Struts2 Action调用过程

1.当客户端有请求过来，Web容器把它传给Struts2的FilterDispather；如果是.html
  等请求，FilterDispacther直接把调用传给Servlet的FilterChain，如果是Action
  请求，则把调用传递给FilterDispachter中dispatcher对象的ServiceAction方法.

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        ServletContext servletContext = getServletContext();

        String timerKey = "FilterDispatcher_doFilter: ";
        try {
            
            // FIXME: this should be refactored better to not duplicate work with the action invocation
            ValueStack stack = dispatcher.getContainer().getInstance(ValueStackFactory.class).createValueStack();
            ActionContext ctx = new ActionContext(stack.getContext());
            ActionContext.setContext(ctx);
            
            UtilTimerStack.push(timerKey);
            request = prepareDispatcherAndWrapRequest(request, response);
            ActionMapping mapping;
            try {
                mapping = actionMapper.getMapping(request, dispatcher.getConfigurationManager());
            } catch (Exception ex) {
                log.error("error getting ActionMapping", ex);
                dispatcher.sendError(request, response, servletContext, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, ex);
                return;
            }

            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 (serveStatic && resourcePath.startsWith("/struts")) {
                    String name = resourcePath.substring("/struts".length());
                    findStaticResource(name, request, response);
                } else {
                    // this is a normal request, let it pass through
                    chain.doFilter(request, response);
                }
                // The framework did its job here
                return;
            }

            dispatcher.serviceAction(request, response, servletContext, mapping);

        } finally {
            try {
                ActionContextCleanUp.cleanUp(req);
            } finally {
                UtilTimerStack.pop(timerKey);
            }
        }
    }

2.在dispatcher方法中，首先创建ActionProxy对象，在ActionProxy对象中创建DefaultActionInvocation对象(此对象包括获取Action，Result的方法，并包括Interceptors列表)

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);
        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 (stack != null) {
                request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack);
            }
        } catch (ConfigurationException e) {
            LOG.error("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);
        }
    }

3.在上面准备工作完成后，调用ActionProxy的execute()方法，它调用
DefaultActionInvocation对象的invoke方法，此方法开启Interceptors链的调用，然后调Action的excute(或其它)方法，最后调用Result的excute方法， 并把执行结果返回给客户端.

public String invoke() throws Exception {
    	String profileKey = "invoke: ";
    	try {
    		UtilTimerStack.push(profileKey);
    		
    		if (executed) {
    			throw new IllegalStateException("Action has already executed");
    		}
    		if (interceptors.hasNext()) {
    			final InterceptorMapping interceptor = (InterceptorMapping) interceptors.next();
    			//UtilTimerStack.profile("interceptor: "+interceptor.getName(), 
    			//		new UtilTimerStack.ProfilingBlock<String>() {
				//			public String doProfiling() throws Exception {
				    			resultCode = interceptor.getInterceptor().intercept(DefaultActionInvocation.this);
				//    			return null;
				//			}
    			//});
    		} else 
    		{
    			resultCode = invokeActionOnly();
    		}

    		// this is needed because the result will be executed, then control will return to the Interceptor, which will
    		// return above and flow through again
    		if (!executed) {
    			if (preResultListeners != null) {
    				for (Iterator iterator = preResultListeners.iterator();
    					iterator.hasNext();) {
    					PreResultListener listener = (PreResultListener) iterator.next();
    					
    					String _profileKey="preResultListener: ";
    					try {
    						UtilTimerStack.push(_profileKey);
    						listener.beforeResult(this, resultCode);
    					}
    					finally {
    						UtilTimerStack.pop(_profileKey);
    					}
    				}
    			}

    			// now execute the result, if we're supposed to
    			if (proxy.getExecuteResult()) {
    				executeResult();
    			}

    			executed = true;
    		}

    		return resultCode;
    	}
    	finally {
    		UtilTimerStack.pop(profileKey);
    	}
    }