超轻量级MVC框架的设计和实现

1.设计背景
   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下载完整的源代码和jar包。

2.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。

3.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。

4.设计各主要功能类
4.1Action接口
   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。

4.2Action返回模型ModelAndView
   Action返回的ModelAndView：
package com.javaeedev.lightweight.mvc;
public final class ModelAndView {

    private String view;
    private Map 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 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 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。

4.3类ActionContext
   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 contextThreadLocal = new ThreadLocal();

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

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

    /**
     * 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;
    }

}

4.4接口Interceptor
   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;
}

4.5类ViewResolver
   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 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。

5.开发MVC具体实现类
   MVC框架的核心是一个DispatcherServlet，用于接收所有的HTTP请求，并根据URL选择合适的Action对其进行处理。在这里，和Struts不同的是，所有的组件均被IoC容器管理，因此，DispatcherServlet需要实例化并持有Guice IoC容器，此外，DispatcherServlet还需要保存URL映射和Action的对应关系，一个Interceptor拦截器链，一个 ExceptionResolver处理异常。

5.1类DispatcherServlet
package com.javaeedev.lightweight.mvc;
/**
 * Core dispatcher servlet.
 *
 * @author Xuefeng
 */
public class DispatcherServlet extends HttpServlet {

    private Log log = LogFactory.getLog(getClass());

    private Map actionMap;
    private Interceptor[] interceptors = null;
    private ExceptionResolver exceptionResolver = null;
    private ViewResolver viewResolver = null;

    private Injector injector = null; // Guice IoC容器

    ...
}
   Guice 的配置完全由Java 5注解完成，而在DispatcherServlet中，我们需要主动从容器中查找某种类型的Bean，相对于客户端被动地使用IoC容器（客户端甚至不能感觉到IoC容器的存在），DispatcherServlet需要使用ServiceLocator模式主动查找Bean，写一个通用方法：

private List> findKeysByType(Injector inj, Class type) {
    Map, Binding> map = inj.getBindings();
    List> keyList = new ArrayList>();
    for(Key key : map.keySet()) {
        Type t = key.getTypeLiteral().getType();
        if(t instanceof Class) {
            Class clazz = (Class) t;
            if(type==null || type.isAssignableFrom(clazz)) {
                keyList.add(key);
            }
        }
    }
    return keyList;
}

DispatcherServlet初始化时就要首先初始化Guice IoC容器：

public void init(ServletConfig config) throws ServletException {
    String moduleClass = config.getInitParameter("module");
    if(moduleClass==null || moduleClass.trim().equals(""))
        throw new ServletException("Cannot find init parameter in web.xml: "
                + "?"
                + getClass().getName()
                + "module"
                + "put-your-config-module-full-class-name-here");
    ServletContext context = config.getServletContext();
    // init guice:
    injector = Guice.createInjector(Stage.PRODUCTION, getConfigModule(moduleClass.trim(), context));
    ...
}

然后，从IoC容器中查找Action和URL的映射关系：

private Map getUrlMapping(List> actionKeys) {
    Map urlMapping = new HashMap();
    for(Key key : actionKeys) {
        Object obj = safeInstantiate(key);
        if(obj==null)
            continue;
        Class actionClass = (Class) obj.getClass();
        Annotation ann = key.getAnnotation();
        if(ann instanceof Named) {
            Named named = (Named) ann;
            String url = named.value();
            if(url!=null)
                url = url.trim();
            if(!"".equals(url)) {
                log.info("Bind action [" + actionClass.getName() + "] to URL: " + url);
                // link url with this action:
                urlMapping.put(url, new ActionAndMethod(key, actionClass));
            }
            else {
                log.warn("Cannot bind action [" + actionClass.getName() + "] to *EMPTY* URL.");
            }
        }
        else {
            log.warn("Cannot bind action [" + actionClass.getName() + "] because no @Named annotation found in config module. Using: binder.bind(MyAction.class).annotatedWith(Names.named(\"/url\"));");
        }
    }
    return urlMapping;
}

