RequestMappingHandlerAdapter详解
RequestMappingHandlerAdapter是日常项目中使用最多的HandlerAdapter实现类。
它还有一个抽象父类AbstractHandlerMethodAdapter，顾名思义，是专门用来处理HandlerMethod类型的handler。具体可以看AbstractHandlerMethodAdapter#supports方法：
public final boolean supports(Object handler) {
return (handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler));
}
通过之前的学习可以知道，RequestMappingHandlerMapping获取的handler就是HandlerMethod类型的。
RequestMappingHandlerMapping和RequestMappingHandlerAdapter就像一对孪生兄弟：
RequestMappingHandlerMapping负责根据request找到映射的handlerRequestMappingHandlerAdapter负责根据handler执行对应的方法
我们先总结RequestMappingHandlerAdapter处理handler的核心流程：
- 将
request和response封装成ServletWebRequest对象。 - 将
handler封装成ServletInvocableHandlerMethod对象invocableMethod。 - 为
invocableMethod设置argumentResolvers、returnValueHandlers、dataBinderFactory和parameterNameDiscoverer等工具。 - 解析请求参数。
- 执行方法。
- 处理返回值。
实际上，RequestMappingHandlerAdapter处理handler过程中还有许多细节，比如前后端不分离项目的视图相关处理（没有必要花费时间深入学习），异步请求的相关处理（会另外写文章）。
0 预备知识
RequestMappingHandlerAdapter中有许多成员变量，在请求处理过程中起着重要的作用。
0.1 argumentResolvers
argumentResolvers是参数解析器，RequestMappingHandlerAdapter使用argumentResolvers进行参数解析。
简单来说，就是将HTTP请求中的数据，转换成handler方法中的形参对象。
argumentResolvers使用了组合模式，它的类型是HandlerMethodArgumentResolverComposite，其内部缓存HandlerMethodArgumentResolver对象，用来进行参数解析。
HandlerMethodArgumentResolverComposite中包含argumentResolvers和argumentResolverCache两个成员变量。在初始化时，会将所有配置的参数解析器缓存到argumentResolvers中。第一次解析参数时，会遍历argumentResolvers获取对应参数解析器，并缓存到argumentResolverCache中，后续再次解析该参数可直接从键值对中获取，提高效率。
实际进行参数解析的是HandlerMethodArgumentResolver实现类。它们使用了策略模式，通过supportsParameter()方法获取支持的参数解析器，通过resolveArgument()方法进行参数解析。
0.2 customArgumentResolvers
customArgumentResolvers是用于缓存开发人员自定义的参数解析器，即通过WebMvcConfigurer#addArgumentResolvers()方法添加的解析器。
在RequestMappingHandlerAdapter初始化时，会将customArgumentResolvers中的自定义参数解析器添加到argumentResolvers中。
0.3 returnValueHandlers
returnValueHandlers是返回值处理器，它可以对控制层业务返回值进行处理。
例如，对@ResponseBody标注的返回值进行JSON格式化，并写到输出流。
returnValueHandlers使用了组合模式，它的类型是HandlerMethodReturnValueHandlerComposite，其内部缓存HandlerMethodReturnValueHandler对象，用来进行返回值处理。
0.4 customReturnValueHandlers
customReturnValueHandlers是用于缓存开发人员自定义的参数解析器，即通过WebMvcConfigurer#addReturnValueHandlers()方法添加的解析器。
在RequestMappingHandlerAdapter初始化时，会将customReturnValueHandlers中的自定义参数解析器添加到returnValueHandlers中。
1 初始化流程
在RequestMappingHandlerAdapter内部，有两个方法用于初始化。一个是构造函数，另一个是实现org.springframework.beans.factory.InitializingBean的afterPropertiesSet()方法。
在Spring Boot中，会在WebMvcConfigurationSupport中进行完整的初始化。
1.1 构造函数
构造函数中主要是对messageConverters进行初始化，添加一些必备的消息转换器。实际上，WebMvcConfigurationSupport中会进行覆盖，因此不过多描述：
public RequestMappingHandlerAdapter() {
this.messageConverters = new ArrayList<>(4);
this.messageConverters.add(new ByteArrayHttpMessageConverter());
this.messageConverters.add(new StringHttpMessageConverter());
if (!shouldIgnoreXml) {
try {
this.messageConverters.add(new SourceHttpMessageConverter<>());
}
catch (Error err) {
// Ignore when no TransformerFactory implementation is available
}
}
this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
}
1.2 afterPropertiesSet()
在RequestMappingHandlerAdapter#afterPropertiesSet()方法中，会对argumentResolvers、initBinderArgumentResolvers和returnValueHandlers等进行初始化：
public void afterPropertiesSet() {
// Do this first, it may add ResponseBody advice beans
initControllerAdviceCache();
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) {
