小架构step系列21：参数和返回值的匹配

1 概述

从前面看到参数是通过HandlerMethodArgumentResolver进行匹配的，参数有各种各样的形式，比如带注解的、自定义类、基本类型、甚至ServletReqest之类的。返回值则是通过ReturnValueHandler来处理的，返回值也有比较多类型，比如String、Void、Model、自定义类等，如果不是前后端分离的，还可能带View之类的。本文了解一下参数和返回值的匹配原理。

2 参数匹配

2.1 参数匹配原理

遍历所有HandlerMethodArgumentResolver，如果Resolver的supportsParameter()接口返回值为true，则匹配此Resolver，不再看下面其它的Resolver。

// 源码位置：org.springframework.web.method.support.HandlerMethodArgumentResolverComposite
private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);if (result == null) {// 遍历所有argumentResolver，调用其supportsParameter()方法，如果返回值为true则匹配上此argumentResolverfor (HandlerMethodArgumentResolver resolver : this.argumentResolvers) {if (resolver.supportsParameter(parameter)) {result = resolver;this.argumentResolverCache.put(parameter, result);break;}}}return result;
}

默认有27个Resolver：

// 源码位置：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>(30);resolvers.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());}// 注意自定义的ArgumentResolver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;
}org.springframework.web.method.annotation.RequestParamMethodArgumentResolver
org.springframework.web.method.annotation.RequestParamMapMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.PathVariableMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.PathVariableMapMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.MatrixVariableMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.MatrixVariableMapMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor
org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor
org.springframework.web.servlet.mvc.method.annotation.RequestPartMethodArgumentResolver
org.springframework.web.method.annotation.RequestHeaderMethodArgumentResolver
org.springframework.web.method.annotation.RequestHeaderMapMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.ServletCookieValueMethodArgumentResolver
org.springframework.web.method.annotation.ExpressionValueMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.SessionAttributeMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.RequestAttributeMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.ServletRequestMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.ServletResponseMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor
org.springframework.web.servlet.mvc.method.annotation.RedirectAttributesMethodArgumentResolver
org.springframework.web.method.annotation.ModelMethodProcessor
org.springframework.web.method.annotation.MapMethodProcessor
org.springframework.web.method.annotation.ErrorsMethodArgumentResolver
org.springframework.web.method.annotation.SessionStatusMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.UriComponentsBuilderMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.PrincipalMethodArgumentResolver
org.springframework.web.method.annotation.RequestParamMethodArgumentResolver
org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor

注意：RequestParamMethodArgumentResolver和ServletModelAttributeMethodProcessor有两个。

2.2 常用的MethodArgumentResolver

下面列一下几个常用的MethodArgumentResolver：

// 源码位置：org.springframework.web.method.annotation.RequestParamMethodArgumentResolver
public boolean supportsParameter(MethodParameter parameter) {// 参数前加了@RequestParam注解时，用此MethodArgumentResolver处理if (parameter.hasParameterAnnotation(RequestParam.class)) {// 如果参数是Map类型，则还需要在@RequestParam注解中指定映射字段名if (Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType())) {RequestParam requestParam = parameter.getParameterAnnotation(RequestParam.class);return (requestParam != null && StringUtils.hasText(requestParam.name()));}else {return true;}}else {// 参数前加了@RequestPart注解时，不用此MethodArgumentResolver处理if (parameter.hasParameterAnnotation(RequestPart.class)) {return false;}parameter = parameter.nestedIfOptional();// 参数类型为MultipartFile或Part时(可以List里放这些类型)，用此MethodArgumentResolver处理if (MultipartResolutionDelegate.isMultipartArgument(parameter)) {return true;}else if (this.useDefaultResolution) {// SpringMVC准备了两个RequestParamMethodArgumentResolver，一个useDefaultResolution=false，另外一个useDefaultResolution=true(倒数第二个Resolver)// 如果useDefaultResolution=true，则普通类型如Void、int/byte/long等基础类型可以用此MethodArgumentResolver处理// 详细类型参考BeanUtils.isSimpleProperty()方法return BeanUtils.isSimpleProperty(parameter.getNestedParameterType());}else {return false;}}
}
// 源码位置：org.springframework.beans.BeanUtils
public static boolean isSimpleValueType(Class<?> type) {return (Void.class != type && void.class != type &&(ClassUtils.isPrimitiveOrWrapper(type) ||Enum.class.isAssignableFrom(type) ||CharSequence.class.isAssignableFrom(type) ||Number.class.isAssignableFrom(type) ||Date.class.isAssignableFrom(type) ||Temporal.class.isAssignableFrom(type) ||URI.class == type ||URL.class == type ||Locale.class == type ||Class.class == type));
}// 源码位置：org.springframework.web.method.annotation.RequestParamMapMethodArgumentResolver
public boolean supportsParameter(MethodParameter parameter) {// 参数前加了@RequestParam注解、且参数类型为Map类型、也没有在@RequestParam注解指定字段映射名时，用此MethodArgumentResolver处理RequestParam requestParam = parameter.getParameterAnnotation(RequestParam.class);return (requestParam != null && Map.class.isAssignableFrom(parameter.getParameterType()) &&!StringUtils.hasText(requestParam.name()));
}// 源码位置：org.springframework.web.servlet.mvc.method.annotation.PathVariableMethodArgumentResolver
public boolean supportsParameter(MethodParameter parameter) {// 参数前没有加@PathVariable注解，不用此MethodArgumentResolver处理if (!parameter.hasParameterAnnotation(PathVariable.class)) {return false;}// 参数前加了@PathVariable注解，且参数为Map类型时，需要在注解指定字段映射名，才用此MethodArgumentResolver处理if (Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType())) {PathVariable pathVariable = parameter.getParameterAnnotation(PathVariable.class);return (pathVariable != null && StringUtils.hasText(pathVariable.value()));}return true;
}// 源码位置：org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor#supportsParameter
public boolean supportsParameter(MethodParameter parameter) {// 参数前加了@RequestBody注解，用此MethodArgumentResolver处理return parameter.hasParameterAnnotation(RequestBody.class);
}// 源码位置：org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor
// 实际由父类org.springframework.web.method.annotation.ModelAttributeMethodProcessor提供supportsParameter()方法
public boolean supportsParameter(MethodParameter parameter) {// 两种情况用此MethodArgumentResolver处理：// 1. 参数前加了@ModelAttribute注解；// 2. 没有加参数注解，参数未非普通类型(普通类型参考上面BeanUtils.isSimpleProperty()的实现)；// 注：有两个ServletModelAttributeMethodProcessor，一个annotationNotRequired=false，另一个annotationNotRequired=true(倒数第一个Resolver)return (parameter.hasParameterAnnotation(ModelAttribute.class) ||(this.annotationNotRequired && !BeanUtils.isSimpleProperty(parameter.getParameterType())));
}// 源码位置：org.springframework.web.servlet.mvc.method.annotation.ServletRequestMethodArgumentResolver
public boolean supportsParameter(MethodParameter parameter) {// 参数是Servlet有关的类型，用此MethodArgumentResolver处理Class<?> paramType = parameter.getParameterType();return (WebRequest.class.isAssignableFrom(paramType) ||ServletRequest.class.isAssignableFrom(paramType) ||MultipartRequest.class.isAssignableFrom(paramType) ||HttpSession.class.isAssignableFrom(paramType) ||(pushBuilder != null && pushBuilder.isAssignableFrom(paramType)) ||(Principal.class.isAssignableFrom(paramType) && !parameter.hasParameterAnnotations()) ||InputStream.class.isAssignableFrom(paramType) ||Reader.class.isAssignableFrom(paramType) ||HttpMethod.class == paramType ||Locale.class == paramType ||TimeZone.class == paramType ||ZoneId.class == paramType);
}

