超轻量级MVC框架的设计和实现 (1)

前段时间准备做一个小网站，但是又不想用Spring/Struts/WebWork这样的大块头，因此决定自己写一个MVC框架。花了3天左右时间完成，目前运行良好，整个MVC框架仅21KB，感兴趣的朋友可以从http://code.google.com/p/lightweight-mvc/downloads/list下载完整的源代码和jar包。

设计目标：

一个最简单最小巧的MVC框架，花哨的功能一个不要，越简洁越好，并且不使用XML配置文件，而是完全用Java 5注解配置。

功能列表：

组件必须用IoC配置；

处理HTTP请求的Action，类似WebWork每个请求都生成一个新实例，并自动填充属性；

类似Filter的Interceptor机制，但是在IoC容器中配置；

统一的异常处理；

多视图支持。

由于组件需要用IoC容器配置，因此，第一步就是寻找小巧的IoC容器，Google Guice是一个很不错的选择，并且完全用Java 5注解配置组件。这个MVC框架唯一依赖的也就是Guice和Commons Logging两个jar包，如果使用Velocity作为视图则还需要Velocity的jar包。

下一步，开始设计各主要功能类：

负责处理Http请求的Action类必须实现的Action接口：

package com.javaeedev.lightweight.mvc;

public interface Action {

ModelAndView execute() throws Exception;

}

从WebWork抄过来，不过返回值由String改成了ModelAndView（从Spring抄过来的），好处是不必再次根据String查找视图的绝对路径，直接在ModelAndView中包含了。用Spring的MVC其实可以发现，ModelAndView同时包含一个Model（本质是一个Map）和View的路径，减少了Struts和WebWork需要的一个XML映射文件，而维护XML配置文件是一件相当令人头疼的问题，往往改了代码还要改配置，索性写死在代码中得了，视图路径又不会经常改变，没必要为了额外的灵活性给自己搞一堆XML配置文件。

Action返回的ModelAndView：

package com.javaeedev.lightweight.mvc;

public final class ModelAndView {

private String view;
private Map<String, Object> model;

/**
* Construct a View with empty model.
*
* @param view View's logic name.
*/
public ModelAndView(String view) {
this.view = view;
this.model = Collections.emptyMap();
}

/**
* Construct a View with model.
*
* @param view View's logic name.
* @param model Model as a Map.
*/
public ModelAndView(String view, Map<String, Object> model) {
this.view = view;
this.model = model;
}

/**
* Return View.
*
* @return View's logic name.
*/
public String getView() {
return view;
}

/**
* Return model.
*
* @return Model as a Map.
*/
public Map<String, Object> getModel() {
return model;
}

}

这个完全是从Spring MVC抄过来的，Map改成了泛型，View路径可以以"redirect:"开头表示重定向，这个和Spring MVC一致。虽然直接调用HttpServletResponse也可以重定向，但是遇到事务处理起来会很麻烦，还是让MVC框架自己来处理会好一些。

WebWork的Action设计的好处是大大简化了参数的绑定，不过很多时候也需要在Action中访问HttpSession等对象，因此还需要设计一个ActionContext类，通过ThreadLocal让Action对象能轻易地访问到这些对象：

package com.javaeedev.lightweight.mvc;

public final class ActionContext {

private static ThreadLocal<ActionContext> contextThreadLocal = new ThreadLocal<ActionContext>();

/**
* Get current ActionContext.
*
* @return ActionContext.
*/
public static ActionContext getActionContext() {
return contextThreadLocal.get();
}

private HttpServletRequest request;
private HttpServletResponse response;
private HttpSession session;
private ServletContext context;

/**
* Initiate all servlet objects as thread local.
*
* @param request HttpServletRequest object.
* @param response HttpServletResponse object.
* @param session HttpSession object.
* @param context ServletContext object.
*/
static void setActionContext(HttpServletRequest request, HttpServletResponse response, HttpSession session, ServletContext context) {
ActionContext actionContext = new ActionContext();
actionContext.setRequest(request);
actionContext.setResponse(response);
actionContext.setSession(session);
actionContext.setServletContext(context);
contextThreadLocal.set(actionContext);
}