List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
}
}
1.2.1 initControllerAdviceCache
在RequestMappingHandlerAdapter#initControllerAdviceCache()方法中，会从容器中获取所有@ControllerAdvice标注的bean。然后缓存这些bean中标注@RequestMapping&@ModelAttribute（modelAttributeAdviceCache）和@InitBinder（initBinderAdviceChache）等注解的方法，并且直接缓存实现RequestBodyAdvice或ResponseBodyAdvice的bean（requestResponseBodyAdvice）。
private void initControllerAdviceCache() {
if (getApplicationContext() == null) {
return;
}
List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
List<Object> requestResponseBodyAdviceBeans = new ArrayList<>();
for (ControllerAdviceBean adviceBean : adviceBeans) {
Class<?> beanType = adviceBean.getBeanType();
if (beanType == null) {
throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
}
Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS);
if (!attrMethods.isEmpty()) {
this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
}
Set<Method> binderMethods = MethodIntrospector.selectMethods(beanType, INIT_BINDER_METHODS);
if (!binderMethods.isEmpty()) {
this.initBinderAdviceCache.put(adviceBean, binderMethods);
}
if (RequestBodyAdvice.class.isAssignableFrom(beanType) || ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
requestResponseBodyAdviceBeans.add(adviceBean);
}
}
if (!requestResponseBodyAdviceBeans.isEmpty()) {
this.requestResponseBodyAdvice.addAll(0, requestResponseBodyAdviceBeans);
}
}
1.2.2 getDefaultXxx()方法
通过getDefaultArgumentResolvers()、getDefaultInitBinderArgumentResolvers()和getDefaultResurnValueHandlers()方法分别对argumentResolvers、initBinderArgumentResolvers和returnValueHandlers进行初始化。
在这些getDefaultXxx()方法中，一方面会按一定顺序添加一系列默认的处理器对象，另一方面会通过getCustomXxx()方法获取开发人员自定义的处理器对象（可通过WevMvcConfigurer添加）。
例如，RequestMappingHandlerAdapter#getDefaultArgumentResolvers()方法会添加一系列默认的参数解析器，并且通过getCustomArgumentResolvers()方法获取开发人员自定义的参数解析器：
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>(30);
// Annotation-based argument resolution
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());
// Type-based argument resolution
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());
}
// Custom arguments
if (getCustomArgumentResolvers() != null) {
resolvers.addAll(getCustomArgumentResolvers());
}
// Catch-all
resolvers.add(new PrincipalMethodArgumentResolver());
resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
resolvers.add(new ServletModelAttributeMethodProcessor(true));
return resolvers;
1.3 WebMvcConfigurationSupport
在WebMvcConfigurationSupport#requestMappingHandlerAdapter()中，会完成requestMappingHandlerAdapter的bean的创建，对contentNegotiationManager、messageConverters、webBindingInitializer、customArgumentResolvers和customReturnValueHandlers等基础成员变量，以及异步请求的taskExecutor、asyncRequestTimeout、callableInterceptors和deferredResultInterceptors等成员变量进行初始化：
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());
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;
}
在初始化过程中，一方面会为这些成员添加一系列默认对象，另一方面会从WebMvcConfigurer中获取开发人员自定义的对象。
2 同步请求处理流程
首先，DispatcherServlet会调用HandlerAdapter接口的handle()方法。
AbstractHandlerMethodAdapter对handle()方法的实现只是做了一个类型转换：
public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
return handleInternal(request, response, (HandlerMethod) handler);
}
AbstractHandlerMethodAdapter#handleInternal()是一个抽象方法，会由子类具体去实现。
RequestMappingHandlerAdapter#handlerInternal()方法中会进行一些请求判断和缓存处理（省略），它的核心是在invokeHandlerMethod()方法：