我们假定客户端是以如下方式配置Action和URL映射的：

public class MyModule implements Module {

    public void configure(Binder binder) {
        // bind actions:
        binder.bind(Action.class)
              .annotatedWith(Names.named("/start.do"))
              .to(StartAction.class);
        binder.bind(Action.class)
              .annotatedWith(Names.named("/register.do"))
              .to(RegisterAction.class);
        binder.bind(Action.class)
              .annotatedWith(Names.named("/signon.do"))
              .to(SignonAction.class);
        ...
    }
}

即通过Guice提供的一个注解Names.named()指定URL。当然还可以用其他方法，比如标注一个@Url注解可能更方便，下一个版本会加上。

Interceptor，ExceptionResolver和ViewResolver也是通过查找获得的。

下面讨论DispatcherServlet如何真正处理用户请求。第一步是根据URL查找对应的Action：

String contextPath = request.getContextPath();
String url = request.getRequestURI().substring(contextPath.length());
if(log.isDebugEnabled())
    log.debug("Handle for URL: " + url);
ActionAndMethod am = actionMap.get(url);
if(am==null) {
    response.sendError(HttpServletResponse.SC_NOT_FOUND); // 404 Not Found
    return;
}

没找到Action就直接给个404 Not Found，找到了进行下一步，实例化一个Action并填充参数：

// init ActionContext:
HttpSession session = request.getSession();
ServletContext context = session.getServletContext();

ActionContext.setActionContext(request, response, session, context);

// 每次创建一个新的Action实例:
Action action = (Action) injector.getInstance(am.getKey());
// 把HttpServletRequest的参数自动绑定到Action的属性中:
List props = am.getProperties();
for(String prop : props) {
    String value = request.getParameter(prop);
    if(value!=null) {
        am.invokeSetter(action, prop, value);
    }
}

注意，为了提高速度，所有的set方法已经预先缓存了，因此避免每次请求都用反射重复查找Action的set方法。

然后要应用所有的Interceptor以便拦截Action：

InterceptorChainImpl chains = new InterceptorChainImpl(interceptors);
chains.doInterceptor(action);
ModelAndView mv = chains.getModelAndView();

实现InterceptorChain看上去复杂，其实就是一个简单的递归，大家看InterceptorChainImpl代码就知道了：

package com.javaeedev.lightweight.mvc;

/**
 * Used for holds an interceptor chain.
 *
 * @author Xuefeng
 */
class InterceptorChainImpl implements InterceptorChain {

    private final Interceptor[] interceptors;
    private int index = 0;
    private ModelAndView mv = null;

    InterceptorChainImpl(Interceptor[] interceptors) {
        this.interceptors = interceptors;
    }

    ModelAndView getModelAndView() {
        return mv;
    }

    public void doInterceptor(Action action) throws Exception {
        if(index==interceptors.length)
            // 所有的Interceptor都执行完毕:
            mv = action.execute();
        else {
            // 必须先更新index，再调用interceptors[index-1]，否则是一个无限递归:
            index++;
            interceptors[index-1].intercept(action, this);
        }
    }
}

把上面的代码用try ... catch包起来，就可以应用ExceptionResolver了。

如果得到了ModelAndView，最后一步就是渲染View了，这个过程极其简单：

// render view:
private void render(ModelAndView mv, HttpServletRequest reqest, HttpServletResponse response) throws ServletException, IOException {
    String view = mv.getView();
    if(view.startsWith("redirect:")) {
        // 重定向:
        String redirect = view.substring("redirect:".length());
        response.sendRedirect(redirect);
        return;
    }
    Map model = mv.getModel();
    if(viewResolver!=null)
        viewResolver.resolveView(view, model, reqest, response);
}

最简单的JspViewResolver的实现如下：

package com.javaeedev.lightweight.mvc.view;

/**
 * Let JSP render the model returned by Action.
 *
 * @author Xuefeng
 */
public class JspViewResolver implements ViewResolver {

    /**
     * Init JspViewResolver.
     */
    public void init(ServletContext context) throws ServletException {
    }

    /**
     * Render view using JSP.
     */
    public void resolveView(String view, Map model, HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        if(model!=null) {
            Set keys = model.keySet();
            for(String key : keys) {
                request.setAttribute(key, model.get(key));
            }
        }
        request.getRequestDispatcher(view).forward(request, response);
    }
}

至此，MVC框架的核心已经完成。