springboot DispatcherServlet

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Springboot中我们引入spring-boot-starter-web依赖后，web就自动配置好了，在web.xml的年代，我们需要在web.xml中手动配置DispatcherServlet，但是Springboot中不需要，Springboot是如何替我们做好这一切的呢?

    我们来看下DispatcherServletAutoConfiguration，这个类在Springboot-autoconfig包中，如下List-1

    List-1

@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
public DispatcherServlet dispatcherServlet() {
    DispatcherServlet dispatcherServlet = new DispatcherServlet();
    dispatcherServlet.setDispatchOptionsRequest(
            this.webMvcProperties.isDispatchOptionsRequest());
    dispatcherServlet.setDispatchTraceRequest(
            this.webMvcProperties.isDispatchTraceRequest());
    dispatcherServlet.setThrowExceptionIfNoHandlerFound(
            this.webMvcProperties.isThrowExceptionIfNoHandlerFound());
    return dispatcherServlet;
}

@Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
@ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
public DispatcherServletRegistrationBean dispatcherServletRegistration(
        DispatcherServlet dispatcherServlet) {
    DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(
            dispatcherServlet, this.serverProperties.getServlet().getPath());
    registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
    registration.setLoadOnStartup(
            this.webMvcProperties.getServlet().getLoadOnStartup());
    if (this.multipartConfig != null) {
        registration.setMultipartConfig(this.multipartConfig);
    }
    return registration;
}

1. 实例化DispatcherServlet，之后注册到Spring容器中。
2. 实例化DispatcherServletRegistrationBean，并将DispatcherServlet传入到构造方法法中，注册到Spring容器中。

       所以说，在Springboot中，有个DispatcherServlet的bean，我们可以写个单元测试验证从BeanFactory中获取DispatcherServlet这个bean，接下来看DispatcherServletRegistrationBean。

                   

                                                                                          图1

    如图1所示，DispatcherServletRegistrationBean继承了ServletContextInitializer——见List-2，其中onStartUp的参数ServletContext是Servlet里面的。

    RegistrationBean实现了ServletContextInitializer，之后调用register方法，这个是抽象方法，由子类DynamicRegistrationBean实现，DynamicRegistrationBean再将ServletContext用方法addRegistration传递给子类ServletRegistrationBean——见List-3，List-3中用addServlet方法加入的就是DispatcherServletRegistrationBean传递到父类ServletRegistrationBean中的。这样Springboot利用Servlet3.0+的特性，自动注册DispatcherServlet到ServletContext中。

    List-2

@FunctionalInterface
public interface ServletContextInitializer {

	/**
	 * Configure the given {@link ServletContext} with any servlets, filters, listeners
	 * context-params and attributes necessary for initialization.
	 * @param servletContext the {@code ServletContext} to initialize
	 * @throws ServletException if any call against the given {@code ServletContext}
	 * throws a {@code ServletException}
	 */
	void onStartup(ServletContext servletContext) throws ServletException;
}

    List-3

@Override
protected ServletRegistration.Dynamic addRegistration(String description,
        ServletContext servletContext) {
    String name = getServletName();
    logger.info("Servlet " + name + " mapped to " + this.urlMappings);
    return servletContext.addServlet(name, this.servlet);
}

    有个问题，实现了ServletContextInitializer的实例，什么时候会调用onStartup方法呢？来看ServletContextInitializerBeans，这个类在Springboot中，如List-4中:

1. 实例化的时候会从BeanFactory中获取所有的ServletContextInitializer——在getOrderedBeansOfType方法中，之后用addServletContextInitializerBean方法，将获取到的ServletContextInitializer类型的Bean，添加到属性initializers中。
2. 这个地方可以看到，实现了ServletContextInitializer的不止是Servlet类型的，还有Listener、Filter类型的，为什么呢，因为他们都需要动态添加到web容器中，即需要ServletContext。

    List-4

public class ServletContextInitializerBeans
		extends AbstractCollection<ServletContextInitializer> {

	private static final String DISPATCHER_SERVLET_NAME = "dispatcherServlet";

	private static final Log logger = LogFactory
			.getLog(ServletContextInitializerBeans.class);

	/**
	 * Seen bean instances or bean names.
	 */
	private final Set<Object> seen = new HashSet<>();

	private final MultiValueMap<Class<?>, ServletContextInitializer> initializers;

	private List<ServletContextInitializer> sortedList;

	public ServletContextInitializerBeans(ListableBeanFactory beanFactory) {
		this.initializers = new LinkedMultiValueMap<>();
		addServletContextInitializerBeans(beanFactory);
		addAdaptableBeans(beanFactory);
		List<ServletContextInitializer> sortedInitializers = this.initializers.values()
				.stream()
				.flatMap((value) -> value.stream()
						.sorted(AnnotationAwareOrderComparator.INSTANCE))
				.collect(Collectors.toList());
		this.sortedList = Collections.unmodifiableList(sortedInitializers);
	}

	private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {
		for (Entry<String, ServletContextInitializer> initializerBean : getOrderedBeansOfType(
				beanFactory, ServletContextInitializer.class)) {
			addServletContextInitializerBean(initializerBean.getKey(),
					initializerBean.getValue(), beanFactory);
		}
	}

	private void addServletContextInitializerBean(String beanName,
			ServletContextInitializer initializer, ListableBeanFactory beanFactory) {
		if (initializer instanceof ServletRegistrationBean) {
			Servlet source = ((ServletRegistrationBean<?>) initializer).getServlet();
			addServletContextInitializerBean(Servlet.class, beanName, initializer,
					beanFactory, source);
		}
		else if (initializer instanceof FilterRegistrationBean) {
			Filter source = ((FilterRegistrationBean<?>) initializer).getFilter();
			addServletContextInitializerBean(Filter.class, beanName, initializer,
					beanFactory, source);
		}
		else if (initializer instanceof DelegatingFilterProxyRegistrationBean) {
			String source = ((DelegatingFilterProxyRegistrationBean) initializer)
					.getTargetBeanName();
			addServletContextInitializerBean(Filter.class, beanName, initializer,
					beanFactory, source);
		}
		else if (initializer instanceof ServletListenerRegistrationBean) {
			EventListener source = ((ServletListenerRegistrationBean<?>) initializer)
					.getListener();
			addServletContextInitializerBean(EventListener.class, beanName, initializer,
					beanFactory, source);
		}
		else {
			addServletContextInitializerBean(ServletContextInitializer.class, beanName,
					initializer, beanFactory, initializer);
		}
	}
...

    接着引出一个问题，ServletContextInitializerBeans这个在哪被调用呢，在ServletWebServerApplicationContext中，在Servlet类型的Sprringboot Web应用中，ApplicationContext是AnnotationConfigServletWebServerApplicationContext，而ServletWebServerApplicationContext正是其父类。

    ServletWebServerApplicationContext中，方法onRefresh()-->createWebServer()-->getSelfInitializer()-->selfInitialize()-->getServletContextInitializerBeans()-->new ServletContextInitializerBeans(getBeanFactory())。

    ServletWebServerApplicationContext的onRefresh方法覆盖了AbstractApplicationContext的onRefresh方法，AbstractApplicationContext中，方法onRefresh被方法refresh调用。SpringApplication的run()-->refreshContext()-->refresh()-->AbstractApplicationContext的refresh()。

    通过上面的分析可以看出，Springboot利用SpringFramework的特性，将DispatcherServlet、Filter或者Listener通过ServletContextInitializer的ServletContext，添加到tomcat之类的web容器中，这些都发生在Springboot启动的过程中。