/**
* Remove all servlet objects from thread local.
*/
static void remove() {
contextThreadLocal.remove();
}

/**
* Get HttpServletRequest object.
*
* @return HttpServletRequest object.
*/
public HttpServletRequest getRequest() {
return request;
}

/**
* Set HttpServletRequest object.
*
* @param request HttpServletRequest object.
*/
void setRequest(HttpServletRequest request) {
this.request = request;
}

/**
* Get HttpServletResponse object.
*
* @return HttpServletResponse object.
*/
public HttpServletResponse getResponse() {
return response;
}

/**
* Set HttpServletResponse object.
*
* @param response HttpServletResponse object.
*/
void setResponse(HttpServletResponse response) {
this.response = response;
}

/**
* Get HttpSession object.
*
* @return HttpSession object.
*/
public HttpSession getSession() {
return session;
}

/**
* Set HttpSession object.
*
* @param session HttpSession object.
*/
void setSession(HttpSession session) {
this.session = session;
}

/**
* Get ServletContext object.
*
* @return ServletContext object.
*/
public ServletContext getServletContext() {
return context;
}

/**
* Set ServletContext object.
*
* @param context ServletContext object.
*/
void setServletContext(ServletContext context) {
this.context = context;
}

}

接下来是定义类似Filter功能的Interceptor接口：

package com.javaeedev.lightweight.mvc;

/**
* Intercept action's execution like servlet Filter, but interceptors are
* configured and managed by IoC container. Another difference from Filter
* is that Interceptor is executed around Action's execution, but before
* rendering view.
*
* @author Xuefeng
*/
public interface Interceptor {

/**
* Do intercept and invoke chain.doInterceptor() to process next interceptor.
* NOTE that process will not continue if chain.doInterceptor() method is not
* invoked.
*
* @param action Action instance to handle http request.
* @param chain Interceptor chain.
* @throws Exception If any exception is thrown, process will not continued.
*/
void intercept(Action action, InterceptorChain chain) throws Exception;

}

InterceptorChain对象和FilterChain是一样的，它允许一个拦截器是否将请求继续交给下一拦截器处理，还是中断当前请求的处理：

package com.javaeedev.lightweight.mvc;

/**
* Holds all interceptors as a chain.
*
* @author Xuefeng
*/
public interface InterceptorChain {

/**
* Apply next interceptor around the execution of Action.
*
* @param action Target Action to execute.
* @throws Exception Any exception if error occured.
*/
void doInterceptor(Action action) throws Exception;

}

最后是支持多种View的ViewResolver，这个也抄自Spring MVC：

package com.javaeedev.lightweight.mvc;

import java.io.IOException;
import java.util.Map;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
* To resolve and render a view.
*
* @author Xuefeng
*/
public interface ViewResolver {

/**
* Init this ViewResolver.
*
* @param context ServletContext object that holds information of current
* web application.
* @throws ServletException If init failed.
*/
void init(ServletContext context) throws ServletException;

/**
* To resolve view's name and render view if necessary.
*
* @param view View's logic name.
* @param model Model represent as a generic Map.
* @param request HttpServletRequest object.
* @param response HttpServletResponse object.
* @throws ServletException If any ServletException occur.
* @throws IOException If any IOException occur.
*/
void resolveView(String view, Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException;

}

第一个版本支持JSP和Velocity两种View，其实支持其他的View完全是可扩展的，只需要参考现有的两种ViewResolver的实现再写一个实现即可，例如支持FreeMarker的ViewResolver。

到此为止，提供给客户端的API准备完毕。下一步是如何实现这些API。虽然概念和结构都来自WebWork和Spring，但是其具体实现却没有参考他们的源代码，因为读大块头的源码本身就是一件非常费力的事情，还不如自己身体力行，写代码往往比读懂代码更快。

后面我们会讲述如何实现该MVC框架。