SpringMVC @RequestBody和@ResponseBody原理解析
前言
@RequestBody作用是将http请求解析为对应的对象。例如：
http请求的参数（application/json格式）：
{
"accountId": 10,
"adGroupId": "12345678",
"campaignId": "12345678",
"dataType": 0,
"sign": "abcdefg",
"site": "us",
"timeStamp": 1453250,
"userId": 10
}
通过@RequestBody可以解析为ProductSyncNegativeDto对象（如下代码所示）
public CustResponse syncNegative(@RequestBody ProductSyncNegativeDto productSyncNegativeDto)
那@RequestBody注解是如何实现http请求报文转对象的呢？
@ResponseBody作用是将返回的对象转为json字符串，例如我们返回一个CustResponse对象，那postman中的结果会是啥？
{
"code": 100,
"msg": "",
"details": [
10,
10,
"us",
"12345678",
"12345678",
0
]
}
我们可以发现，结果是一个json字符串，那@ResponseBody注解到底是如何将对象转为json字符串返回的呢？
接下来老师会带童鞋们一些来揭秘,@RequestBody、@ResponseBody的底层实现原理。
概述
@Controller注解
在开始之前，我们先来介绍一下@Controller，做过ssm/ssh项目的同学肯定都接触过springMVC，那必然会用到@Controller注解。Controller方法被封装成ServletInvocableHandlerMethod类，并且由invokeAndHandle方法完成请求处理。
HttpMessageConverter
SpringMVC处理请求和响应时，支持多种类型的请求参数和返回类型，而此种功能的实现就需要对HTTP消息体和参数及返回值进行转换，为此SpringMVC提供了大量的转换类，所有转换类都实现了HttpMessageConverter接口。
public interface HttpMessageConverter<T> {
// 当前转换器是否能将HTTP报文转换为对象类型
boolean canRead(Class<?> clazz, MediaType mediaType);
// 当前转换器是否能将对象类型转换为HTTP报文
boolean canWrite(Class<?> clazz, MediaType mediaType);
// 转换器能支持的HTTP媒体类型
List<MediaType> getSupportedMediaTypes();
// 转换HTTP报文为特定类型
T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException;
// 将特定类型对象转换为HTTP报文
void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException;
}
HttpMessageConverter接口定义了5个方法，用于将HTTP请求报文转换为java对象，以及将java对象转换为HTTP响应报文。
HandlerMethodArgumentResolver与HandlerMethodReturnValueHandler
对应到SpringMVC的Controller方法，read方法即是读取HTTP请求转换为参数对象，write方法即是将返回值对象转换为HTTP响应报文。SpringMVC定义了两个接口来操作这两个过程：参数解析器HandlerMethodArgumentResolver和返回值处理器HandlerMethodReturnValueHandler。
// 参数解析器接口
public interface HandlerMethodArgumentResolver {
// 解析器是否支持方法参数
boolean supportsParameter(MethodParameter parameter);
// 解析HTTP报文中对应的方法参数
Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception;
}
// 返回值处理器接口
public interface HandlerMethodReturnValueHandler {
// 处理器是否支持返回值类型
boolean supportsReturnType(MethodParameter returnType);
// 将返回值解析为HTTP响应报文
void handleReturnValue(Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;
}
参数解析器和返回值处理器在底层处理时，都是通过HttpMessageConverter进行转换。流程如下：
@RequestBody解析过程
所有的http请求都会进入ServletInvocableHandlerMethod类（继承InvocableHandlerMethod，所有的参数解析器都会在在这里面进行初始化）的invokeAndHandle方法中，我们来具体看看invokeAndHandle方法是干什么的。
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 执行http请求
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
setResponseStatus(webRequest);
if (returnValue == null) {
if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
disableContentCachingIfNecessary(webRequest);
mavContainer.setRequestHandled(true);
return;
}
}
else if (StringUtils.hasText(getResponseStatusReason())) {
mavContainer.setRequestHandled(true);
return;
}
mavContainer.setRequestHandled(false);
Assert.state(this.returnValueHandlers != null, "No return value handlers");
// 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;
}
}
我们可以看到invokeAndHandle方法都会进入invokeForRequest方法中，invokeForRequest方法就是实现@RequestBody注解的功能，将http请求报文解析为我们设置的对象。我们进入该方法看看，里面具体做了哪些事情。
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// http报文解析为对象数组
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Arguments: " + Arrays.toString(args));
}
//执行@PostMapping、@GetMapping等接口
return doInvoke(args);
}
我们可以看到invokeForRequest中主要做了两件事情，一个是通过getMethodArgumentValues方法返回http解析后的对象数组，然后通过doInvoke方法执行接口的具体业务逻辑代码。
我们接着进入getMethodArgumentValues方法，细看一下@RequestBody的具体解析过程。
protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 获取http请求参数
MethodParameter[] parameters = getMethodParameters();
