超轻量级MVC框架的设计和实现 (2)

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在设计完API后，我们就需要实现这个MVC框架。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。DispatcherServlet定义如下：

package com.javaeedev.lightweight.mvc;

/**
* Core dispatcher servlet.
*
* @author Xuefeng
*/
public class DispatcherServlet extends HttpServlet {

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

private Map<string actionandmethod> actionMap;<br> private Interceptor[] interceptors = null;<br> private ExceptionResolver exceptionResolver = null;<br> private ViewResolver viewResolver = null;</string>

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

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

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<string> props = am.getProperties();<br>for(String prop : props) {<br> String value = request.getParameter(prop);<br> if(value!=null) {<br> am.invokeSetter(action, prop, value);<br> }<br>}</string>

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<string object> model = mv.getModel();<br> if(viewResolver!=null)<br><font color="#ff0000">viewResolver.resolveView</font>(view, model, reqest, response);<br>}</string>

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<string object> model, HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {<br> if(model!=null) {<br> Set<string> keys = model.keySet();<br> for(String key : keys) {<br> request.setAttribute(key, model.get(key));<br> }<br> }<br> request.getRequestDispatcher(view).<font color="#ff0000">forward</font>(request, response);<br> }<br>}</string></string>

至此，MVC框架的核心已经完成。