。
6.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" />

< 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" /> 

7.一旦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………