1: 原始的Spring mvc 项目是要使用web.xml 来把DispatchServlet注入到Tomcat中。

    2：Spring 3+后支持不使用xml配置，技术依赖Servlet3.0 SPI

    1. 新建Springmvc项目，引入依赖 ```xml org.springframework spring-webmvc 5.2.5.RELEASE
    org.apache.tomcat.embed tomcat-embed-core 8.5.43 org.apache.tomcat.embed tomcat-embed-jasper 8.5.34

    javax.servlet javax.servlet-api 4.0.0 provided 

    2. 2. 添加启动配置类  AppConfig
    3. ```java
    4. @Configuration
    5. @ComponentScan("com.mkb")
    6. public class AppConfig {
    7. }

    1. 添加内置tomcat

      1. public class TomcatApplication {
      3.  public static void run()throws Exception{
      4.      /**
      5.       * 这么启动tomcat不知道项目是web项目。所以不能执行MyWebApplicationInitializer 方法
      6.       */
      7.      Tomcat tomcat = new Tomcat();
      8.      tomcat.setPort(8080);
      9.      tomcat.addWebapp("/", "D:\\workspace\\webapp");
      10.      tomcat.start();
      11.      tomcat.getServer().await();
      12.  }
      13. }

    2. 启动主函数

      1. public class MyWebApplicationInitializer implements WebApplicationInitializer {
      2.  @Override
      3.  public void onStartup(ServletContext servletCxt) {
      4.      System.out.println("start");
      5.      // Load Spring web application configuration
      6.      AnnotationConfigWebApplicationContext ac = new AnnotationConfigWebApplicationContext();
      7.      ac.register(AppConfig.class);
      8.      ac.refresh();  //初始化Spring容器
      10.      // Create and register the DispatcherServlet
      11.      DispatcherServlet servlet = new DispatcherServlet(ac);
      12.      ServletRegistration.Dynamic registration = servletCxt.addServlet("app", servlet);
      13.      registration.setLoadOnStartup(1);
      14.      registration.addMapping("/*");
      15.  }
      16. }

    3. 测试

      1. @RestController
      2. public class IndexController {
      4.  @RequestMapping("/test")
      5.  public String index1(){
      6.      return "1";
      7.  }
      8. }

    总结：Spring 3.0后支持0配置，为后期springboot的演变做了铺垫。

    在Spring-web jar 下面， META-INF\services目录下会有一个javax.servlet.ServletContainerInitializer 类，按照SPI的规范，就是Tomcat的启动的时候 会加载这个目录下的这个配置。

    1. @HandlesTypes({WebApplicationInitializer.class})
    2. public class SpringServletContainerInitializer implements ServletContainerInitializer {
    3.     public SpringServletContainerInitializer() {
    4.     }
    6.     public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException {
    7.         List<WebApplicationInitializer> initializers = new LinkedList();
    8.         Iterator var4;
    9.         if (webAppInitializerClasses != null) {
    10.             var4 = webAppInitializerClasses.iterator();
    12.             while(var4.hasNext()) {
    13.                 Class<?> waiClass = (Class)var4.next();
    14.                 if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
    15.                     try {
    16.                         initializers.add((WebApplicationInitializer)ReflectionUtils.accessibleConstructor(waiClass, new Class[0]).newInstance());
    17.                     } catch (Throwable var7) {
    18.                         throw new ServletException("Failed to instantiate WebApplicationInitializer class", var7);
    19.                     }
    20.                 }
    21.             }
    22.         }
    24.         if (initializers.isEmpty()) {
    25.             servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
    26.         } else {
    27.             servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
    28.             AnnotationAwareOrderComparator.sort(initializers);
    29.             var4 = initializers.iterator();
    31.             while(var4.hasNext()) {
    32.                 WebApplicationInitializer initializer = (WebApplicationInitializer)var4.next();
    33.                 initializer.onStartup(servletContext);
    34.             }
    36.         }
    37.     }
    38. }

    @HandlesTypes({WebApplicationInitializer.class})
    就是说，凡是实现 WebApplicationInitializer 接口的类 都会被执行，我们的项目中 MyWebApplicationInitializer 实现了 WebApplicationInitializer 接口，所以在Tomcat启动的时候会加载MyWebApplicationInitializer，这个方法中，实现了Spring容器的初始化，和DispatcherServlet 的初始化。