菜鸟之路——Spring MVC（二）再次深入

最新推荐文章于 2021-02-01 20:00:44 发布

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本文详细介绍了Spring MVC的核心组件，包括DispatcherServlet、HandlerMapping、HandlerAdapter、Controller、HandlerInterceptor和ViewResolver。同时，阐述了Spring MVC的启动过程，涉及ContextLoaderListener和DispatcherServlet的初始化。接着，解析了请求处理流程，展示了从客户端请求到返回响应的过程中，各组件如何协同工作。最后，提供了一个配置实例，包括目录结构和相关配置文件。

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Spring MVC 的核心类与接口：

- DispatcherServlet 前置控制器

- HandlerMapping 请求映射（到Controller）

HandlerMapping接口的实现类：
SimpleUrlHandlerMapping 通过配置文件，把一个URL映射到Controller
DefaultAnnotationHandlerMapping 通过注解，把一个URL映射到Controller类上

- HandlerAdapter 请求映射（到Controller类的方法上）

AnnotationMethodHandlerAdapter类，通过注解，把一个URL映射到Controller类的方法上

- Controller 控制器

由于我们使用了@Controller注解，添加了@Controller注解注解的类就可以担任控制器（Action）的职责,
所以我们并没有用到这个接口。
- HandlerIntercepter 拦截器

需要自己实现这个接口，来完成拦截的器的工作。
- ViewResolver 视图映射

UrlBasedViewResolver类通过配置文件，把一个视图名交给到一个View来处理
InternalResourceViewResolver类，比上面的类，加入了JSTL的支持

- View 视图处理

启动过程

Spring MVC启动过程大致分为两个过程：
- ContextLoaderListener初始化，读取context-param中的contextConfigLocation指定的配置文件，创建ROOT Context，通过调用继承自ContextLoader的initWebApplicationContext方法实例化Spring IoC容器，并将此容器实例注册到ServletContext中。
- ContextLoaderListener初始化完毕后，开始初始化web.xml中配置的DispatcherServlet，DispatcherServlet的初始化又包括了视图管理器、异常处理器、映射管理等等。

/**
     * Initialize the strategy objects that this servlet uses.
     * <p>May be overridden in subclasses in order to initialize further strategy objects.
     */
    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        initHandlerMappings(context);
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }

ContextLoaderListener初始化的是 WebApplicationContext, 创建后可以从ServletContext中获取，WebApplicationContext是应用程序内共享的，最多只有一个，如果寻求简单也可以不初始化此容器。与之不同 DispatcherServlet可以有多个，并共享一个WebApplicationContext容器，每一个DispatcherServlet有不同的配置，控制不同的WEB访问。一般将 DAO、Service 层Bean共享的放在ContextLoaderListener配置的容器中，将WEB层的Bean放在特定的DispatcherServlet配置的容器中。

SpringMVC利用Spring的注入特性初始化资源文件，只需要调用setPropertyValues方法就可将contextConfigLocation属性设置到对应实例中，也就是以依赖注入的方式初始化属性。

时序图如下：

请求处理流程

官网上的图

涉及到核心类与接口的过程描述：

客户端浏览器发送http请求，被`DispatcherServlet`捕获，调用关键的doDispatch方法，遍历所有注册为`Controller`的bean，为请求寻找关联映射，其中遍历查找的函数getHandler和getHandlerAdapter的源码：

/**
     * Return the HandlerExecutionChain for this request.
     * <p>Tries all handler mappings in order.
     * @param request current HTTP request
     * @return the HandlerExecutionChain, or {@code null} if no handler could be found
     */
    protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        for (HandlerMapping hm : this.handlerMappings) {
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
            }
            HandlerExecutionChain handler = hm.getHandler(request);
            if (handler != null) {
                return handler;
            }
        }
        return null;
    }
    
    
    /**
     * Return the HandlerAdapter for this handler object.
     * @param handler the handler object to find an adapter for
     * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
     */
    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        for (HandlerAdapter ha : this.handlerAdapters) {
            if (logger.isTraceEnabled()) {
                logger.trace("Testing handler adapter [" + ha + "]");
            }
            if (ha.supports(handler)) {
                return ha;
            }
        }
        throw new ServletException("No adapter for handler [" + handler +
                "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    }