if (ObjectUtils.isEmpty(parameters)) {
return EMPTY_ARGS;
}
Object[] args = new Object[parameters.length];
// 遍历所有参数，挨个解析
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 {
// 参数解析器解析HTTP报文
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;
}
其中this.resolvers.supportsParameter(parameter)用来判断请求参数是否合法，this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory)方法最终实现@RequestBody 解析操作。我们来看看 this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory) 中做了什么。
@Override
@Nullable
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
// 获取对应的解析器
HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
if (resolver == null) {
throw new IllegalArgumentException(
"Unsupported parameter type [" + parameter.getParameterType().getName() + "]." +
" supportsParameter should be called first.");
}
// 通过HandlerMethodArgumentResolver 解析器解析http报文
return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
}
getArgumentResolver方法来获取对应的HandlerMethodArgumentResolver参数解析器，参数解析器最终通过RequestResponseBodyMethodProcessor类来具体执行解析过程，我们接着来看看RequestResponseBodyMethodProcessor中resolveArgument方法又是怎样的一个处理过程。
不同的resolvers（HandlerMethodArgumentResolver接口）会对应不同的参数解析器，例如public String testDemo(String name)，解析器就会变成ServletRequestMethodArgumentResolver，如果是@RequestBody，参数解析器就是RequestResponseBodyMethodProcessor
@Override
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
parameter = parameter.nestedIfOptional();
// 通过HttpMessageConverter来解析http报文为Object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) {
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);
}
readWithMessageConverters方法中，HttpMessageConverter（接口对应实现类）的read方法实现了http报文解析，我们来看看最终http参数解析部分的代码。
protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
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);
// 判断转换器是否支持参数类型
if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
(targetClass != null && converter.canRead(targetClass, contentType))) {
if (message.hasBody()) {
HttpInputMessage msgToUse =
getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
// read方法执行HTTP报文到参数的转换
body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));
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);
}
}
代码部分省略了，关键部分即是遍历所有的HttpMessageConverter，然后通过canRead方法判断解析器是否支持，最后执行AbstractJackson2HttpMessageConverter对象（HttpMessageConverter实现类）的read方法完成最后的参数解析。
AbstractJackson2HttpMessageConverter对象的read方法，核心是利用了jackson工具，将http报文的json字符串转换为object对象并返回。
@ResponseBody返回值序列化过程
执行完doInvoke逻辑代码之后，通过ServletInvocableHandlerMethod对象的invokeAndHandle方法，利用返回值处理器对返回值进行序列化输出。
this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
returnValueHandlers为HandlerMethodReturnValueHandlerComposite对象，该对象实现了HandlerMethodReturnValueHandler接口，我们接着来看看handleReturnValue方法的具体实现。
@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
// 选择合适的HandlerMethodReturnValueHandler返回值处理器
HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
if (handler == null) {
throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
}
// 执行返回值处理
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}
selectHandler方法会提供合适的HandlerMethodReturnValueHandler，用来处理返回值。
我们看到的HandlerMethodReturnValueHandler处理器最终也是由RequestResponseBodyMethodProcessor实现的，我们具体来看看handleReturnValue方法。
handler（HandlerMethodReturnValueHandler）接口会根据不同类型选择不同的返回值处理器，例如页面跳转类型的处理器就是ViewNameMethodReturnValueHandler。
@Override
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);
// 调用HttpMessageConverter执行
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
for (HttpMessageConverter<?> converter : this.messageConverters) {
GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
(GenericHttpMessageConverter<?>) converter : null);
// 判断是否支持返回值类型，返回值类型很有可能不同，如String，Map，List等
if (genericConverter != null ?
