Spring Boot中Spring MVC自动配置原理分析

​ 了解Spring Boot启动原理的童鞋都知道Spring Boot程序在启动时，会加载classpath中的spring.factories文件,进行组件自动装配,那么Spring MVC是怎么被Spring Boot程序启动的,本文就对此进行简要分析。

​ 在spring.factories文件中可以找到DispatcherServerlet类的自动配置类DispatcherServletAutoConfiguration

···

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)//指定该配置类的加载顺序
@Configuration(proxyBeanMethods = false)//声明该类为配置类
@ConditionalOnWebApplication(type = Type.SERVLET)//指定配置类加载条件为Servlet
@ConditionalOnClass(DispatcherServlet.class)//指定classpath中存在DispatcherServlet时才会加载
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)//指定该配置类在ServletWebServer装配完成后进行
public class DispatcherServletAutoConfiguration {

	
    //DispatcherServlet默认的bean名称
	public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";

	
     //DispatcherServletRegistration默认的bean名称
	public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";

	@Configuration(proxyBeanMethods = false)
	@Conditional(DefaultDispatcherServletCondition.class)
	@ConditionalOnClass(ServletRegistration.class)
	@EnableConfigurationProperties(WebMvcProperties.class)
	protected static class DispatcherServletConfiguration {
        //配置DispatcherServlet
		@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
		public DispatcherServlet dispatcherServlet(WebMvcProperties webMvcProperties) {
			DispatcherServlet dispatcherServlet = new DispatcherServlet();
//设置配置DispatcherServlet参数
dispatcherServlet.setDispatchOptionsRequest(webMvcProperties.isDispatchOptionsRequest());
			dispatcherServlet.setDispatchTraceRequest(webMvcProperties.isDispatchTraceRequest());
			dispatcherServlet.setThrowExceptionIfNoHandlerFound(webMvcProperties.isThrowExceptionIfNoHandlerFound());
			dispatcherServlet.setPublishEvents(webMvcProperties.isPublishRequestHandledEvents());
			dispatcherServlet.setEnableLoggingRequestDetails(webMvcProperties.isLogRequestDetails());
			return dispatcherServlet;
		}

        //修改自己配置的文件上传解析器名称（如果配置过）
		@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;
		}

	}

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

        //配置dispatcherServletRegistrationBean
		@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 multipartConfig) {
			DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(dispatcherServlet,
					webMvcProperties.getServlet().getPath());
			registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
			registration.setLoadOnStartup(webMvcProperties.getServlet().getLoadOnStartup());
			multipartConfig.ifAvailable(registration::setMultipartConfig);
			return registration;
		}

	}

	其他代码略...
}

DispatcherServletRegistrationBean的作用是自动配置DispatcherServlet，该类是ServletContextInitializer的实现类。

Servlet容器在创建时，会调用IOC容器中的所有ServletContextInitializer对象的onStartup方法。

public abstract class RegistrationBean implements ServletContextInitializer, Ordered {

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

	private int order = Ordered.LOWEST_PRECEDENCE;

	private boolean enabled = true;

	@Override
	public final void onStartup(ServletContext servletContext) throws ServletException {
		String description = getDescription();
		if (!isEnabled()) {
			logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
			return;
		}
        //向ServletContext中注册Servlet
		register(description, servletContext);
	}
	代码略...

}

最终调用ServletRegistrationBean的addRegistration添加dispatcherServlet到servletContext中

public class ServletRegistrationBean extends DynamicRegistrationBean {
    ....
	@Override
	protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
		String name = getServletName();
        //将dispatcherServlet添加到servletContext中
		return servletContext.addServlet(name, this.servlet);
	}
    ....
}

启动Servlet容器时，容器会创建自动启动的Servlet。

public void start() throws WebServerException {
		synchronized (this.monitor) {
			if (this.started) {
				return;
			}
			try {
				addPreviouslyRemovedConnectors();
				Connector connector = this.tomcat.getConnector();
				if (connector != null && this.autoStart) {
                    //启动需要自启动的Servlet
					performDeferredLoadOnStartup();
				}
				checkThatConnectorsHaveStarted();
				this.started = true;
				logger.info("Tomcat started on port(s): " + getPortsDescription(true) + " with context path '"
						+ getContextPath() + "'");
			}
			catch (ConnectorStartFailedException ex) {
				stopSilently();
				throw ex;
			}
			catch (Exception ex) {
				PortInUseException.throwIfPortBindingException(ex, () -> this.tomcat.getConnector().getPort());
				throw new WebServerException("Unable to start embedded Tomcat server", ex);
			}
			finally {
				Context context = findContext();
				ContextBindings.unbindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
			}
		}
	}