How to implement Springboot automatic assembly by injecting DispatcherServlet

Principle Overview

Springboot provides web services to the outside world, and the bottom layer relies on the web module in springframework (including but not limited to spring mvc core class DispatcherServlet) to achieve

So when does springboot inject the core class DispatcherServlet into the container?
The injection process still follows the automatic assembly process, and the support for automatic assembly is provided by default in the springboot framework.

In There is an org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration configuration in the spring.factories file in the jar package

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {
	/*
	 * The bean name for a DispatcherServlet that will be mapped to the root URL "/"
	 */
	public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";
	/*
	 * The bean name for a ServletRegistrationBean for the DispatcherServlet "/"
	 *
	public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";
	// ... 省略代码
	}

There are two internal classes in DispatcherServletAutoConfiguration ( Configuration class) DispatcherServletConfiguration, DispatcherServletRegistrationConfiguration and two corresponding Condition classes DefaultDispatcherServletCondition, DispatcherServletRegistrationCondition

DispatcherServletConfiguration

DispatcherServletConfiguration is an internal class of DispatcherServletAutoConfiguration and is annotated with @Configuration and will be automatically scanned by the container; It has two methods: 1. dispatcherServlet; 2. multipartResolver

	@Configuration(proxyBeanMethods = false)
	@Conditional(DefaultDispatcherServletCondition.class)
	@ConditionalOnClass(ServletRegistration.class)
	@EnableConfigurationProperties({ HttpProperties.class, WebMvcProperties.class })
	protected static class DispatcherServletConfiguration {

// @A
		@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
		public DispatcherServlet dispatcherServlet(HttpProperties httpProperties, WebMvcProperties webMvcProperties) {
			DispatcherServlet dispatcherServlet = new DispatcherServlet();
			dispatcherServlet.setDispatchOptionsRequest(webMvcProperties.isDispatchOptionsRequest());
			dispatcherServlet.setDispatchTraceRequest(webMvcProperties.isDispatchTraceRequest());
			dispatcherServlet.setThrowExceptionIfNoHandlerFound(webMvcProperties.isThrowExceptionIfNoHandlerFound());
			dispatcherServlet.setPublishEvents(webMvcProperties.isPublishRequestHandledEvents());
			dispatcherServlet.setEnableLoggingRequestDetails(httpProperties.isLogRequestDetails());
			return dispatcherServlet;
		}

// @B
		@Bean
		@ConditionalOnBean(MultipartResolver.class)
		@ConditionalOnMissingBean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
		public MultipartResolver multipartResolver(MultipartResolver resolver) {
			// Detect if the user has created a MultipartResolver but named it incorrectly
			return resolver;
		}
	}

@A：

• @Configuration(proxyBeanMethods = false): The configuration class adopts Lite mode

• @Conditional(DefaultDispatcherServletCondition.class): Calculate whether to inject based on the return value of the DefaultDispatcherServletCondition class. The main logic is to check whether there is already a DispatcherServlet named "dispatcherServlet" in the Spring container

• @ConditionalOnClass(ServletRegistration.class): There must be a ServletRegistration class in the container

• @EnableConfigurationProperties({ HttpProperties.class, WebMvcProperties.class }): Loading HttpProperties and WebMvcProperties

@B: Inject MultipartResolver parsing class

DispatcherServletRegistrationConfiguration

The injected logic of this class is the same as DispatcherServletConfiguration, but it is annotated on the class @Import(DispatcherServletConfiguration.class), then the two class definitions have a sequence

@Configuration(proxyBeanMethods = false)
	@Conditional(DispatcherServletRegistrationCondition.class)
	@ConditionalOnClass(ServletRegistration.class)
	@EnableConfigurationProperties(WebMvcProperties.class)
	@Import(DispatcherServletConfiguration.class) //@A
	protected static class DispatcherServletRegistrationConfiguration {

		@Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
		@ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
		public DispatcherServletRegistrationBean dispatcherServletRegistration(DispatcherServlet dispatcherServlet,
				WebMvcProperties webMvcProperties, ObjectProvider<MultipartConfigElement> multipartConfig) {
			DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(dispatcherServlet,
					webMvcProperties.getServlet().getPath());
			// 设置名称		
			registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
			registration.setLoadOnStartup(webMvcProperties.getServlet().getLoadOnStartup());
			multipartConfig.ifAvailable(registration::setMultipartConfig);
			return registration;
		}
	}

Among them, DispatcherServletRegistrationBean inherits from ServletRegistrationBean and mainly provides services for DispatcherServlet. Both DispatcherServletRegistrationBean and DispatcherServlet provide the function of registering Servlet and exposing DispatcherServletPath information

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