((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
converter.canWrite(valueType, selectedMediaType)) {
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);
if (genericConverter != null) {
// 执行返回值转换
genericConverter.write(body, targetType, selectedMediaType, outputMessage);
}
else {
// 执行返回值转换
((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Nothing to write: null body");
}
}
return;
}
}
}
我们看到最终还是由HttpMessageConverter（AbstractGenericHttpMessageConverter实现类）的write方法来进行对象的序列化输出。
大家都知道@ResponseBody需要通过io流来读取，也就HttpMessageConverter最终的write会写入到io输出流中，上面的createOutputMessage(webRequest)方法就是创建一个输出流，我们来具体看看它的实现。
protected ServletServerHttpResponse createOutputMessage(NativeWebRequest webRequest) {
HttpServletResponse response = webRequest.getNativeResponse(HttpServletResponse.class);
Assert.state(response != null, "No HttpServletResponse");
return new ServletServerHttpResponse(response);
}
public class ServletServerHttpResponse implements ServerHttpResponse {
private final HttpServletResponse servletResponse;
private final HttpHeaders headers;
private boolean headersWritten = false;
private boolean bodyUsed = false;
/**
* Construct a new instance of the ServletServerHttpResponse based on the given {@link HttpServletResponse}.
* @param servletResponse the servlet response
*/
public ServletServerHttpResponse(HttpServletResponse servletResponse) {
Assert.notNull(servletResponse, "HttpServletResponse must not be null");
this.servletResponse = servletResponse;
this.headers = new ServletResponseHttpHeaders();
}
}
public interface ServletResponse {
String getCharacterEncoding();
String getContentType();
ServletOutputStream getOutputStream() throws IOException;
PrintWriter getWriter() throws IOException;
void setCharacterEncoding(String var1);
void setContentLength(int var1);
void setContentLengthLong(long var1);
void setContentType(String var1);
void setBufferSize(int var1);
int getBufferSize();
void flushBuffer() throws IOException;
void resetBuffer();
boolean isCommitted();
void reset();
void setLocale(Locale var1);
Locale getLocale();
}
createOutputMessage方法中创建了ServletServerHttpResponse ，然后通过 ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage)方法写入到输出流中。write方法的核心也是通过Jackson工具将对象解析为json字符串。我们最后来看看write的核心处理方法writeInternal。
protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException {
MediaType contentType = outputMessage.getHeaders().getContentType();
JsonEncoding encoding = getJsonEncoding(contentType);
JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputMessage.getBody(), encoding);
try {
writePrefix(generator, object);
Object value = object;
Class<?> serializationView = null;
FilterProvider filters = null;
JavaType javaType = null;
if (object instanceof MappingJacksonValue) {
MappingJacksonValue container = (MappingJacksonValue) object;
value = container.getValue();
serializationView = container.getSerializationView();
filters = container.getFilters();
}
if (type != null && TypeUtils.isAssignable(type, value.getClass())) {
javaType = getJavaType(type, null);
}
ObjectWriter objectWriter = (serializationView != null ?
this.objectMapper.writerWithView(serializationView) : this.objectMapper.writer());
if (filters != null) {
objectWriter = objectWriter.with(filters);
}
if (javaType != null && javaType.isContainerType()) {
objectWriter = objectWriter.forType(javaType);
}
SerializationConfig config = objectWriter.getConfig();
if (contentType != null && contentType.isCompatibleWith(MediaType.TEXT_EVENT_STREAM) &&
config.isEnabled(SerializationFeature.INDENT_OUTPUT)) {
objectWriter = objectWriter.with(this.ssePrettyPrinter);
}
objectWriter.writeValue(generator, value);
writeSuffix(generator, object);
generator.flush();
}
catch (InvalidDefinitionException ex) {
throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
}
catch (JsonProcessingException ex) {
throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getOriginalMessage(), ex);
}
}
@Override
public OutputStream getBody() throws IOException {
this.bodyUsed = true;
writeHeaders();
return this.servletResponse.getOutputStream();
}
objectWriter.writeValue(generator, value)方法中将value对象通过serialize序列化方法，将对象转为json字符串，然后设置到io流中。我们最后看看Jackson最终的序列化是怎么样的？
@Override
public final void serialize(Object bean, JsonGenerator gen, SerializerProvider provider)
throws IOException
{
if (_objectIdWriter != null) {
gen.setCurrentValue(bean); // [databind#631]
_serializeWithObjectId(bean, gen, provider, true);
return;