protected ModelAndView handleInternal(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ModelAndView mav;
mav = invokeHandlerMethod(request, response, handlerMethod);
return mav;
}
2.1 预处理：添加处理器
在RequestMappingHandlerAdapter#invokeHandlerMethod()方法中，会进行如下处理：
- 将
request和response封装成ServletWebRequest对象，便于后续处理。 - 将
handler封装成ServletInvocableHandlerMethod对象invocableMethod。 - 为
invocableMethod设置argumentResolvers（参数解析）、returnValueHandlers（返回值处理）、dataBinderFactory（数据绑定和校验）和parameterNameDiscoverer（形参名字解析）等组件，用作后续方法处理的工具。这些组件都来自RequestMappingHandlerAdapter的成员变量。 - 最后会调用
invocableMethod的invokeAndHandle()方法进行实际处理。
RequestMappingHandlerAdapter#invokeHandlerMethod()具体源码如下：
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
// 1、将`request`和`response`封装成`ServletWebRequest`对象
ServletWebRequest webRequest = new ServletWebRequest(request, response);
try {
WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
// 2、将`handler`封装成`ServletInvocableHandlerMethod`对象`invocableMethod`
ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
// 3、为`invocableMethod`设置`argumentResolvers`、`returnValueHandlers`、`dataBinderFactory`和`parameterNameDiscoverer`等工具
if (this.argumentResolvers != null) {
invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
}
if (this.returnValueHandlers != null) {
invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
}
invocableMethod.setDataBinderFactory(binderFactory);
invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
// 4、处理请求
invocableMethod.invokeAndHandle(webRequest, mavContainer);
return getModelAndView(mavContainer, modelFactory, webRequest);
}
finally {
webRequest.requestCompleted();
}
}
ServletInvocableHandlerMethod#invokeAndHandle()方法会调用请求，并且对返回值进行处理：
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
// 1、调用请求
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
// 省略相关代码
// 2、返回值处理
try {
this.returnValueHandlers.handleReturnValue(
returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
}
catch (Exception ex) {
if (logger.isTraceEnabled()) {
logger.trace(formatErrorForReturnValue(returnValue), ex);
}
throw ex;
}
}
2.2 形参对象解析
在InvocableHandlerMethod#invokeForRequest()方法中，会进行参数解析（将request中的数据解析成handler方法的形参对象），然后通过反射调用对应方法，获取返回值：
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
// 1、参数解析
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
// 2、调用方法
return doInvoke(args);
}
在InvocableHandlerMethod#getMethodArgumentValues()方法中，会通过反射获取handler方法的形参，然后使用resolvers对一个个形参进行解析。
根据形参的类型不同（HttpServletRequest等），形参上标注的注解不同（@RequestBody等），会调用不同的解析器实现类进行处理。
根据解析器实现类的不同，在解析过程中，会进行数据绑定、消息转换和参数校验：
protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
// 1、获取方法的形参信息
MethodParameter[] parameters = getMethodParameters();
if (ObjectUtils.isEmpty(parameters)) {
return EMPTY_ARGS;
}
Object[] args = new Object[parameters.length];
// 遍历方法形参
for (int i = 0; i < parameters.length; i++) {
MethodParameter parameter = parameters[i];
parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
args[i] = findProvidedArgument(parameter, providedArgs);
if (args[i] != null) {
continue;
}
if (!this.resolvers.supportsParameter(parameter)) {
throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
}
try {
// 2、形参解析
args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
}
catch (Exception ex) {
// Leave stack trace for later, exception may actually be resolved and handled...
