MVC架构探究及其源码实现(4)-前端控制器

前端控制器是整个MVC框架中最为核心的一块，它主要用来拦截符合要求的外部请求，并把请求分发到不同的控制器去处理，根据控制器处理后的结果，生成相应的响应发送到客户端。前端控制器既可以使用Filter实现（Struts2采用这种方式），也可以使用Servlet来实现。这里我们就采用后一种方式来实现我们的MVC框架。

 

1.配置web.xml，使得我们的前端控制器可以拦截所有符合要求的用户请求，这里我们的前端控制器能处理所有以.do结尾的用户请求。 

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
   version="2.5">

    <description>MVC Sample</description>
    <display-name>MVC</display-name>

    <servlet>
      <servlet-name>DispatcherServlet</servlet-name>
      <servlet-class>com.google.mvc.web.servlet.DispatcherServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>DispatcherServlet</servlet-name>
        <url-pattern>*.do</url-pattern>
    </servlet-mapping>
</web-app>

  2.FrameworkServlet实现。
FrameworkServlet是DispatcherServlet的直接父类，继承自HttpServlet，主要用来初始话WebApplicationContext，把不同的Http请求操作委托给同一个方法去处理。

package com.google.mvc.web.servlet;

import java.io.IOException;

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

import org.apache.log4j.Logger;
import com.google.mvc.web.context.WebApplicationContext;

public abstract class FrameworkServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;
    private static final Logger LOGGER = Logger.getLogger(FrameworkServlet.class);
    private WebApplicationContext webApplicationContext;

    @Override
    public void init() throws ServletException {

        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("----------Initializing servlet '" + getServletName() + "'----------");
        }

        this.webApplicationContext = initWebApplicationContext();

        initServletBean();

        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("----------Servlet '" + getServletName() + "' configured successfully----------/n/n");
        }

    }

    private WebApplicationContext initWebApplicationContext() {
        WebApplicationContext wac = new WebApplicationContext(getServletContext());
        wac.init();
        onRefresh(wac);
        return wac;
    }

    protected void onRefresh(WebApplicationContext context) {
        // For subclasses: do nothing by default.
    }

    protected void initServletBean(){

    }

    protected abstract void doService(HttpServletRequest request, HttpServletResponse response) throws Exception;


    protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        long startTime = System.currentTimeMillis();
        Throwable failureCause = null;

        try {
            doService(request, response);
        } catch (ServletException ex) {
            failureCause = ex;
            throw ex;
        } catch (IOException ex) {
            failureCause = ex;
            throw ex;
        } catch (Throwable ex) {
            failureCause = ex;
            throw new NestedServletException("Request processing failed", ex);
        } finally {
            if (failureCause != null) {
                LOGGER.error("Could not complete request", failureCause);
            } else {
                long processingTime = System.currentTimeMillis() - startTime;
                if (LOGGER.isDebugEnabled()) {
                    LOGGER.info("Successfully completed request, cost " + processingTime + " ms/n");
                }
            }
        }
    }

    @Override
    protected void doDelete(HttpServletRequest request, HttpServletResponse response) throws ServletException,
            IOException {
        processRequest(request, response);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        processRequest(request, response);
    }

    @Override
    protected void doHead(HttpServletRequest request, HttpServletResponse response) throws ServletException,
            IOException {
        processRequest(request, response);
    }

    @Override
    protected void doOptions(HttpServletRequest request, HttpServletResponse response) throws ServletException,
            IOException {
        processRequest(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException,
            IOException {
        processRequest(request, response);
    }

    @Override
    protected void doPut(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        processRequest(request, response);
    }

    @Override
    protected void doTrace(HttpServletRequest request, HttpServletResponse response) throws ServletException,
            IOException {
        processRequest(request, response);
    }

    @Override
    public void destroy() {
        if(LOGGER.isDebugEnabled()){
            LOGGER.info("Servlet destory");
        }
        super.destroy();
    }

    public WebApplicationContext getWebApplicationContext() {
        return webApplicationContext;
    }



}

  3.DispatcherServlet实现。

package com.google.mvc.web.servlet;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Properties;
import java.util.StringTokenizer;

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

import org.apache.log4j.Logger;
import com.google.mvc.web.context.WebApplicationContext;

public class DispatcherServlet extends FrameworkServlet {

    private static final long serialVersionUID = 1L;
    private static final Logger LOGGER = Logger.getLogger(DispatcherServlet.class);
    private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";
    private static final Properties defaultStrategies = new Properties();

    private List<HandlerMapping> handlerMappings;
    private List<HandlerAdapter> handlerAdapters;
    private List<ViewResolver> viewResolvers;

    static {
        try {
            defaultStrategies.load(DispatcherServlet.class.getResourceAsStream(DEFAULT_STRATEGIES_PATH));
        } catch (IOException ex) {
            throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());
        }
    }