}
// 设置json的开始符号（"{"）
gen.writeStartObject(bean);
if (_propertyFilterId != null) {
// 循环将对象设置为json字符串 serializeFieldsFiltered(bean, gen, provider);
} else {
serializeFields(bean, gen, provider);
}
// 设置json的结束符号（"}"）
gen.writeEndObject();
}
在serialize方法中通过JsonGenerator将要返回的对象转为json格式的字符串。
springMVC初始化
至此我们就基本走完了一个HTTP请求和响应的过程。现在你可能有个疑惑，SpringMVC我们都是开箱即用，这些参数解析器和返回值处理器在哪里定义的呢？在核心的HandlerAdapter实现类RequestMappingHandlerAdapter的初始化方法中定义的。
而在RequestMappingHandlerAdapter构造时，也同时初始化了众多的HttpMessageConverter，以支持多样的转换需求。
WebMvcConfigurationSupport.java
protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
stringConverter.setWriteAcceptCharset(false);
messageConverters.add(new ByteArrayHttpMessageConverter());
messageConverters.add(stringConverter);
messageConverters.add(new ResourceHttpMessageConverter());
messageConverters.add(new SourceHttpMessageConverter<Source>());
messageConverters.add(new AllEncompassingFormHttpMessageConverter());
if (romePresent) {
messageConverters.add(new AtomFeedHttpMessageConverter());
messageConverters.add(new RssChannelHttpMessageConverter());
}
if (jackson2XmlPresent) {
ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.xml().applicationContext(this.applicationContext).build();
messageConverters.add(new MappingJackson2XmlHttpMessageConverter(objectMapper));
}
else if (jaxb2Present) {
messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
}
if (jackson2Present) {
ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.json().applicationContext(this.applicationContext).build();
messageConverters.add(new MappingJackson2HttpMessageConverter(objectMapper));
}
else if (gsonPresent) {
messageConverters.add(new GsonHttpMessageConverter());
}
}
五、相关依赖
大家可能会发现springboot项目都没有jackson相关的依赖，那为什么可以进行jackson的序列化呢，那是因为在spring-boot-starter-web依赖中其实已经包含了jackson相关的依赖。
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.8</version>
<scope>compile</scope>
<optional>true</optional>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.dataformat</groupId>
<artifactId>jackson-dataformat-cbor</artifactId>
<version>2.9.8</version>
<scope>compile</scope>
<optional>true</optional>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.dataformat</groupId>
<artifactId>jackson-dataformat-smile</artifactId>
<version>2.9.8</version>
<scope>compile</scope>
<optional>true</optional>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.dataformat</groupId>
<artifactId>jackson-dataformat-xml</artifactId>
<version>2.9.8</version>
<scope>compile</scope>
<optional>true</optional>
</dependency>
总结
看似简简单单的@RequestBody和@ResponseBody两个注解，其实内部做了大量的准备工作。现在童鞋们明白这整个过程的实现原理吧。
举例使用
@RequestBody是把整个HttpServletRequest的输入(request.getInputStream())，转换成一个对象，常用的转换是采用json方式，在spring中是RequestResponseBodyMethodProcessor利用HttpMessageConventer做的。
前端代码必须采用POST请求(GET请求的输入是空):
<!DOCTYPE html>
<html>
<head>
<title></title>
<script type="text/javascript" src="http://www.w3school.com.cn/jquery/jquery-1.11.1.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var saveDataAry=[];
var data1={
"userName":"test","address":"gz"};
var data2={
"userName":"ququ","address":"gr"};
saveDataAry.push(data1);
saveDataAry.push(data2);
$.ajax({
type:"POST",
url:"http://localhost:8080/test/mvc/saveUser",
dataType:"json",
contentType:"application/json",
data:JSON.stringify(saveDataAry),
success:function(data){
alert("succes" +data);
},
error:function(argument) {
alert("error" + argument);
}
});
});
</script>
</head>
<body>
test
</body>
</html>
后端代码
@Controller
public class UserController {
@RequestMapping(value = "saveUser")
@ResponseBody
public String saveUser(@RequestBody List<User> users) {
System.out.println("UserController.saveUser users size:"+users.size());
return "ok";
}
}
整体springmvc 调用链路
@RequestBody
在处理参数时候各种HandlerMethodArgumentResolver来解决参数实际需要传递参数，其中默认的有24中；而处理@RequestBody的是RequestResponseBodyMethodProcessor。相对的，处理@CookieValue注释的则是ServletCookieValueMethodArgumentResolver。
@ResponseBody
而responseBody同样是RequestResponseBodyMethodProcessor作为HandlerMethodReturnValueHandler，处理实际执行的controller HandlerMethod函数的return value.
public void handleReturnValue(Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException {
mavContainer.setRequestHandled(true); //最重要一步，设置请求已经处理过
if (returnValue != null) {
writeWithMessageConverters(returnValue, returnType, webRequest);
}
}
RequestResponseBodyMethodProcessor 的handleReturnValue（）函数处理时，首先ModelAndViewContainer.requestHandled设置成功，表明整个http请求已经处理，后续就不会执行veiw渲染了（跳过render view）.