if (logger.isDebugEnabled()) {
String exMsg = ex.getMessage();
if (exMsg != null && !exMsg.contains(parameter.getExecutable().toGenericString())) {
logger.debug(formatArgumentError(parameter, exMsg));
}
}
throw ex;
}
}
return args;
}
2.3 执行方法
回到InvocableHandlerMethod#invokeForRequest()方法，解析方法形参后，会调用InvocableHandlerMethod#doInvoke()方法，通过反射调用方法，并传入handler对应的控制层bean作为触发对象，以及上述形参对象：
protected Object doInvoke(Object... args) throws Exception {
Method method = getBridgedMethod();
try {
return method.invoke(getBean(), args);
}
catch (IllegalArgumentException ex) {
// 省略相关代码
}
}
2.4 返回值处理
回到ServletInvocableHandlerMethod#invokeAndHandle()方法，此时获取了handler方法执行完成的返回值，会调用HandlerMethodReturnValueHandlerComposite#handleReturnValue()方法对返回值进行处理。首先会根据返回值信息MethodParameter对象查找支持的返回值处理器HandlerMethodReturnValueHandler，然后使用该处理器对返回值进行处理：
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
// 1、查找返回值处理器
HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
// 2、返回值处理
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}
在HandlerMethodReturnValueHandlerComposite#selectHandler方法中，会遍历returnValueHandlers，调用其HandlerMethodReturnValueHandler#supportsReturnType实现方法找到对应返回值处理器。：
private HandlerMethodReturnValueHandler selectHandler(@Nullable Object value, MethodParameter returnType) {
boolean isAsyncValue = isAsyncReturnValue(value, returnType);
for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
continue;
}
if (handler.supportsReturnType(returnType)) {
return handler;
}
}
return null;
}
找到返回值处理器后，就可以通过其handleReturnValue()方法对返回值进行处理。
举个有实战意义的例子，@ResponseBody的HandlerMethodReturnValueHandler实现类是RequestResponseBodyMethodProcessor。
RequestResponseBodyMethodProcessor的supportsReturnType()方法会判断返回值是否标有ResponseBody注解：
public boolean supportsReturnType(MethodParameter returnType) {
return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||
returnType.hasMethodAnnotation(ResponseBody.class));
}
RequestResponseBodyMethodProcessor的handleReturnValue()方法会根据返回的Content-Type对返回值进行对应格式化，并写入到输出流中：
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
mavContainer.setRequestHandled(true);
ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
// Try even with null return value. ResponseBodyAdvice could get involved.
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
至此，我们走完了RequestMappingHandlerAdapter对同步请求的完整处理流程（前后端分离）。简单来说，会经过一下主要步骤：
- 初始化请求处理的工具：
argumentResolvers、returnValueHandlers、binderFactory和parameterNameDiscoverer等。 - 解析形参对象
- 执行方法
- 返回值处理
实际上RequestMappingHandlerAdapter中还会对异步请求进行处理，这部分我们会在之后的文章进行详细介绍。
3 HandlerMethodArgumentResolver实现类
3.1 RequestResponseBodyMethodProcessor
RequestResponseBodyMethodProcessor是前后端分离项目中使用最多的HandlerMethodArgumentResolver实现类，它可以处理@RequestBody标注的形参。
3.1.1 supportsParameter()方法
RequestResponseBodyMethodProcessor#supportsParameter()方法会判断形参上是否标注@RequestBody注解：
public boolean supportsParameter(MethodParameter parameter) {
return parameter.hasParameterAnnotation(RequestBody.class);
}
3.1.2 resolveArgument()方法
RequestResponseBodyMethodProcessor#resolveArgument()方法会从输入流中读取数据，转换成形参对象，并且对其进行数据校验：
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
parameter = parameter.nestedIfOptional();
// 从输入流中读取数据，并构造成形参对象
Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
String name = Conventions.getVariableNameForParameter(parameter);
if (binderFactory != null) {
WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);
if (arg != null) {
// 数据校验
validateIfApplicable(binder, parameter);
if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {
throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());
}
}
if (mavContainer != null) {
mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
}
}
return adaptArgumentIfNecessary(arg, parameter);
}
AbstractMessageConverterMethodArgumentResolver#readWithMessageConverters()方法会根据Content-Type从输入流读取数据，并创建成形参对象：
protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
