1 概述

通过上一篇了解请求和响应的流程，Spring在设计上留了不少扩展点。里面通过查找接口的方式获取的地方，都可以成为一种扩展点，因为只要实现这类接口就可以成为Spring加载的一部分。本文了解一下这些扩展点，方便后面进行扩展。

2 原理

2.1 HandleInterceptor的扩展点

加载HandleInterceptor的方法getInterceptors是在各个HandlerMapping初始化的时候调用的，该方法的调用会加载所有实现了WebMvcConfigurer接口的类，通过WebMvcConfigurer提供的addInterceptors()可以自定义增加HandleInterceptor：

// 继承关系：EnableWebMvcConfiguration < DelegatingWebMvcConfiguration < WebMvcConfigurationSupport
// EnableWebMvcConfiguration在开启WebMvc后由Spring触发加载
// 源码位置：org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport
protected final Object[] getInterceptors(FormattingConversionService mvcConversionService, ResourceUrlProvider mvcResourceUrlProvider) {if (this.interceptors == null) {InterceptorRegistry registry = new InterceptorRegistry();// 加载自定义HandleInterceptor，自定义的Interceptor在最前面addInterceptors(registry);registry.addInterceptor(new ConversionServiceExposingInterceptor(mvcConversionService));registry.addInterceptor(new ResourceUrlProviderExposingInterceptor(mvcResourceUrlProvider));this.interceptors = registry.getInterceptors();}return this.interceptors.toArray();
}
// 源码位置：org.springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration
protected void addInterceptors(InterceptorRegistry registry) {// configurers为WebMvcConfigurerCompositethis.configurers.addInterceptors(registry);
}
// org.springframework.web.servlet.config.annotation.WebMvcConfigurerComposite
public void addInterceptors(InterceptorRegistry registry) {// delegates里注入了所有实现了WebMvcConfigurer接口的类，// 也就是说如果希望自定义一个HandlerInterceptor，则可以通过实现WebMvcConfigurer接口来完成for (WebMvcConfigurer delegate : this.delegates) {delegate.addInterceptors(registry);}
}

2.2 HandlerMethodArgumentResolver扩展点

在初始化RequestMappingHandlerAdapter的时候，就把实现WebMvcConfigurer接口来增加的HandlerMethodArgumentResolver存放到了RequestMappingHandlerAdapter，在RequestMappingHandlerAdapter执行afterPropertiesSet()的时候，再把自定义的HandlerMethodArgumentResolver和默认的一起加到argumentResolvers中：

