来源：/java-chen-hao/p/11187914.html
1. 概述
在SpringMVC的使用时，往往会用到@RequestBody和@ResponseBody两个注解，尤其是处理ajax请求必然要使用@ResponseBody注解。这两个注解对应着Controller方法的参数解析和返回值处理，开始时都是只知其用，不知原理。我们来看个例子。
@RequestMapping("/requestBody")public void requestBody(@RequestBody String body, Writer writer) throws IOException{writer.write(body);}@RequestMapping(value="/responseBody", produces="application/json")@ResponseBodypublic Map<String, Object> responseBody(){Map<String, Object> retMap = new HashMap<>();retMap.put("param1", "abc");return retMap;}
第一个requestBody请求，使用@RequestBody将HTTP请求体转换成String类型，第二个responseBody请求，将Map对象转换成json格式输出到HTTP响应中。这两个请求方法没有什么特殊，就是一个在参数前加了@RequestBody注解，一个在方法上加了@ResponseBody注解。而这两个注解是怎么完成HTTP报文信息同Controller方法中对象的转换的呢？
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。
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：
SpringMVC为@RequestBody和@ResponseBody两个注解实现了统一处理类RequestResponseBodyMethodProcessor，实现了HandlerMethodArgumentResolver和HandlerMethodReturnValueHandler两个接口。
由上一篇文章我们可以知道，Controller方法被封装成ServletInvocableHandlerMethod类，并且由invokeAndHandle方法完成请求处理。
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {// 执行请求Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);// 返回值处理try {this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);}catch (Exception ex) {if (logger.isTraceEnabled()) {logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);}throw ex;}}public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {// 参数解析Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);// invoke Controller方法Object returnValue = doInvoke(args);return returnValue;}
在invoke Controller方法的前后分别执行了方法参数的解析和返回值的处理，我们分别来看。
2. 参数解析
private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {MethodParameter[] parameters = getMethodParameters();Object[] args = new Object[parameters.length];// 遍历所有参数，逐个解析for (int i = 0; i < parameters.length; i++) {MethodParameter parameter = parameters[i];parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);args[i] = resolveProvidedArgument(parameter, providedArgs);if (args[i] != null) {continue;}// 参数解析器解析HTTP报文到参数if (this.argumentResolvers.supportsParameter(parameter)) {args[i] = this.argumentResolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);continue;}}return args;}
getMethodArgumentValues方法中的argumentResolvers就是多个HandlerMethodArgumentResolver的集合体，supportsParameter方法寻找参数合适的解析器，resolveArgument调用具体解析器的resolveArgument方法执行。
我们从RequestResponseBodyMethodProcessor看看@RequestBody的解析过程。RequestResponseBodyMethodProcessor的supportsParameter定义了它支持的参数类型，即必须有@RequestBody注解。
public boolean supportsParameter(MethodParameter parameter) {return parameter.hasParameterAnnotation(RequestBody.class);}
再来看resolveArgument方法
public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {parameter = parameter.nestedIfOptional();// 通过HttpMessageConverter读取HTTP报文Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());String name = Conventions.getVariableNameForParameter(parameter);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());}}mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());return adaptArgumentIfNecessary(arg, parameter);}
具体实现由HttpMessageConverter来完成
protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {....try {inputMessage = new EmptyBodyCheckingHttpInputMessage(inputMessage);for (HttpMessageConverter<?> converter : this.messageConverters) {Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();....// 判断转换器是否支持参数类型if (converter.canRead(targetClass, contentType)) {if (inputMessage.getBody() != null) {inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType);// read方法执行HTTP报文到参数的转换body = ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType);}else {body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType);}break;}...}}catch (IOException ex) {throw new HttpMessageNotReadableException("I/O error while reading input message", ex);}....return body;}
代码部分省略了，关键部分即是遍历所有的HttpMessageConverter，通过canRead方法判断转换器是否支持对参数的转换，然后执行read方法完成转换。
3.返回值处理
完成Controller方法的调用后，在ServletInvocableHandlerMethod的invokeAndHandle中，使用返回值处理器对返回值进行转换。
