聊聊、Spring ServletContainerInitializer

  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  

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public class WebInitializer implements WebApplicationInitializer{
@Override
public void onStartup(ServletContext servletContext) throws ServletException {}
}

二、继承 AbstractAnnotationConfigDispatcherServletInitializer 

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 public class WebMvcInitializer  extends AbstractAnnotationConfigDispatcherServletInitializer {}

三、容器启动 

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  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。

  下一篇来聊聊，这些注解和标签被调用了，怎么识别的呢？我怎么知道要去扫描这个包下面的类？哪里有定义吗？