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Spring MVC 源码解析（二）— 容器初始化

1、什么是 `servlet` 容器

servlet 我们可以把它理解成是基于 java 语言的 web 组件，每一个 servlet 都有自己的生命周期包括 init() 、 service() 、 destroy() 而这些都是通过 servlet container 管理的。客户端通过servlet 容器实现的 request/response paradigm（请求/应答模式）与servlet进行交互。Servlet Container 是 Web 服务器或者应用服务器的一部分，用于提供基于请求/响应发送模式的网络服务，解码基于 MIME（全称是"Multipurpose Internet Mail Extensions"，中译为"多用途互联网邮件扩展"，指的是一系列的电子邮件技术规范）的请求，并且格式化基于 MIME 的响应。Servlet 容器可以嵌入到宿主的 Web 服务器中，或者通过 Web 服务器的本地扩展 API 单独作为附加组件安装。Servelt 容器也可能内嵌或安装到启用 Web 功能的应用服务器中。目前 Spring boot 就内嵌了 tomcat、jetty等web容器。

2、Servlet 容器初始化时 Spring Mvc 都做了什么

在web.xml的文件中可看到如下配置：

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

这个配置的意思是在 servlet 容器中增加一个监听器，当容器初始化的时候会抛出 ServletContextEvent 事件，监听器监听到该事件就会调用 ContextLoaderListener.contextInitialized(ServletContextEvent event) 方法来初始化 Spring mvc rootContext

/**
	 * Initialize the root web application context.
	 */
	@Override
	public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}

3、Spring Mvc 容器结构

Spring Mvc 的容器是分层的，当我们的 web 应用有多个 servlet 的时候，一些公共的资源（bean）就可以放在 root WebApplicationContext 中比如 dataSource 、 service 等，在 Spring Mvc 中每一 servlet 都有对应的属于自己的一个 servlet WebApplicationContext 这些 Controllers 、 ViewResolver 等就可以放到与之相关连的 WebapplicationContext 中：

当我们只有一个 servlet 时，当然可以把所以的 bean 都交由 root WebApplicationContext 来管理，这样 Spring Mvc

就会通过代理的形式生成一个空的与之对应的容器。

4、java 配置容器

servlet 3.0 以后支持用过 java 代码来配置容器， servlet 提供了一个接口：

public interface ServletContainerInitializer {

    /**
     * Receives notification during startup of a web application of the classes
     * within the web application that matched the criteria defined via the
     * {@link javax.servlet.annotation.HandlesTypes} annotation.
     *
     * @param c     The (possibly null) set of classes that met the specified
     *              criteria
     * @param ctx   The ServletContext of the web application in which the
     *              classes were discovered
     *
     * @throws ServletException If an error occurs
     */
    void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException;
}

servlet 容器在启动的时候回到 classpath 下扫描这个接口的实现。 spring Mvc 分装了一层代理实现了这个接口：

@Override
	public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {

		List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();

		if (webAppInitializerClasses != null) {
			for (Class<?> waiClass : webAppInitializerClasses) {
				// Be defensive: Some servlet containers provide us with invalid classes,
				// no matter what @HandlesTypes says...
				if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
						WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
					try {
						initializers.add((WebApplicationInitializer) waiClass.newInstance());
					}
					catch (Throwable ex) {
						throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
					}
				}
			}
		}

		if (initializers.isEmpty()) {
			servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
			return;
		}

		servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
		AnnotationAwareOrderComparator.sort(initializers);
		for (WebApplicationInitializer initializer : initializers) {
			initializer.onStartup(servletContext);
		}
	}

在实际使用中只要实现 Spring Mvc 为我们提供的接口即可;

public interface WebApplicationInitializer {

	/**
	 * Configure the given {@link ServletContext} with any servlets, filters, listeners
	 * context-params and attributes necessary for initializing this web application. See
	 * examples {@linkplain WebApplicationInitializer above}.
	 * @param servletContext the {@code ServletContext} to initialize
	 * @throws ServletException if any call against the given {@code ServletContext}
	 * throws a {@code ServletException}
	 */
	void onStartup(ServletContext servletContext) throws ServletException;

}

我们可以通过这个接口来完成servlet 容器以及 Spring Mvc 容器的初始化工作，这样做就可以将项目中的配置文件彻底消灭掉。

5、如何消灭配置文件

去除 web.xml

<web-app id="WebApp_ID" version="2.4"
         xmlns="http://java.sun.com/xml/ns/j2ee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
   http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd
http://java.sun.com/xml/ns/j2ee ">