// RequestMappingHandlerAdapter初始化后，触发afterPropertiesSet()方法，在里面初始化HandlerMethodArgumentResolver
// 源码位置：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter
public void afterPropertiesSet() {// Do this first, it may add ResponseBody advice beansinitControllerAdviceCache();if (this.argumentResolvers == null) {// 加载默认的HandlerMethodArgumentResolverList<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.initBinderArgumentResolvers == null) {List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.returnValueHandlers == null) {List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);}
}
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>(30);// 创建了二十多个HandlerMethodArgumentResolverresolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));resolvers.add(new RequestParamMapMethodArgumentResolver());resolvers.add(new PathVariableMethodArgumentResolver());resolvers.add(new PathVariableMapMethodArgumentResolver());resolvers.add(new MatrixVariableMethodArgumentResolver());resolvers.add(new MatrixVariableMapMethodArgumentResolver());resolvers.add(new ServletModelAttributeMethodProcessor(false));resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));resolvers.add(new RequestHeaderMapMethodArgumentResolver());resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));resolvers.add(new SessionAttributeMethodArgumentResolver());resolvers.add(new RequestAttributeMethodArgumentResolver());resolvers.add(new ServletRequestMethodArgumentResolver());resolvers.add(new ServletResponseMethodArgumentResolver());resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));resolvers.add(new RedirectAttributesMethodArgumentResolver());resolvers.add(new ModelMethodProcessor());resolvers.add(new MapMethodProcessor());resolvers.add(new ErrorsMethodArgumentResolver());resolvers.add(new SessionStatusMethodArgumentResolver());resolvers.add(new UriComponentsBuilderMethodArgumentResolver());if (KotlinDetector.isKotlinPresent()) {resolvers.add(new ContinuationHandlerMethodArgumentResolver());}// 加载自定义的HandlerMethodArgumentResolver，注意自定义的Resolver是在中间偏后的位置if (getCustomArgumentResolvers() != null) {resolvers.addAll(getCustomArgumentResolvers());}resolvers.add(new PrincipalMethodArgumentResolver());resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));resolvers.add(new ServletModelAttributeMethodProcessor(true));return resolvers;
}
public List<HandlerMethodArgumentResolver> getCustomArgumentResolvers() {// 自定义的HandlerMethodArgumentResolver是在之前赋值的return this.customArgumentResolvers;
}// 源码位置：org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.EnableWebMvcConfiguration
@Bean
@Override
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcValidator") Validator validator) {// 父类为WebMvcConfigurationSupportRequestMappingHandlerAdapter adapter = super.requestMappingHandlerAdapter(contentNegotiationManager, conversionService, validator);adapter.setIgnoreDefaultModelOnRedirect(this.mvcProperties == null || this.mvcProperties.isIgnoreDefaultModelOnRedirect());return adapter;
}// 源码位置：org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport
@Bean
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcValidator") Validator validator) {RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();adapter.setContentNegotiationManager(contentNegotiationManager);adapter.setMessageConverters(getMessageConverters());adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer(conversionService, validator));// 设置自定义的HandlerMethodArgumentResolveradapter.setCustomArgumentResolvers(getArgumentResolvers());adapter.setCustomReturnValueHandlers(getReturnValueHandlers());if (jackson2Present) {adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));}AsyncSupportConfigurer configurer = getAsyncSupportConfigurer();if (configurer.getTaskExecutor() != null) {adapter.setTaskExecutor(configurer.getTaskExecutor());}if (configurer.getTimeout() != null) {adapter.setAsyncRequestTimeout(configurer.getTimeout());}adapter.setCallableInterceptors(configurer.getCallableInterceptors());adapter.setDeferredResultInterceptors(configurer.getDeferredResultInterceptors());return adapter;
}
protected final List<HandlerMethodArgumentResolver> getArgumentResolvers() {if (this.argumentResolvers == null) {this.argumentResolvers = new ArrayList<>();// addArgumentResolvers()接口由DelegatingWebMvcConfiguration实现addArgumentResolvers(this.argumentResolvers);}return this.argumentResolvers;
}// 源码位置：org.springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration
protected void addArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers) {// configurers为WebMvcConfigurerCompositethis.configurers.addArgumentResolvers(argumentResolvers);
}// 源码位置：org.springframework.web.servlet.config.annotation.WebMvcConfigurerComposite
public void addArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers) {// delegates里注入了所有实现了WebMvcConfigurer接口的类，// 也就是说如果希望自定义一个HandlerMethodArgumentResolver，则可以通过实现WebMvcConfigurer接口来完成for (WebMvcConfigurer delegate : this.delegates) {delegate.addArgumentResolvers(argumentResolvers);}
}

2.3 RequestMappingHandlerAdapter的扩展点

在初始化RequestMappingHandlerAdapter的时候，就把实现WebMvcConfigurer接口来增加的HandlerMethodReturnValueHandler存放到了RequestMappingHandlerAdapter，在RequestMappingHandlerAdapter执行afterPropertiesSet()的时候，再把自定义的HandlerMethodReturnValueHandler和默认的一起加到returnValueHandlers中：

