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 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();
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 result
" + 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 operate
ActionContext 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 list
List<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 action
String 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：
1、分析问题的方法；
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。