    <servlet>
        <servlet-name>demo</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>/WEB-INF/spring-mvc-servlet.xml</param-value>
        </init-param>
    </servlet>

    <servlet-mapping>
        <servlet-name>demo</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

    <!-- 如果不想用默认的配置文件名，可以在这里指定. 核心文件名规则：xxx-servlet.xml，xxx是<servlet-name>的名字 -->
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/spring-mvc-servlet.xml</param-value>
    </context-param>
    <!-- Spring ContextLoaderListener -->
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <!-- Post请求中文乱码 -->
    <filter>
        <filter-name>CharacterEncoding</filter-name>
        <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
        <init-param>
            <param-name>encoding</param-name>
            <param-value>UTF-8</param-value>
        </init-param>
        <init-param>
            <param-name>forceEncoding</param-name>
            <param-value>true</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncoding</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
</web-app>

通过java 代码可以将其改造为：

public class DemoServletinitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {

        //初始化root WebApplicationContext
        AnnotationConfigWebApplicationContext rootContext = new AnnotationConfigWebApplicationContext();
        rootContext.register(DemoRootApplicationContextConfiguration.class);
        servletContext.addListener(new ContextLoaderListener(rootContext));

        //初始化 servlet WebApplicationContext
        AnnotationConfigWebApplicationContext webApplicationContext = new AnnotationConfigWebApplicationContext();
        webApplicationContext.register(DemoWebMvcConfiguration.class);

        //注册 servlet
        ServletRegistration.Dynamic registration = servletContext.addServlet("demo", new DispatcherServlet(webApplicationContext));
        registration.setLoadOnStartup(1);
        registration.addMapping("/");

        //注册 filter
        FilterRegistration.Dynamic characterEncoding = servletContext.addFilter("characterEncoding", CharacterEncodingFilter.class);
        characterEncoding.setInitParameter("encoding", "UTF-8");
        characterEncoding.setInitParameter("forceEncoding", "true");

    }
}

去除 xxx-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:context="http://www.springframework.org/schema/context"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
      http://www.springframework.org/schema/beans/spring-beans.xsd
      http://www.springframework.org/schema/context
      http://www.springframework.org/schema/context/spring-context.xsd
      http://www.springframework.org/schema/mvc
      http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <context:component-scan base-package="demo" />


    <mvc:annotation-driven />

    <!-- 启用注解配置 -->
    <context:annotation-config />

    <bean class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver">
        <property name="defaultViews">
            <list>
                <bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView">
                    <property name="extractValueFromSingleKeyModel" value="true" />
                </bean>
            </list>
        </property>
    </bean>

    <mvc:interceptors>
        <bean class="demo.DemoInterceptor"/>
    </mvc:interceptors>

</beans>

等价于java 配置： Root WebAppicationContext 配置

@Configuration
@ComponentScan("demo")
public class DemoRootApplicationContextConfiguration {

    @Bean
    public ContentNegotiatingViewResolver contentNegotiatingViewResolver() {
        ContentNegotiatingViewResolver viewResolver = new ContentNegotiatingViewResolver();
        viewResolver.setDefaultViews(Lists.<View>newArrayList(mappingJackson2JsonView()));
        return viewResolver;
    }

    @Bean
    public MappingJackson2JsonView mappingJackson2JsonView() {
        MappingJackson2JsonView jsonView = new MappingJackson2JsonView();
        jsonView.setExtractValueFromSingleKeyModel(true);
        return jsonView;
    }

    @Bean
    public DataSource dataSource() {
        // add data source config
        return null;
    }
}

servlet WebApplicationContext 配置

@ComponentScan("demo")
@Configuration
@EnableWebMvc
public class DemoWebMvcConfiguration extends WebMvcConfigurerAdapter {

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new DemoInterceptor());
    }

    @Bean
    public InternalResourceViewResolver internalResourceViewResolver() {
        InternalResourceViewResolver viewResolver = new InternalResourceViewResolver();
        viewResolver.setPrefix("/WEB-INF/jsp/");
        viewResolver.setSuffix(".jsp");
        return viewResolver;
    }
}

这样就可以将web.xml、xxx-servlet去除掉了，再也不用看到烦人的配置文件了。当然 spring Mvc 还提供了一些抽象类来简化配置工作，这里为了更方便的解释java 配置的过程所以没有直接使用。对这一部分有兴趣的同学可以自己查看 AbstractAnnotationConfigDispatcherServletInitializer 源码。

原文 https://juejin.im/post/5a3f7ece518825696f7e36fe

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