[Java知识库]Tomcat启动时，加载Spring容器的过程(基于JavaConfig配置形式)

1.根据servlet3.0约定，提供Web服务的框架，必须实现ServletContainerInitializer接口，重写onStartUp方法。Spring根据约定，提供了ServletContainerInitializer接口的子接口SpringServletContainerInitializer。
2.在Spring根据SpringServletContainerInitializer接口的实现类在配置Web环境时，又向我们甩出了WebApplicationInitializer接口，用于我们做具体的配置（SpringMVC，Spring的核心配置，以及监听器，过滤器等等）。在WebApplicationInitializer接口下又提供了AbstractContextLoaderInitializer抽象子类，主要用于在服务器启动时注册ContextLoaderListener监听器，用于加载关于Spring核心配置相关的配置类信息，创建ROOT WebApplicationContext容器。，具体代码如下(AbstractContextLoaderInitializer类部分源码)：

public abstract class AbstractContextLoaderInitializer implements WebApplicationInitializer {

	/** Logger available to subclasses. */
	protected final Log logger = LogFactory.getLog(getClass());


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

3.在AbstractContextLoaderInitializer抽象子类下又提供了AbstractDispatcherServletInitializer抽象子类。在AbstractDispatcherServletInitializer类的onStartUp方法中主要提供了DispatcherServlet（核心控制器）的创建方法（用于创建Servlet WebApplicationContext容器—SpringMVC核心配置容器），以及过滤器的注入。

public abstract class AbstractDispatcherServletInitializer extends AbstractContextLoaderInitializer {

	/**
	 * The default servlet name. Can be customized by overriding {@link #getServletName}.
	 */
	public static final String DEFAULT_SERVLET_NAME = "dispatcher";


	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		super.onStartup(servletContext);
		registerDispatcherServlet(servletContext);
	}
}

4.针对Root WebApplicationContext与Servlet WebApplicationContext两个容器的具体配置信息的提供，spring又向我们提供了AbstractAnnotationConfigDispatcherServletInitializer，即我们实现SpringMVCP配置类的需要继承实现的最终抽象类，在AbstractAnnotationConfigDispatcherServletInitializer类中主要提供了我们SpringMVC配置(getServletConfigClasses)，Spring配置(getRootConfigClasses)，Dispatcher映射关系(getServletMappings)的配置信息的提交方法。

public abstract class AbstractAnnotationConfigDispatcherServletInitializer
		extends AbstractDispatcherServletInitializer {

	/**
	 * {@inheritDoc}
	 * <p>This implementation creates an {@link AnnotationConfigWebApplicationContext},
	 * providing it the annotated classes returned by {@link #getRootConfigClasses()}.
	 * Returns {@code null} if {@link #getRootConfigClasses()} returns {@code null}.
	 */
	@Override
	@Nullable
	protected WebApplicationContext createRootApplicationContext() {
		Class<?>[] configClasses = getRootConfigClasses();
		if (!ObjectUtils.isEmpty(configClasses)) {
			AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
			context.register(configClasses);
			return context;
		}
		else {
			return null;
		}
	}

	/**
	 * {@inheritDoc}
	 * <p>This implementation creates an {@link AnnotationConfigWebApplicationContext},
	 * providing it the annotated classes returned by {@link #getServletConfigClasses()}.
	 */
	@Override
	protected WebApplicationContext createServletApplicationContext() {
		AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
		Class<?>[] configClasses = getServletConfigClasses();
		if (!ObjectUtils.isEmpty(configClasses)) {
			context.register(configClasses);
		}
		return context;
	}

	/**
	 * Specify {@code @Configuration} and/or {@code @Component} classes for the
	 * {@linkplain #createRootApplicationContext() root application context}.
	 * @return the configuration for the root application context, or {@code null}
	 * if creation and registration of a root context is not desired
	 */
	@Nullable
	protected abstract Class<?>[] getRootConfigClasses();

	/**
	 * Specify {@code @Configuration} and/or {@code @Component} classes for the
	 * {@linkplain #createServletApplicationContext() Servlet application context}.
	 * @return the configuration for the Servlet application context, or
	 * {@code null} if all configuration is specified through root config classes.
	 */
	@Nullable
	protected abstract Class<?>[] getServletConfigClasses();

}

5.综上：在相应方法中提交相应配置信息后，Tomcat服务器在启动时，会追层调用各个子类的onStartUp()方法，先完成Root WebApplicationContext容器的创建及配置，再完成Servlet WebApplicationContext容器的创建及配置。