问题提出：

1、Struts2框架在什么时候将Action对象放到了值栈ValueStack的栈顶了？

2、在哪里设置Debug断点能够最恰当的观察到这一过程？

问题解决：

2、我们知道，在值栈ValueStack中有两个逻辑部分Map栈和对象栈ObjectStack，而Action对象是被默认放在了对象栈的栈顶的（这一点我们通过<s:debug/>标签可以在页面中看到），因此我们将该断点设置在对象栈所对应的类CompoundRoot的push方法中最合适。

1、通过Debug过程来分析值栈ValueStack的变化过程：

1）当我们在CompoundRoot类的push方法中打上断点后，程序执行到此处，如下图：

2）这时，我们从Struts2框架的入口StrutsPrepareAndExecuteFilter类开始分析。在Debug视图中，我们将程序定位到StrutsPrepareAndExecuteFilter类，如下图：

附上StrutsPrepareAndExecuteFilter中doFilter()方法的源代码：

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);}}} finally {prepare.cleanupRequest(request);}}

这里我们可以看到，doFilter()方法中通过调用execute对象的executeAction()方法来执行Action，接下来我们继续看一看executeAction()方法，如下图：

可以看出executeAction()方法中只是调用了dispatcher对象的serviceAction()方法，因此，我们再继续查看serviceAction()方法，如下图：

可以看到，在serviceAction()方法中实际上是创建了一个Action的代理类ActionProxy的对象，而并没有直接去执行Action。这是因为在执行Action之前Struts2还要去调用许多的拦截器，因此创建了Action的代理类。

附Dispatcher类中serviceAction()方法的源代码：

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 ActionValueStack 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();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 requestif (!nullStack) {request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack);}} catch (ConfigurationException e) {// WW-2874 Only log error if in devModeif (devMode) {String reqStr = request.getRequestURI();if (request.getQueryString() != null) {reqStr = reqStr + "?" + request.getQueryString();}LOG.error("Could not find action or result\n" + 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) {if (handleException || devMode) {sendError(request, response, context, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, e);} else {throw new ServletException(e);}} finally {UtilTimerStack.pop(timerKey);}}

继续往下进行，如下图：

我们可以看到在上面的执行过程中，主要工作都是在创建Action的代理类对象proxy，在创建完成之后，该代理类对象proxy调用prepare()方法做了一些初始化的工作。

继续往下查看prepare()方法：

在prepare()方法（这是StrutsActionProxy的父类DefaultActionProxy的prepare()方法）中可以看到，其执行了invocation对象的init()方法，在该方法中this就是StrutsActionProxy类的对象，也即ActionProxy类的对象，就是Action的代理类。继续来看init()方法的具体实现：

到这里我们已经能够比较清楚的看到了Struts2框架将Action对象放到值栈中。注意，这里的Action对象中的相关属性（如提交的表单中的参数）并没有被赋上值，直到真正的执行Action对象中相应的方法时，其相关属性才被赋值。

附DefaultActionInvocation类中init()方法与createAction()方法的源代码：

public void <span style="background-color: rgb(255, 0, 0);">init</span>(ActionProxy proxy) {this.proxy = proxy;Map<String, Object> contextMap = createContextMap();// Setting this so that other classes, like object factories, can use the ActionProxy and other// contextual information to operateActionContext actionContext = ActionContext.getContext();if (actionContext != null) {actionContext.setActionInvocation(this);}<span style="background-color: rgb(51, 255, 51);">createAction(contextMap);</span>if (pushAction) {<span style="background-color: rgb(51, 255, 51);">stack.push(action);</span><span style="background-color: rgb(51, 255, 51);">contextMap.put("action", action);</span>}invocationContext = new ActionContext(contextMap);invocationContext.setName(proxy.getActionName());// get a new List so we don't get problems with the iterator if someone changes the listList<InterceptorMapping> interceptorList = new ArrayList<InterceptorMapping>(proxy.getConfig().getInterceptors());interceptors = interceptorList.iterator();}

protected void <span style="background-color: rgb(255, 0, 0);">createAction</span>(Map<String, Object> contextMap) {// load actionString timerKey = "actionCreate: " + proxy.getActionName();try {UtilTimerStack.push(timerKey);<span style="background-color: rgb(51, 255, 51);">action = objectFactory.buildAction(proxy.getActionName(), proxy.getNamespace(), proxy.getConfig(), contextMap);</span>} catch (InstantiationException e) {throw new XWorkException("Unable to intantiate Action!", e, proxy.getConfig());} catch (IllegalAccessException e) {throw new XWorkException("Illegal access to constructor, is it public?", e, proxy.getConfig());} catch (Exception e) {String gripe;if (proxy == null) {gripe = "Whoa!  No ActionProxy instance found in current ActionInvocation.  This is bad ... very bad";} else if (proxy.getConfig() == null) {gripe = "Sheesh.  Where'd that ActionProxy get to?  I can't find it in the current ActionInvocation!?";} else if (proxy.getConfig().getClassName() == null) {gripe = "No Action defined for '" + proxy.getActionName() + "' in namespace '" + proxy.getNamespace() + "'";} else {gripe = "Unable to instantiate Action, " + proxy.getConfig().getClassName() + ",  defined for '" + proxy.getActionName() + "' in namespace '" + proxy.getNamespace() + "'";}gripe += (((" -- " + e.getMessage()) != null) ? e.getMessage() : " [no message in exception]");throw new XWorkException(gripe, e, proxy.getConfig());} finally {UtilTimerStack.pop(timerKey);}if (actionEventListener != null) {<span style="background-color: rgb(51, 255, 51);">action = actionEventListener.prepare(action, stack);</span>}}

查看值栈OgnlValueStack类的push()方法，如下图：

可以看到，它确实是调用了CompoundRoot对象的push()方法将Action对象放到了值栈中对象栈ObjectStack的栈顶。

收获：

2、通过该过程清楚了Struts2值栈的变化过程；

2、进一步学习了Eclipse的debug使用方法。