@EnableWebMvc是什么

直接看源码,@EnableWebMvc实际上引入一个DelegatingWebMvcConfiguration。

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

DelegatingWebMvcConfiguration继承了WebMvcConfigurationSupport

@Configurationpublic class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {...

所以@EnableWebMvc=继承DelegatingWebMvcConfiguration=继承WebMvcConfigurationSupport

@EnableWebMvc和@EnableAutoConfiguration的关系

@EnableAutoConfiguration是springboot项目的启动类注解@SpringBootApplication的子元素，主要功能为自动配置。

@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 {...}

@EnableAutoConfiguration实际是导入了EnableAutoConfigurationImportSelector和Registrar两个类

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

@Target({ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@Import({Registrar.class})public @interface AutoConfigurationPackage {} 这两个类的具体原理有些复杂，不太清除，主要内容是通过SpringFactoriesLoader.loadFactoryNames()导入jar下面的配置文件META-INF/spring.factories

protected List
    
      getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {        List
     
       configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());        Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");        return configurations;    }
     
    

配置文件中的内容如下

# Auto Configureorg.springframework.boot.autoconfigure.EnableAutoConfiguration=\org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\...org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\...

其中有WebMvcAutoConfiguration,WebMvcAutoConfiguration源码如下

@Configuration@ConditionalOnWebApplication(    type = Type.SERVLET)@ConditionalOnClass({Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class})@ConditionalOnMissingBean({WebMvcConfigurationSupport.class})@AutoConfigureOrder(-2147483638)@AutoConfigureAfter({DispatcherServletAutoConfiguration.class, ValidationAutoConfiguration.class})public class WebMvcAutoConfiguration {...}

@ConditionalOnMissingBean({WebMvcConfigurationSupport.class})意思是如果存在它修饰的类的bean

,则不需要再创建这个bean。

由此可得出结论：

如果有配置文件继承了DelegatingWebMvcConfiguration，

或者WebMvcConfigurationSupport，或者配置文件有@EnableWebMvc，那么 @EnableAutoConfiguration 中的

WebMvcAutoConfiguration 将不会被自动配置，而是使用WebMvcConfigurationSupport的配置。

@EnableWebMvc,WebMvcConfigurationSupport,WebMvcConfigurer和WebMvcConfigurationAdapter使用

WebMvcConfigurationAdapter已经废弃，最好用implements WebMvcConfigurer代替

@Configurationpublic class MyConfig implements WebMvcConfigurer {    }

如果使用继承,WebMvcConfigurationSupport，DelegatingWebMvcConfiguration，或者使用@EnableWebMvc，

需要注意会覆盖application.properties中关于WebMvcAutoConfiguration的设置，需要在自定义配置中实现，如

示例如下

Configuration@EnableWebMvcpublic class MyConfig implements WebMvcConfigurer {}

@Configurationpublic class MyConfig extends WebMvcConfigurationSupport {}

@Configurationpublic class MyConfig extends DelegatingWebMvcConfiguration {} 上面代码中需要在类中实现关于WebMvcAutoConfiguration的配置，而不是在application.properties中。 总结 implements WebMvcConfigurer ： 不会覆盖@EnableAutoConfiguration关于WebMvcAutoConfiguration的配置 @EnableWebMvc + implements WebMvcConfigurer ： 会覆盖@EnableAutoConfiguration关于WebMvcAutoConfiguration的配置 extends WebMvcConfigurationSupport ：会覆盖@EnableAutoConfiguration关于WebMvcAutoConfiguration的配置 extends DelegatingWebMvcConfiguration ：会覆盖@EnableAutoConfiguration关于WebMvcAutoConfiguration的配置