// 获取请求Content-Type
MediaType contentType;
boolean noContentType = false;
try {
contentType = inputMessage.getHeaders().getContentType();
}
catch (InvalidMediaTypeException ex) {
throw new HttpMediaTypeNotSupportedException(ex.getMessage());
}
if (contentType == null) {
noContentType = true;
contentType = MediaType.APPLICATION_OCTET_STREAM;
}
// 获取形参类型
Class<?> contextClass = parameter.getContainingClass();
Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);
if (targetClass == null) {
ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);
targetClass = (Class<T>) resolvableType.resolve();
}
HttpMethod httpMethod = (inputMessage instanceof HttpRequest ? ((HttpRequest) inputMessage).getMethod() : null);
Object body = NO_VALUE;
// 根据Content-Type使用对应messageConverter读取并转换数据
EmptyBodyCheckingHttpInputMessage message = null;
try {
message = new EmptyBodyCheckingHttpInputMessage(inputMessage);
for (HttpMessageConverter<?> converter : this.messageConverters) {
Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
GenericHttpMessageConverter<?> genericConverter =
(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
// 根据Content-Type获取对应的messageConverter
if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
(targetClass != null && converter.canRead(targetClass, contentType))) {
if (message.hasBody()) {
// RequestBodyAdvice#beforeBodyRead()处理
HttpInputMessage msgToUse =
getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
// 读取并转换数据
body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));
// RequestBodyAdvice#afterBodyRead()处理
body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);
}
else {
body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);
}
break;
}
}
}
catch (IOException ex) {
throw new HttpMessageNotReadableException("I/O error while reading input message", ex, inputMessage);
}
finally {
if (message != null && message.hasBody()) {
closeStreamIfNecessary(message.getBody());
}
}
if (body == NO_VALUE) {
if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||
(noContentType && !message.hasBody())) {
return null;
}
throw new HttpMediaTypeNotSupportedException(contentType,
getSupportedMediaTypes(targetClass != null ? targetClass : Object.class));
}
MediaType selectedContentType = contentType;
Object theBody = body;
LogFormatUtils.traceDebug(logger, traceOn -> {
String formatted = LogFormatUtils.formatValue(theBody, !traceOn);
return "Read \"" + selectedContentType + "\" to [" + formatted + "]";
});
return body;
}
AbstractMessageConverterMethodArgumentResolver#validateIfApplicable()方法会对标注javax.validation.Valid、org.springframework.validation.annotation.Validated以及以Valid开头的自定义注解进行参数校验：
protected void validateIfApplicable(WebDataBinder binder, MethodParameter parameter) {
Annotation[] annotations = parameter.getParameterAnnotations();
for (Annotation ann : annotations) {
Object[] validationHints = ValidationAnnotationUtils.determineValidationHints(ann);
if (validationHints != null) {
binder.validate(validationHints);
break;
}
}
}
4 HandlerMethodReturnValueHandler实现类
4.1 RequestResponseBodyMethodProcessor
RequestResponseBodyMethodProcessor是前后端分离项目中使用最多的HandlerMethodReturnValueHandler实现类，它可以处理@ResponseBody标注的返回值。
4.1.1 supportsReturnType()方法
RequestResponseBodyMethodProcessor#supportsReturnType()方法会判断类或方法上是否标注@RequestBody注解：
public boolean supportsReturnType(MethodParameter returnType) {
return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||
returnType.hasMethodAnnotation(ResponseBody.class));
}
4.1.2 handleReturnValue()方法
RequestResponseBodyMethodProcessor#handleReturnValue()方法会根据响应的Content-Type，将返回值格式化成对应数据格式，写道输出流进行响应：
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
mavContainer.setRequestHandled(true);
ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
// Try even with null return value. ResponseBodyAdvice could get involved.