    @Override
    protected void onRefresh(WebApplicationContext wac) {
        initHandlerMappings(wac);
        initHandlerAdapters(wac);
        initViewResolvers(wac);
    }

    private void initHandlerMappings(WebApplicationContext wac) {
        Map<String, HandlerMapping> map = wac.beansOfType(HandlerMapping.class);
        if (!map.isEmpty()) {
            this.handlerMappings = new ArrayList<HandlerMapping>(map.values());
        }
        if (this.handlerMappings == null) {
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
            }
            this.handlerMappings = getDefaultStrategies(wac, HandlerMapping.class);
        }
    }

    private void initHandlerAdapters(WebApplicationContext wac) {
        Map<String, HandlerAdapter> map = wac.beansOfType(HandlerAdapter.class);
        if (!map.isEmpty()) {
            this.handlerAdapters = new ArrayList<HandlerAdapter>(map.values());
        }
        if (this.handlerAdapters == null) {
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default");
            }
            this.handlerAdapters = getDefaultStrategies(wac, HandlerAdapter.class);
        }
    }

    private void initViewResolvers(WebApplicationContext wac) {
        Map<String, ViewResolver> map = wac.beansOfType(ViewResolver.class);
        if (!map.isEmpty()) {
            this.viewResolvers = new ArrayList<ViewResolver>(map.values());
        }
        if (this.viewResolvers == null) {
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("No ViewResolvers found in servlet '" + getServletName() + "': using default");
            }
            this.viewResolvers = getDefaultStrategies(wac, ViewResolver.class);
        }
    }

    @Override
    protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {

        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("DispatcherServlet with name '" + getServletName() + "' received request for ["
                    + request.getRequestURI() + "]");
        }
        doDispatch(request, response);
    }

    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Bound request context to thread: " + request);
        }

        Object handler = getHandler(request);
        HandlerAdapter ha = getHandlerAdapter(handler);
        ModelAndView mv = ha.handle(request, response, handler);

        // Do we need view name translation?
        if (mv != null && !mv.hasView()) {
            mv.setViewName(getDefaultViewName(request));
        }

        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) {
            render(mv, request, response);
        }
    }

    protected <T> List<T> getDefaultStrategies(WebApplicationContext wac, Class<T> strategyInterface) {
        String key = strategyInterface.getName();
        List<T> strategies = new ArrayList<T>();
        String value = defaultStrategies.getProperty(key);
        if (value != null) {
            StringTokenizer token = new StringTokenizer(value, ",");
            while (token.hasMoreTokens()) {
                String className = token.nextToken();
                try {
                    Class<?> clazz = this.getClass().getClassLoader().loadClass(className);
                    strategies.add((T) wac.createBean(clazz));
                } catch (Exception e) {
                    LOGGER.error("Can't load class " + className + "", e);
                }
            }
        } else {
            strategies = Collections.emptyList();
        }
        return strategies;
    }

    protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
        View view = null;
        if (mv.isReference()) {
            // We need to resolve the view name.
            view = resolveViewName(mv.getViewName(), mv.getModelInternal(), request);
            if (view == null) {
                throw new ServletException("Could not resolve view with name '" + mv.getViewName()
                        + "' in servlet with name '" + getServletName() + "'");
            }
        } else {
            // No need to lookup: the ModelAndView object contains the actual
            // View object.
            view = mv.getView();
            if (view == null) {
                throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a "
                        + "View object in servlet with name '" + getServletName() + "'");
            }
        }
        // Delegate to the View object for rendering.
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
        }
        view.render(mv.getModelInternal(), request, response);
    }

    protected View resolveViewName(String viewName, Map<String, Object> model, HttpServletRequest request)
            throws Exception {
        for (Iterator<ViewResolver> it = this.viewResolvers.iterator(); it.hasNext();) {
            ViewResolver viewResolver = it.next();
            View view = viewResolver.resolveViewName(viewName);
            if (view != null) {
                return view;
            }
        }
        return null;
    }

    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        Iterator<HandlerAdapter> it = this.handlerAdapters.iterator();
        while (it.hasNext()) {
            HandlerAdapter ha = it.next();
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Testing handler adapter [" + ha + "]");
            }
            if (ha.supports(handler)) {
                return ha;
            }
        }
        throw new ServletException("No adapter for handler [" + handler
                + "]: Does your handler implement a supported interface like Controller?");
    }

    protected Object getHandler(HttpServletRequest request) throws Exception {
        Iterator<HandlerMapping> it = this.handlerMappings.iterator();
        while (it.hasNext()) {
            HandlerMapping hm = it.next();
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName()
                        + "'");
            }
            return hm.getHandler(request);
        }
        return null;
    }

    private String getDefaultViewName(HttpServletRequest request) {
        String url = request.getServletPath();
        url = url.replaceAll("/", "");
        url = url.replaceAll(".html", "");
        url = url.replaceAll(".htm", "");

        url = "WEB-INF/" + url + ".jsp";

        return url;
    }

}

 

初始化操作.

1. 检查系统中是否已经定义HandlerMapping。如果没有定义，则使用默认配置。
2. 检查系统中是否已经定义HandlerAdapter。如果没有定义，则使用默认配置。
3. 检查系统中是否已经定义ViewResolover。如果没有定义，则使用默认配置。

 

请求处理.

1. 根据特定的请求，使用HandlerMapping找到相应的控制器Handler。
2. 找到支持此种handler的HandlerAdapter，handler处理完响应业务后，HandlerAdapter把它转化为ModelAndView对象。
3. 利用ViewResolver对ModelAndView进行分析，生成相应的View对象。
4. 生成响应。

 

 

默认配置

com.google.mvc.web.servlet.HandlerMapping=com.google.mvc.web.servlet.handler.URLHandlerMapping
com.google.mvc.web.servlet.HandlerAdapter=com.google.mvc.web.servlet.mvc.HttpRequestHandlerAdapter,/
com.google.mvc.web.servlet.mvc.ControllerHandlerAdapter
com.google.mvc.web.servlet.ViewResolver=com.google.mvc.web.servlet.mvc.DefaultViewResolver

 

文章来源:http://blog.csdn.net/zhiqiangzhan/article/details/4767758