// RequestMappingHandlerAdapter初始化后，触发afterPropertiesSet()方法，在里面初始化HandlerMethodReturnValueHandler
// 源码位置：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter
public void afterPropertiesSet() {// Do this first, it may add ResponseBody advice beansinitControllerAdviceCache();if (this.argumentResolvers == null) {List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.initBinderArgumentResolvers == null) {List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.returnValueHandlers == null) {// 加载默认的HandlerMethodReturnValueHandlerList<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);}
}
private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(20);handlers.add(new ModelAndViewMethodReturnValueHandler());handlers.add(new ModelMethodProcessor());handlers.add(new ViewMethodReturnValueHandler());handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));handlers.add(new StreamingResponseBodyReturnValueHandler());handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),this.contentNegotiationManager, this.requestResponseBodyAdvice));handlers.add(new HttpHeadersReturnValueHandler());handlers.add(new CallableMethodReturnValueHandler());handlers.add(new DeferredResultMethodReturnValueHandler());handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));handlers.add(new ServletModelAttributeMethodProcessor(false));handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),this.contentNegotiationManager, this.requestResponseBodyAdvice));handlers.add(new ViewNameMethodReturnValueHandler());handlers.add(new MapMethodProcessor());// 加载自定义的HandlerMethodReturnValueHandler，注意自定义的Handler是在中间偏后的位置if (getCustomReturnValueHandlers() != null) {handlers.addAll(getCustomReturnValueHandlers());}if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));}else {handlers.add(new ServletModelAttributeMethodProcessor(true));}return handlers;
}
public List<HandlerMethodReturnValueHandler> getCustomReturnValueHandlers() {// 自定义的HandlerMethodReturnValueHandler是在之前赋值的return this.customReturnValueHandlers;
}// 初始化RequestMappingHandlerAdapter的时候，加载自定义的HandlerMethodReturnValueHandler
// 源码位置：org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport
@Bean
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcValidator") Validator validator) {RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();adapter.setContentNegotiationManager(contentNegotiationManager);adapter.setMessageConverters(getMessageConverters());adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer(conversionService, validator));adapter.setCustomArgumentResolvers(getArgumentResolvers());// 设置自定义的RequestMappingHandlerAdapteradapter.setCustomReturnValueHandlers(getReturnValueHandlers());if (jackson2Present) {adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));}AsyncSupportConfigurer configurer = getAsyncSupportConfigurer();if (configurer.getTaskExecutor() != null) {adapter.setTaskExecutor(configurer.getTaskExecutor());}if (configurer.getTimeout() != null) {adapter.setAsyncRequestTimeout(configurer.getTimeout());}adapter.setCallableInterceptors(configurer.getCallableInterceptors());adapter.setDeferredResultInterceptors(configurer.getDeferredResultInterceptors());return adapter;
}
protected final List<HandlerMethodReturnValueHandler> getReturnValueHandlers() {if (this.returnValueHandlers == null) {this.returnValueHandlers = new ArrayList<>();// addReturnValueHandlers()接口由DelegatingWebMvcConfiguration实现addReturnValueHandlers(this.returnValueHandlers);}return this.returnValueHandlers;
}// 源码位置：org.springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration
protected void addReturnValueHandlers(List<HandlerMethodReturnValueHandler> returnValueHandlers) {// configurers为WebMvcConfigurerCompositethis.configurers.addReturnValueHandlers(returnValueHandlers);
}// 源码位置：org.springframework.web.servlet.config.annotation.WebMvcConfigurerComposite
public void addReturnValueHandlers(List<HandlerMethodReturnValueHandler> returnValueHandlers) {for (WebMvcConfigurer delegate : this.delegates) {// delegates里注入了所有实现了WebMvcConfigurer接口的类，// 也就是说如果希望自定义一个RequestMappingHandlerAdapter，则可以通过实现WebMvcConfigurer接口来完成delegate.addReturnValueHandlers(returnValueHandlers);}
}