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
实际业务在AbstractMessageConverterMethodProcessor#writeWithMessageConverters()方法，
protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
Object body;
Class<?> valueType;
Type targetType;
// 如果返回值是CharSequence类型，valueType和targetType都设置成String类型
if (value instanceof CharSequence) {
body = value.toString();
valueType = String.class;
targetType = String.class;
}
// 如果返回值不是CharSequence，valueType设置成对应返回值类型，targetType会设置成解析泛型后的返回值类型
else {
body = value;
valueType = getReturnValueType(body, returnType);
targetType = GenericTypeResolver.resolveType(getGenericType(returnType), returnType.getContainingClass());
}
// 如果是返回值继承自Resource
if (isResourceType(value, returnType)) {
outputMessage.getHeaders().set(HttpHeaders.ACCEPT_RANGES, "bytes");
if (value != null && inputMessage.getHeaders().getFirst(HttpHeaders.RANGE) != null &&
outputMessage.getServletResponse().getStatus() == 200) {
Resource resource = (Resource) value;
try {
List<HttpRange> httpRanges = inputMessage.getHeaders().getRange();
outputMessage.getServletResponse().setStatus(HttpStatus.PARTIAL_CONTENT.value());
body = HttpRange.toResourceRegions(httpRanges, resource);
valueType = body.getClass();
targetType = RESOURCE_REGION_LIST_TYPE;
}
catch (IllegalArgumentException ex) {
outputMessage.getHeaders().set(HttpHeaders.CONTENT_RANGE, "bytes */" + resource.contentLength());
outputMessage.getServletResponse().setStatus(HttpStatus.REQUESTED_RANGE_NOT_SATISFIABLE.value());
}
}
}
// 获取响应的Content-Type
MediaType selectedMediaType = null;
MediaType contentType = outputMessage.getHeaders().getContentType();
boolean isContentTypePreset = contentType != null && contentType.isConcrete();
if (isContentTypePreset) {
if (logger.isDebugEnabled()) {
logger.debug("Found 'Content-Type:" + contentType + "' in response");
}
selectedMediaType = contentType;
}
else {
HttpServletRequest request = inputMessage.getServletRequest();
List<MediaType> acceptableTypes;
try {
acceptableTypes = getAcceptableMediaTypes(request);
}
catch (HttpMediaTypeNotAcceptableException ex) {
int series = outputMessage.getServletResponse().getStatus() / 100;
if (body == null || series == 4 || series == 5) {
if (logger.isDebugEnabled()) {
logger.debug("Ignoring error response content (if any). " + ex);
}
return;
}
throw ex;
}
List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);
if (body != null && producibleTypes.isEmpty()) {
throw new HttpMessageNotWritableException(
"No converter found for return value of type: " + valueType);
}
List<MediaType> mediaTypesToUse = new ArrayList<>();
for (MediaType requestedType : acceptableTypes) {
for (MediaType producibleType : producibleTypes) {
if (requestedType.isCompatibleWith(producibleType)) {
mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
}
}
}
if (mediaTypesToUse.isEmpty()) {
if (logger.isDebugEnabled()) {
logger.debug("No match for " + acceptableTypes + ", supported: " + producibleTypes);
}
if (body != null) {
throw new HttpMediaTypeNotAcceptableException(producibleTypes);
}
return;
}
MediaType.sortBySpecificityAndQuality(mediaTypesToUse);
for (MediaType mediaType : mediaTypesToUse) {
if (mediaType.isConcrete()) {
selectedMediaType = mediaType;
break;
}
else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
break;
}
}
if (logger.isDebugEnabled()) {
logger.debug("Using '" + selectedMediaType + "', given " +
acceptableTypes + " and supported " + producibleTypes);
}
}
// 根据响应Content-Type格式化返回值，并写到输出流
if (selectedMediaType != null) {
selectedMediaType = selectedMediaType.removeQualityValue();
for (HttpMessageConverter<?> converter : this.messageConverters) {
GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
(GenericHttpMessageConverter<?>) converter : null);
// 根据响应Content-Type获取对应的messageConverter
if (genericConverter != null ?
((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
converter.canWrite(valueType, selectedMediaType)) {
// ResponseBodyAdvice的beforeBodyWrite()处理
body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) converter.getClass(),
inputMessage, outputMessage);
if (body != null) {
Object theBody = body;
LogFormatUtils.traceDebug(logger, traceOn ->
"Writing [" + LogFormatUtils.formatValue(theBody, !traceOn) + "]");
addContentDispositionHeader(inputMessage, outputMessage);
// 通过messageConverter格式化返回值，并写到输出流
if (genericConverter != null) {
genericConverter.write(body, targetType, selectedMediaType, outputMessage);
}
else {
((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Nothing to write: null body");
}
}
return;
}
}
}
}