// 源码位置：org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite
private HandlerMethodReturnValueHandler selectHandler(@Nullable Object value, MethodParameter returnType) {boolean isAsyncValue = isAsyncReturnValue(value, returnType);for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {continue;}// 遍历所有支持的ReturnValueHandler，调用supportsReturnType()接口，如果返回值为true，则选中此ReturnValueHandlerif (handler.supportsReturnType(returnType)) {return handler;}}return null;
}

// 源码位置：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter
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());// 注意自定义的ReturnValueHandler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;
}org.springframework.web.servlet.mvc.method.annotation.ModelAndViewMethodReturnValueHandler
org.springframework.web.method.annotation.ModelMethodProcessor
org.springframework.web.servlet.mvc.method.annotation.ViewMethodReturnValueHandler
org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitterReturnValueHandler
org.springframework.web.servlet.mvc.method.annotation.StreamingResponseBodyReturnValueHandler
org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor
org.springframework.web.servlet.mvc.method.annotation.HttpHeadersReturnValueHandler
org.springframework.web.servlet.mvc.method.annotation.CallableMethodReturnValueHandler
org.springframework.web.servlet.mvc.method.annotation.DeferredResultMethodReturnValueHandler
org.springframework.web.servlet.mvc.method.annotation.AsyncTaskMethodReturnValueHandler
org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor
org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor
org.springframework.web.servlet.mvc.method.annotation.ViewNameMethodReturnValueHandler
org.springframework.web.method.annotation.MapMethodProcessor
org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor

// 源码位置：org.springframework.web.servlet.mvc.method.annotation.ModelAndViewMethodReturnValueHandler
public boolean supportsReturnType(MethodParameter returnType) {// 返回值类型为ModelAndView，用此ReturnValueHandlerreturn ModelAndView.class.isAssignableFrom(returnType.getParameterType());
}// 源码位置：org.springframework.web.method.annotation.ModelMethodProcessor
public boolean supportsParameter(MethodParameter parameter) {// 返回值类型为Model，用此ReturnValueHandlerreturn Model.class.isAssignableFrom(parameter.getParameterType());
}// 源码位置：org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor
public boolean supportsReturnType(MethodParameter returnType) {// 返回值对象对应的类指定了@ResponseBody注解，或者返回值有指定@ResponseBody注解，用此ReturnValueHandlerreturn (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||returnType.hasMethodAnnotation(ResponseBody.class));
}// 源码位置：org.springframework.web.servlet.mvc.method.annotation.ViewMethodReturnValueHandler
public boolean supportsReturnType(MethodParameter returnType) {// 返回值类型为View，用此ReturnValueHandlerreturn View.class.isAssignableFrom(returnType.getParameterType());
}// 源码位置：org.springframework.web.servlet.mvc.method.annotation.ViewNameMethodReturnValueHandler
public boolean supportsReturnType(MethodParameter returnType) {Class<?> paramType = returnType.getParameterType();// 如果没有返回值或者返回值是字符串类型，则用此ReturnValueHandlerreturn (void.class == paramType || CharSequence.class.isAssignableFrom(paramType));
}// 源码位置：org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor
// 实际用父类org.springframework.web.method.annotation.ModelAttributeMethodProcessor的实现
public boolean supportsReturnType(MethodParameter returnType) {// 返回值类型为ModelAttribute，或者在annotationNotRequired=true的时候不是普通类型// ServletModelAttributeMethodProcessor有两个，第二个(在列表最后一个)annotationNotRequired=truereturn (returnType.hasMethodAnnotation(ModelAttribute.class) ||(this.annotationNotRequired && !BeanUtils.isSimpleProperty(returnType.getParameterType())));
}