2.4 MessageConverter的扩展点

在初始化RequestMappingHandlerAdapter的时候，就把实现WebMvcConfigurer接口来增加的MessageConverter存放到了RequestMappingHandlerAdapter，在RequestMappingHandlerAdapter执行afterPropertiesSet()的时候，再把自定义的MessageConverter加到messageConverters中：

// RequestMappingHandlerAdapter初始化后，触发afterPropertiesSet()方法，在里面初始化HandlerMethodReturnValueHandler
// 源码位置：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter
public void afterPropertiesSet() {// Do this first, it may add ResponseBody advice beansinitControllerAdviceCache();if (this.argumentResolvers == null) {List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.initBinderArgumentResolvers == null) {List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.returnValueHandlers == null) {// 加载默认的HandlerMethodReturnValueHandlerList<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);}
}
private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(20);handlers.add(new ModelAndViewMethodReturnValueHandler());handlers.add(new ModelMethodProcessor());handlers.add(new ViewMethodReturnValueHandler());// 有设置MessageConverterhandlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));handlers.add(new StreamingResponseBodyReturnValueHandler());// 有设置MessageConverterhandlers.add(new HttpEntityMethodProcessor(getMessageConverters(),this.contentNegotiationManager, this.requestResponseBodyAdvice));handlers.add(new HttpHeadersReturnValueHandler());handlers.add(new CallableMethodReturnValueHandler());handlers.add(new DeferredResultMethodReturnValueHandler());handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));handlers.add(new ServletModelAttributeMethodProcessor(false));// 有设置MessageConverterhandlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),this.contentNegotiationManager, this.requestResponseBodyAdvice));handlers.add(new ViewNameMethodReturnValueHandler());handlers.add(new MapMethodProcessor());if (getCustomReturnValueHandlers() != null) {handlers.addAll(getCustomReturnValueHandlers());}if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));}else {handlers.add(new ServletModelAttributeMethodProcessor(true));}return handlers;
}
public List<HttpMessageConverter<?>> getMessageConverters() {// 自定义的MessageConverter是在之前赋值的return this.messageConverters;
}// 初始化RequestMappingHandlerAdapter的时候，加载自定义的MessageConverter
// 源码位置：org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport
@Bean
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcValidator") Validator validator) {RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();adapter.setContentNegotiationManager(contentNegotiationManager);// 设置自定义的MessageConverteradapter.setMessageConverters(getMessageConverters());adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer(conversionService, validator));adapter.setCustomArgumentResolvers(getArgumentResolvers());adapter.setCustomReturnValueHandlers(getReturnValueHandlers());if (jackson2Present) {adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));}AsyncSupportConfigurer configurer = getAsyncSupportConfigurer();if (configurer.getTaskExecutor() != null) {adapter.setTaskExecutor(configurer.getTaskExecutor());}if (configurer.getTimeout() != null) {adapter.setAsyncRequestTimeout(configurer.getTimeout());}adapter.setCallableInterceptors(configurer.getCallableInterceptors());adapter.setDeferredResultInterceptors(configurer.getDeferredResultInterceptors());return adapter;
}
protected final List<HttpMessageConverter<?>> getMessageConverters() {if (this.messageConverters == null) {this.messageConverters = new ArrayList<>();// 配置自定义的MessageConverterconfigureMessageConverters(this.messageConverters);if (this.messageConverters.isEmpty()) {addDefaultHttpMessageConverters(this.messageConverters);}extendMessageConverters(this.messageConverters);}return this.messageConverters;
}// 源码位置：org.springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration
protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {// configurers为WebMvcConfigurerCompositethis.configurers.configureMessageConverters(converters);
}// 源码位置：org.springframework.web.servlet.config.annotation.WebMvcConfigurerComposite
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {for (WebMvcConfigurer delegate : this.delegates) {// delegates里注入了所有实现了WebMvcConfigurer接口的类，// 也就是说如果希望自定义一个MessageConverter，则可以通过实现WebMvcConfigurer接口来完成delegate.configureMessageConverters(converters);}
}

3 架构一小步