this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
这里的returnValueHandlers也是HandlerMethodReturnValueHandler的集合体HandlerMethodReturnValueHandlerComposite
public void handleReturnValue(Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {// 选择合适的HandlerMethodReturnValueHandler，如果没有用@ResposeBody注解和用了注解其返回值处理器肯定不同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方法遍历所有HandlerMethodReturnValueHandler，调用其supportsReturnType方法选择合适的HandlerMethodReturnValueHandler，然后调用其handleReturnValue方法完成处理。
这里还是以RequestResponseBodyMethodProcessor来分析下@ResponseBody的处理，它的具体实现在AbstractMessageConverterMethodProcessor抽象基类中。
public boolean supportsReturnType(MethodParameter returnType) {return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||returnType.hasMethodAnnotation(ResponseBody.class));}
RequestResponseBodyMethodProcessor要求方法上有@ResponseBody注解或者方法所在的Controller类上有@ResponseBody的注解。这就是常常用@RestController注解代替@Controller注解的原因，因为@RestController注解自带@ResponseBody。
handleReturnValue方法实际也是调用HttpMessageConverter来完成转换处理
public void handleReturnValue(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(T value, MethodParameter returnType,ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {....if (selectedMediaType != null) {selectedMediaType = selectedMediaType.removeQualityValue();for (HttpMessageConverter<?> messageConverter : this.messageConverters) {// 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 (messageConverter.canWrite(valueType, selectedMediaType)) {outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),inputMessage, outputMessage);if (outputValue != null) {addContentDispositionHeader(inputMessage, outputMessage);// 执行返回值转换((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);...}return;}}}....}
使用canWrite方法选择合适的HttpMessageConverter，然后调用write方法完成转换。
我们看看传入的参数 ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
protected ServletServerHttpResponse createOutputMessage(NativeWebRequest webRequest) {//获取HttpServletResponseHttpServletResponse response = (HttpServletResponse)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;public ServletServerHttpResponse(HttpServletResponse servletResponse) {Assert.notNull(servletResponse, "HttpServletResponse must not be null");//将获取的HttpServletResponse作为ServletServerHttpResponse的属性值this.servletResponse = servletResponse;this.headers = new ServletServerHttpResponse.ServletResponseHttpHeaders();}}public interface ServletResponse {String getCharacterEncoding();String getContentType();//ServletResponse有一个输出流对象，保存需要相应客户端的字节流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();}
我们具体看看 ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);
protected void writeInternal(Object obj, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {HttpHeaders headers = outputMessage.getHeaders();//创建一个数组字节流缓冲对象ByteArrayOutputStream outnew = new ByteArrayOutputStream();//将obj对象转换成JSON并写入ByteArrayOutputStream中int len = JSON.writeJSONString(outnew, this.fastJsonConfig.getCharset(), obj, this.fastJsonConfig.getSerializeConfig(), this.fastJsonConfig.getSerializeFilters(), this.fastJsonConfig.getDateFormat(), JSON.DEFAULT_GENERATE_FEATURE, this.fastJsonConfig.getSerializerFeatures());headers.setContentLength((long)len);//获取ServletResponse的输出流对象 OutputStream out = outputMessage.getBody();//将转换后的outnew写入ServletResponse的输出流对象，这样就可以给客户端响应数据了outnew.writeTo(out);outnew.close();}public OutputStream getBody() throws IOException {this.bodyUsed = true;this.writeHeaders();//获取ServletResponse的输出流对象 //ServletOutputStream getOutputStream() throws IOException;return this.servletResponse.getOutputStream();}
最后我们看看JSON是怎么将obj对象转换成JSON对象的流
就是做一些循环拼接。
至此我们基本走完了一个HTTP请求报文经过处理后到HTTP响应报文的转换过程。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.javaprotected 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());}}
对于json或xml的转换方式，只要引入了jackson的依赖，即可自动发现，并注册相关的转换器。
<dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-databind</artifactId><version>2.9.0</version></dependency><dependency><groupId>com.fasterxml.jackson.dataformat</groupId><artifactId>jackson-dataformat-xml</artifactId><version>2.9.0</version></dependency>
