我们平时用 Java 注解很多，例如 @Configuration、@Component、@Service，我们习惯于通过 XML 方式来实现 Web，而用 Java 注解方式来实现 Web 却很少。今天来说下，Spring 是怎么通过 Java 注解方式来实现 Web 和 Mvc。
一、实现 WebApplicationInitializer 接口
public class WebInitializer implements WebApplicationInitializer{
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
}
}
二、继承 AbstractAnnotationConfigDispatcherServletInitializer
public class WebMvcInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
}
三、容器启动
这里必须说下容器，容器就是运行程序的器件。像我们最熟悉的 Tomcat，Jetty 等等，要让容器知道怎么运行你的程序，那么你的程序必须符合容器的规范，或者说遵循规矩。
Tomcat 是怎么做到的呢？我们不用 web.xml 来配置 Servlet，我们就用 Java 代码，继承 AbstractAnnotationConfigDispatcherServletInitializer 就行了。怎么做到的？
这里就不去分析 catalina 启动脚本，后面单独分析。
我们只说下 Spring 代码层面的逻辑，首先 javax.servlet.ServletContainerInitializer 这个接口类。这个类的存在就是为了实现 Java 代码配置。而且是 Servlet 3.0 版本才提供，低于 3.0 都没法用
Java 代码实现配置。看看源代码
package javax.servlet;
public interface ServletContainerInitializer {
public void onStartup(Set<Class<?>> c, ServletContext ctx)
throws ServletException;
}
其中有这样一段注解：
* Interface which allows a library /runtime to be notified of a web
* application's startup phase and perform any required programmatic
* registration of servlets, filters, and listeners in response to it.
*
* <p>Implementations of this interface may be annotated with
* {@link javax.servlet.annotation.HandlesTypes HandlesTypes}, in order to
* receive (at their {@link #onStartup} method) the Set of application
* classes that implement, extend, or have been annotated with the class
* types specified by the annotation.
*
* <p>If an implementation of this interface does not use this annotation,
* or none of the application classes match the ones specified
* by the annotation, the container must pass a <tt>null</tt> Set of classes
* to {@link #onStartup}.
*
* <p>Implementations of this interface must be declared by a JAR file
* resource located inside the <tt>META-INF/services</tt> directory and
* named for the fully qualified class name of this interface, and will be
* discovered using the runtime's service provider lookup mechanism
* or a container specific mechanism that is semantically equivalent to
* it. In either case, ServletContainerInitializer services from web
* fragment JAR files excluded from an absolute ordering must be ignored,
* and the order in which these services are discovered must follow the
* application's classloading delegation model.
意思是，这个接口是为了程序启动时，实现 Servlet，Filter，Listener 注册的，但是实现这个接口的类必须在一个 Jar包 的 META-INF/services 目录下面的 javax.servlet.ServletContainerInitializer 文件中有定义。而且实现类必须有 @HandlesTypes 注解。什么意思？
@HandlesTypes
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface HandlesTypes {
Class[] value();
}我们来看看，Spring-web-xxx.jar 包下面的 META-INF/services 文件夹，就有 javax.servlet.ServletContainerInitializer 这么一个文件，里面内容是 org.springframework.web.SpringServletContainerInitializer。这里可能需要了解JDK SPI机制，SPI全称为(Service Provider Interface) ，是JDK内置的一种服务提供发现机制。后面文章单独分析。也就是说，启动容器之后，会调用org.springframework.web.SpringServletContainerInitializer中的onStartup方法。
那么 org.springframework.web.SpringServletContainerInitializer 这个类又是什么样？来看看
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
@Override
public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
throws ServletException {
List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();
if (webAppInitializerClasses != null) {
for (Class<?> waiClass : webAppInitializerClasses) {
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);
}
}
}有这么一段注解：
* Delegate the {@code ServletContext} to any {@link WebApplicationInitializer}
* implementations present on the application classpath.
* <p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},
* Servlet 3.0+ containers will automatically scan the classpath for implementations
* of Spring's {@code WebApplicationInitializer} interface and provide the set of all
* such types to the {@code webAppInitializerClasses} parameter of this method.
* <p>If no {@code WebApplicationInitializer} implementations are found on the classpath,
* this method is effectively a no-op. An INFO-level log message will be issued notifying
* the user that the {@code ServletContainerInitializer} has indeed been invoked but that
* no {@code WebApplicationInitializer} implementations were found.
* <p>Assuming that one or more {@code WebApplicationInitializer} types are detected,
* they will be instantiated (and <em>sorted</em> if the @{@link
* org.springframework.core.annotation.Order @Order} annotation is present or
* the {@link org.springframework.core.Ordered Ordered} interface has been
* implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)}
* method will be invoked on each instance, delegating the {@code ServletContext} such
* that each instance may register and configure servlets such as Spring's
* {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener},
* or any other Servlet API componentry such as filters.
* @param webAppInitializerClasses all implementations of
* {@link WebApplicationInitializer} found on the application classpath
* @param servletContext the servlet context to be initialized
* @see WebApplicationInitializer#onStartup(ServletContext)
* @see AnnotationAwareOrderComparator原来如此，Servlet 3.0+ 容器会自动去扫描 classpath 下所有实现了 WebApplicationInitializer 接口的类，如果没有找到相关类，那么 An INFO-level log message will be issued notifying the user，会有一条INFO级别 的日志。
如果找到了多个实现类，那么都会被实例化，如果实现了 org.springframework.core.Ordered 接口或者添加了 @Order注解，那么就按照顺序来。
（1）org.springframework.core.Ordered
public interface Ordered {
/**
* Useful constant for the highest precedence value.
* @see java.lang.Integer#MIN_VALUE
*/
int HIGHEST_PRECEDENCE = Integer.MIN_VALUE;
/**
* Useful constant for the lowest precedence value.
* @see java.lang.Integer#MAX_VALUE
*/
int LOWEST_PRECEDENCE = Integer.MAX_VALUE;
int getOrder();
}
（2）@Order
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
@Documented
public @interface Order {
/**
* The order value.
* <p>Default is {@link Ordered#LOWEST_PRECEDENCE}.
* @see Ordered#getOrder()
*/
int value() default Ordered.LOWEST_PRECEDENCE;
}
为什么 @MapperScan 和 @ComponentScan 会被调用？到这里就很清晰了，<mybatis:scan> 和 <context:component-scan> 标签的调用就很简单了，因为我们在 xml 里面配置了。
