先贴总结

1. 实现WebMvcConfigurer： 不会覆盖WebMvcAutoConfiguration的配置
2. 实现WebMvcConfigurer+注解@EnableWebMvc：会覆盖WebMvcAutoConfiguration的配置
3. 继承WebMvcConfigurationSupport：会覆盖WebMvcAutoConfiguration的配置
4. 继承DelegatingWebMvcConfiguration：会覆盖WebMvcAutoConfiguration的配置

源码分析

注解-@EnableWebMVC

源码

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DelegatingWebMvcConfiguration.class)
public @interface EnableWebMvc {
}

意思就是说：如果引用了@EnableWebMVC注解，就会往spring容器中注入了一个DelegatingWebMvcConfiguration来统一管理所有的配置类

@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
	private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();
	...
}

注解-@SpringBootApplication

源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM,
				classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
	@AliasFor(annotation = EnableAutoConfiguration.class)
	Class<?>[] exclude() default {};
	@AliasFor(annotation = EnableAutoConfiguration.class)
	String[] excludeName() default {};
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
	String[] scanBasePackages() default {};
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
	Class<?>[] scanBasePackageClasses() default {};
}

这里引用了@EnableAutoConfiguration注解

注解-@EnableAutoConfiguration

源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
	Class<?>[] exclude() default {};
	String[] excludeName() default {};

}

注解@AutoConfigurationPackage的主要作用就是：将主程序类所在包及所有子包下的组件到扫描到spring容器中。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
}

AutoConfigurationImportSelector的作用可以参考：springboot源码自动装配-AutoConfigurationImportSelector

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
		ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
	...
	@Override
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		// 是否启用自动配置
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		// 获取META-INF/spring-autoconfigure-metadata.properties文件的配置，返回AutoConfigurationMetadata类
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
		// 获取自动配置类，返回AutoConfigurationEntry
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
				autoConfigurationMetadata, annotationMetadata);
		// 返回要自动配置的类名
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}
	...
}

意思就是：如果引用了@EnableAutoConfiguration注解，就会往spring容器中注入两个类

1. AutoConfigurationPackages.Registrar：扫包
2. AutoConfigurationImportSelector：等所有类全加载到spring容器之后扫描配置类

配置类-WebMvcAutoConfiguration

源码

@Configuration
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class,
		TaskExecutionAutoConfiguration.class, ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {
	...
}

！！！！！！敲黑板！！！！！！敲黑板！！！！！！敲黑板！！！！！！

• @ConditionalOnMissingBean({WebMvcConfigurationSupport.class})，当Spring容器中不存在WebMvcConfigurationSupportbean，WebMvcAutoConfiguration才会注入
• 如果有配置文件继承了DelegatingWebMvcConfiguration，或者WebMvcConfigurationSupport，或者配置类注解了@EnableWebMvc，那么WebMvcAutoConfiguration 将不会被自动配置，而是使用WebMvcConfigurationSupport的配置。那么！！！！所有实现了WebMvcConfigurer的配置类有可能会 全部失效！！！！！

配置类-WebMvcConfigurationAdapter

WebMvcConfigurationAdapter已经废弃，最好用WebMvcConfigurer代替