找到匹配的映射后`HandlerAdapter`会依次调用preHandle、handle（返回ModelAndView）、postHandle方法，所有步骤完成后调用processDispatchResult函数处理结果，并返回View给客户端。postDispatchResult函数和其中调用的render函数源码如下：

/**
     * Handle the result of handler selection and handler invocation, which is
     * either a ModelAndView or an Exception to be resolved to a ModelAndView.
     */
    private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
            HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

        boolean errorView = false;

        if (exception != null) {
            if (exception instanceof ModelAndViewDefiningException) {
                logger.debug("ModelAndViewDefiningException encountered", exception);
                mv = ((ModelAndViewDefiningException) exception).getModelAndView();
            }
            else {
                Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                mv = processHandlerException(request, response, handler, exception);
                errorView = (mv != null);
            }
        }

        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) {
            render(mv, request, response);
            if (errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        }
        else {
            if (logger.isDebugEnabled()) {
                logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                        "': assuming HandlerAdapter completed request handling");
            }
        }

        if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Concurrent handling started during a forward
            return;
        }

        if (mappedHandler != null) {
            mappedHandler.triggerAfterCompletion(request, response, null);
        }
    }


    /**
     * Render the given ModelAndView.
     * <p>This is the last stage in handling a request. It may involve resolving the view by name.
     * @param mv the ModelAndView to render
     * @param request current HTTP servlet request
     * @param response current HTTP servlet response
     * @throws ServletException if view is missing or cannot be resolved
     * @throws Exception if there's a problem rendering the view
     */
    protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
        // Determine locale for request and apply it to the response.
        Locale locale = this.localeResolver.resolveLocale(request);
        response.setLocale(locale);

        View view;
        if (mv.isReference()) {
            // We need to resolve the view name.
            view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, 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() + "'");
        }
        try {
            if (mv.getStatus() != null) {
                response.setStatus(mv.getStatus().value());
            }
            view.render(mv.getModelInternal(), request, response);
        }
        catch (Exception ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
                        getServletName() + "'", ex);
            }
            throw ex;
        }
    }

这就是一个完整的处理http请求的过程：

时序图如下

配置实例

这里放的是最简单的配置，可以通过这个简单的配置实例回顾一下上面的过程。

目录结构

-SpringMVCDemo
-src
-me.cyan
-WelcomeController
-web
-WEB-INF
-applicationContext.xml
-dispatcher-servlet.xml
-web.xml
-index.jsp
-pom.xml

pom.xml

引入的包

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>me.cyan</groupId>
    <artifactId>SpringMVCDemo</artifactId>
    <name>SpringMVCDemo</name>
    <packaging>war</packaging>
    <version>1.0.0</version>

    <properties>
        <spring-version>4.2.6.RELEASE</spring-version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>${spring-version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>${spring-version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
            <version>${spring-version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-beans</artifactId>
            <version>${spring-version}</version>
        </dependency>
        <dependency>
            <groupId>commons-logging</groupId>
            <artifactId>commons-logging</artifactId>
            <version>1.2</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>${spring-version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-aop</artifactId>
            <version>${spring-version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-expression</artifactId>
            <version>${spring-version}</version>
        </dependency>
    </dependencies>
</project>

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
         
    <!--配置文件路径-->
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/applicationContext.xml</param-value>
    </context-param>
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
    
    <!--SpringMVC核心servlet-->
    <servlet>
        <servlet-name>dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
</web-app>

dispatcher-servlet.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:mvc="http://www.springframework.org/schema/mvc"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">

    <!-- 默认的注解映射的支持 -->
    <mvc:annotation-driven />

    <!-- 自动扫描的包名 -->
    <context:component-scan base-package="me.cyan" />

</beans>

WelcomeController

package me.cyan;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

/**
 * Created by cyan on 16/5/23.
 */

@Controller
public class welcomeController {

    @RequestMapping("/hello")
    public @ResponseBody String sayhello(){
        return "hello Spring MVC!";
    }
}