实践Spring Enable系列之EnableWebMVC

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Adding this annotation to an @Configuration class imports the Spring MVC configuration from WebMvcConfigurationSupport, e.g.:

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 @Configuration
 @EnableWebMvc
 @ComponentScan(basePackageClasses = { MyConfiguration.class })
 public class MyWebConfiguration {

 }

Similar to support found in <mvc:annotation-driven/> namespace.

To customize the imported configuration, implement the interface WebMvcConfigurer or more likely extend the empty method base class WebMvcConfigurerAdapter and override individual methods, e.g.:

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 @Configuration
 @EnableWebMvc
 @ComponentScan(basePackageClasses = { MyConfiguration.class })
 public class MyConfiguration extends WebMvcConfigurerAdapter {

 	   @Override
 	   public void addFormatters(FormatterRegistry formatterRegistry) {
         formatterRegistry.addConverter(new MyConverter());
 	   }

 	   @Override
 	   public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
         converters.add(new MyHttpMessageConverter());
 	   }

     // More overridden methods ...
 }

If WebMvcConfigurer does not expose some advanced setting that needs to be configured, consider removing the @EnableWebMvc annotation and extending directly from WebMvcConfigurationSupportor DelegatingWebMvcConfiguration, e.g.:

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 @Configuration
 @ComponentScan(basePackageClasses = { MyConfiguration.class })
 public class MyConfiguration extends WebMvcConfigurationSupport {

 	   @Override
	   public void addFormatters(FormatterRegistry formatterRegistry) {
         formatterRegistry.addConverter(new MyConverter());
	   }

	   @Bean
	   public RequestMappingHandlerAdapter requestMappingHandlerAdapter() {
         // Create or delegate to "super" to create and
         // customize properties of RequestMappingHandlerAdapter
	   }
 }

原理分析

@EnableWebMvc实际导入了DelegatingWebMvcConfiguration

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

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@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
...
}

以下仅以interceptor为例分析原理，其他类似。

WebMvcConfigurationSupport中有如下拦截器相关的方法：

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protected final Object[] getInterceptors() {
	if (this.interceptors == null) {
		InterceptorRegistry registry = new InterceptorRegistry();
    	// 添加拦截器
		addInterceptors(registry);
		registry.addInterceptor(new ConversionServiceExposingInterceptor(mvcConversionService()));
		registry.addInterceptor(new ResourceUrlProviderExposingInterceptor(mvcResourceUrlProvider()));
		this.interceptors = registry.getInterceptors();
	}
	return this.interceptors.toArray();
}
// 由子类覆盖添加拦截器
protected void addInterceptors(InterceptorRegistry registry) {
}

DelegatingWebMvcConfiguration覆盖了addInterceptors(InterceptorRegistry)方法

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@Override
protected void addInterceptors(InterceptorRegistry registry) {
	this.configurers.addInterceptors(registry);
}

且

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private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();

// 自动注入所有WebMvcConfigurer
@Autowired(required = false)
public void setConfigurers(List<WebMvcConfigurer> configurers) {
	if (configurers == null || configurers.isEmpty()) {
		return;
	}
	this.configurers.addWebMvcConfigurers(configurers);
}

WebMvcConfigurerComposite的addInterceptors实现如下：

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@Override
public void addInterceptors(InterceptorRegistry registry) {
	for (WebMvcConfigurer delegate : this.delegates) {
		delegate.addInterceptors(registry);
	}
}

可见添加拦截器最终代理给了WebMvcConfigurer，以及WebMvcConfigurerAdapter。

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public interface WebMvcConfigurer {
...
	void addInterceptors(InterceptorRegistry registry);
...
}

自定义拦截器通过InterceptorRegistry添加即可，参考如下：

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// 定义拦截器
@Bean
public DemoInterceptor demoInterceptor() {
	return new DemoInterceptor();
}

// 配置自定义拦截器
@Override
public void addInterceptors(InterceptorRegistry registry) {
	registry.addInterceptor(demoInterceptor());
}

Spring boot中参见自动配置之WebMvcAutoConfiguration。

WebMvcConfigurationSupport

This is the main class providing the configuration behind the MVC Java config. It is typically imported by adding @EnableWebMvc to an application @Configuration class. An alternative more advanced option is to extend directly from this class and override methods as necessary remembering to add @Configuration to the subclass and @Bean to overridden @Bean methods. For more details see the Javadoc of @EnableWebMvc.

This class registers the following HandlerMappings:

  • RequestMappingHandlerMapping ordered at 0 for mapping requests to annotated controller methods.
  • HandlerMapping ordered at 1 to map URL paths directly to view names.
  • BeanNameUrlHandlerMapping ordered at 2 to map URL paths to controller bean names.
  • HandlerMapping ordered at Integer.MAX_VALUE-1 to serve static resource requests.
  • HandlerMapping ordered at Integer.MAX_VALUE to forward requests to the default servlet.

Registers these HandlerAdapters:

  • RequestMappingHandlerAdapter for processing requests with annotated controller methods.
  • HttpRequestHandlerAdapter for processing requests with HttpRequestHandlers.
  • SimpleControllerHandlerAdapter for processing requests with interface-based Controllers.

Registers a HandlerExceptionResolverComposite with this chain of exception resolvers:

  • ExceptionHandlerExceptionResolver for handling exceptions through @ExceptionHandlermethods.
  • ResponseStatusExceptionResolver for exceptions annotated with @ResponseStatus.
  • DefaultHandlerExceptionResolver for resolving known Spring exception types

Both the RequestMappingHandlerAdapter and the ExceptionHandlerExceptionResolver are configured with default instances of the following by default:

  • A ContentNegotiationManager
  • A DefaultFormattingConversionService
  • A LocalValidatorFactoryBean if a JSR-303 implementation is available on the classpath
  • A range of HttpMessageConverters depending on the 3rd party libraries available on the classpath.