阅读了struts2的部分源码, 才知道, 原来struts2是不用servlet的, 只用了filter就可以实现所有的功能, 包括action和interceptor.

首先, 一个请求过来, filter会拦截到, 通过解析请求URL, 得到请求的target(哪个类的哪个方法), 放在actionMapping里边, 这个通过actionMapping可以得到要执行的类的方法, 然后作为对象放到DefaultActionInvocation这个类中, 这个类是实现了ActionInvocation的类, 没错, 就是在struts2的拦截器方法intercept(ActionInvocation invocation)的参数类型.把代码罗列出来比较容易看.

先看看FilterDispatcher这个filter的doFilter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();
           //这里就是为什么我们可以在action中直接使用ActionContext.getContext().getRequest()
           //来得到当前的request等对象了. 用的是ThreadLocal的存储.
            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 (staticResourceLoader.canHandle(resourcePath)) {
                    staticResourceLoader.findStaticResource(resourcePath, 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);
            }
        }
    }