struts2 的set get方法取值原理

struts2的action会自动的把本身的属性设置到request中去,这部分的代码应该是在动态代理中实现的 

它是用了params这个拦截器来实现的。当然，拦截器里用到了反射机制，如果你去研究代码的话。

页面上的form表单或者ajax提交action后首先会经过此filter

<filter>
    <filter-name>struts2</filter-name>
      <filter-class>
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
       </filter-class>
    </filter>
  <filter-mapping>
  <filter-name>struts2</filter-name>
  <url-pattern>*.action</url-pattern>
  </filter-mapping>

之后会进入StrutsPrepareAndExecuteFilter

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;

        try {
            prepare.setEncodingAndLocale(request, response);
            prepare.createActionContext(request, response);
            prepare.assignDispatcherToThread();
			if ( excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) {
				chain.doFilter(request, response);
			} else {
				request = prepare.wrapRequest(request);
				ActionMapping mapping = prepare.findActionMapping(request, response, true);
				if (mapping == null) {
					boolean handled = execute.executeStaticResourceRequest(request, response);
					if (!handled) {
						chain.doFilter(request, response);
					}
				} else {
					execute.executeAction(request, response, mapping);//代码走到这				}
			}
        } finally {
            prepare.cleanupRequest(request);
        }
    }

通过这句代码execute.executeAction依次调用到org.apache.struts2.dispatcher.Dispatcher

public void serviceAction(HttpServletRequest request, HttpServletResponse response, ServletContext context,
                              ActionMapping mapping) throws ServletException {

        Map<String, Object> extraContext = createContextMap(request, response, mapping, context);

        // If there was a previous value stack, then create a new copy and pass it in to be used by the new Action
        ValueStack stack = (ValueStack) request.getAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY);
        boolean nullStack = stack == null;
        if (nullStack) {
            ActionContext ctx = ActionContext.getContext();
            if (ctx != null) {
                stack = ctx.getValueStack();
            }
        }
        if (stack != null) {
            extraContext.put(ActionContext.VALUE_STACK, valueStackFactory.createValueStack(stack));
        }

        String timerKey = "Handling request from Dispatcher";
        try {
            UtilTimerStack.push(timerKey);
            String namespace = mapping.getNamespace();
            String name = mapping.getName();
            String method = mapping.getMethod();

            Configuration config = configurationManager.getConfiguration();
             //这里使用的action的代理
            ActionProxy proxy = config.getContainer().getInstance(ActionProxyFactory.class).createActionProxy(
                    namespace, name, method, extraContext, true, false);
            
            request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, proxy.getInvocation().getStack());

            // if the ActionMapping says to go straight to a result, do it!
            if (mapping.getResult() != null) {
                Result result = mapping.getResult();
                result.execute(proxy.getInvocation());
            } else {
                proxy.execute();
            }

            // If there was a previous value stack then set it back onto the request
            if (!nullStack) {
                request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack);
            }
        } catch (ConfigurationException e) {
        	// WW-2874 Only log error if in devMode
        	if(devMode) {
                String reqStr = request.getRequestURI();
                if (request.getQueryString() != null) {
                    reqStr = reqStr + "?" + request.getQueryString();
                }
                LOG.error("Could not find action or resultn" + reqStr, e);
            }
        	else {
                    if (LOG.isWarnEnabled()) {
        		LOG.warn("Could not find action or result", e);
                    }
        	}
            sendError(request, response, context, HttpServletResponse.SC_NOT_FOUND, e);
        } catch (Exception e) {
            sendError(request, response, context, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, e);
        } finally {
            UtilTimerStack.pop(timerKey);
        }
    }

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） 

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