1.写一个Mvc框架_超轻量级MVC框架的设计和实现

梦里梦外; 2022-08-02 05:55 172阅读 0赞

[超轻量级MVC框架的设计和实现][MVC]  2009-11-17 16:01:15

分类： Java

**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http://code.google.com/p/lightweight-mvc/downloads/list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<key<? style="word-wrap: break-word;">> findKeysByType(Injector inj, Class type) \{  
    Map<key<? style="word-wrap: break-word;">, Binding> map = inj.getBindings();  
    List<key<? style="word-wrap: break-word;">> keyList = new ArrayList<key<? style="word-wrap: break-word;">>();  
    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<key<? style="word-wrap: break-word;">> 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。

[MVC]: http://blog.chinaunix.net/uid-122937-id-142926.html