[Java知识库]在SpringMVC框架中，Servlet3.0 容器Tomcat中自动加载Filter和Listener的方式

1、前言：

在SpringMVC中，需要我们实现WebApplicationInitializer接口，来配置SpringMVC核心控制器DispatcherServlet。
Tomcat(Servlet3.0规范的web容器)启动时，会查找ServletContainerInitializer接口实现类

• => SpringServletContainerInitializer类(Spring提供)
• => WebApplicationInitializer接口实现类
• => AbstractContextLoaderInitializer抽象实现类(准备监听器的加载)
• =>AbstractDispatcherServletInitializer抽象实现类(准备过滤器的加载)
• =>AbstractAnnotationConfigDispatcherServletInitializer(准备 RootApplicationContext容器和ServletApplicationContext容器)
• => DispatcherServletConfig(当前类)
• 来完成DispatcherServlet核心控制器的配置

2、过滤器的加载：

而抽象实现类AbstractContextLoaderInitializer，有直接继承的抽象实现子类
AbstractDispatcherServletInitializer，该类完成了过滤器的加载工作。
在该类中，有onStartup方法，在此方法中调用了自己的父类的onStartup()方法，
准备了监听器的加载（详见3），更调用了本类的registerContextLoaderListener()方法完成了监听器加载。

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

	protected void registerDispatcherServlet(ServletContext servletContext) {
		String servletName = getServletName();
		Assert.hasLength(servletName, "getServletName() must not return null or empty");

		WebApplicationContext servletAppContext = createServletApplicationContext();
		Assert.notNull(servletAppContext, "createServletApplicationContext() must not return null");

		FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
		Assert.notNull(dispatcherServlet, "createDispatcherServlet(WebApplicationContext) must not return null");
		dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());

		ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
		if (registration == null) {
			throw new IllegalStateException("Failed to register servlet with name '" + servletName + "'. " +
					"Check if there is another servlet registered under the same name.");
		}

		registration.setLoadOnStartup(1);
		registration.addMapping(getServletMappings());
		registration.setAsyncSupported(isAsyncSupported());

		Filter[] filters = getServletFilters();
		if (!ObjectUtils.isEmpty(filters)) {
			for (Filter filter : filters) {
				registerServletFilter(servletContext, filter);
			}
		}

		customizeRegistration(registration);
	}

3、监听器的加载：

WebApplicationInitializer接口，有直接继承的抽象实现子类AbstractContextLoaderInitializer，其中有实例方法完成了监听器的加载工作。
在该类中，有onStartup方法，

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

该方法中，调用了registerContextLoaderListener()方法完成了监听器加载。

	protected void registerContextLoaderListener(ServletContext servletContext) {
		WebApplicationContext rootAppContext = createRootApplicationContext();
		if (rootAppContext != null) {
			ContextLoaderListener listener = new ContextLoaderListener(rootAppContext);
			listener.setContextInitializers(getRootApplicationContextInitializers());
			servletContext.addListener(listener);
		}
		else {
			logger.debug("No ContextLoaderListener registered, as " +
					"createRootApplicationContext() did not return an application context");
		}
	}