超轻量级MVC框架的设计和实现 (2)

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;
    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<?>> findKeysByType(Injector inj, Class<?> type) {
    Map<Key<?>, Binding<?>> map = inj.getBindings();
    List<Key<?>> keyList = new ArrayList<Key<?>>();
    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: <servlet>"
                + "<servlet-name>?</servlet-name><servlet-class>"
                + getClass().getName()
                + "</servlet-class><init-param><param-name>module</param-name><param-value>"
                + "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<?>> actionKeys) {
    Map<String, ActionAndMethod> urlMapping = new HashMap<String, ActionAndMethod>();
    for(Key<?> key : actionKeys) {
        Object obj = safeInstantiate(key);
        if(obj==null)
            continue;
        Class<Action> actionClass = (Class<Action>) 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<String> 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<String, Object> 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<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        if(model!=null) {
            Set<String> 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。