上一篇博客springMVC源码分析--HttpMessageConverter参数read操作中我们已经简单介绍了参数值转换的read操作，接下来我们介绍一下返回值的处理操作。同样返回值的操作操作也是在HandlerMethodReturnValueHandler中处理的，可以参考一下老田写的springMVC源码分析--HandlerMethodReturnValueHandler返回值解析器（一）
简单的返回值处理示例使用@ResponseBody进行注解：
@ResponseBody
@RequestMapping("/get")
public Object get(){
Product product = new Product();
product.setDescription("hello springMVC RestFul");
product.setId(10);
product.setName("springMVC");
product.setPrice(10);
return product; //在页面中返回json数据
}
这里返回值是一个Product对象，但真正浏览器获取的数据是json数据，处理的过程就是在HttpMessageConverter中实现的 。
配置一下消息处理使用FastJSON处理的配置
<mvc:annotation-driven> <mvc:message-converters register-defaults="true"> <bean class="com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter"> <property name="supportedMediaTypes"> <array> <value>text/html;charset=UTF-8</value> </array> </property> <property name="features"> <array> <value>WriteMapNullValue</value> <value>WriteNullStringAsEmpty</value> <value>DisableCircularReferenceDetect</value> </array> </property> </bean> </mvc:message-converters> </mvc:annotation-driven>
这样返回值的处理操作就是使用FastJsonHttpMessageConverter来进行处理，将返回值转为json数据返回。
返回值的处理是在HandlerMethodReturnValueHandlerComposite中handleReturnValue中实现的。
@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
Assert.notNull(handler, "Unknown return value type [" + returnType.getParameterType().getName() + "]");
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}
接下来是在子类RequestResponseBodyMethodProcessor的handleReturnValue中处理操作
@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
mavContainer.setRequestHandled(true);
// Try even with null return value. ResponseBodyAdvice could get involved.
writeWithMessageConverters(returnValue, returnType, webRequest);
}
在RequestResponseBodyMethodProcessor的父类AbstractMessageConverterMethodProcessor的writeWithMessageConverters中处理操作
protected <T> void writeWithMessageConverters(T returnValue, MethodParameter returnType, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
接下来就是通过职责链模式选择HttpMessageConverter的实现类来进行数据转换操作。
@SuppressWarnings("unchecked")
protected <T> void writeWithMessageConverters(T returnValue, MethodParameter returnType,
ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
Class<?> returnValueClass = getReturnValueType(returnValue, returnType);
Type returnValueType = getGenericType(returnType);
HttpServletRequest servletRequest = inputMessage.getServletRequest();
List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(servletRequest);
List<MediaType> producibleMediaTypes = getProducibleMediaTypes(servletRequest, returnValueClass, returnValueType);
if (returnValue != null && producibleMediaTypes.isEmpty()) {
throw new IllegalArgumentException("No converter found for return value of type: " + returnValueClass);
}
Set<MediaType> compatibleMediaTypes = new LinkedHashSet<MediaType>();
for (MediaType requestedType : requestedMediaTypes) {
for (MediaType producibleType : producibleMediaTypes) {
if (requestedType.isCompatibleWith(producibleType)) {
compatibleMediaTypes.add(getMostSpecificMediaType(requestedType, producibleType));
}
}
}
if (compatibleMediaTypes.isEmpty()) {
if (returnValue != null) {
throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
}
return;
}
List<MediaType> mediaTypes = new ArrayList<MediaType>(compatibleMediaTypes);
MediaType.sortBySpecificityAndQuality(mediaTypes);
MediaType selectedMediaType = null;
for (MediaType mediaType : mediaTypes) {
if (mediaType.isConcrete()) {
selectedMediaType = mediaType;
break;
}
else if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION)) {
selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
break;
}
}
if (selectedMediaType != null) {
selectedMediaType = selectedMediaType.removeQualityValue();
//匹配消息数据转换器
for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
if (messageConverter instanceof GenericHttpMessageConverter) {
if (((GenericHttpMessageConverter<T>) messageConverter).canWrite(returnValueType,
returnValueClass, selectedMediaType)) {
returnValue = (T) getAdvice().beforeBodyWrite(returnValue, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
inputMessage, outputMessage);
if (returnValue != null) {
addContentDispositionHeader(inputMessage, outputMessage);
//进行消息转换成配置的格式
((GenericHttpMessageConverter<T>) messageConverter).write(returnValue,
returnValueType, selectedMediaType, outputMessage);
if (logger.isDebugEnabled()) {
logger.debug("Written [" + returnValue + "] as \"" +
selectedMediaType + "\" using [" + messageConverter + "]");
}
}
return;
}
}
else if (messageConverter.canWrite(returnValueClass, selectedMediaType)) {
returnValue = (T) getAdvice().beforeBodyWrite(returnValue, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
inputMessage, outputMessage);
if (returnValue != null) {
addContentDispositionHeader(inputMessage, outputMessage);
//将数据转换为配置的数据格式
((HttpMessageConverter<T>) messageConverter).write(returnValue,
selectedMediaType, outputMessage);
if (logger.isDebugEnabled()) {
logger.debug("Written [" + returnValue + "] as \"" +
selectedMediaType + "\" using [" + messageConverter + "]");
}
}
return;
}
}
}
if (returnValue != null) {
throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
}
}
最终会选择FastJsonHttpMessageConverter 的write方法中进行处理操作，就是将数据转换为json写到输出流中
@Override
protected void writeInternal(Object obj, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException {
HttpHeaders headers = outputMessage.getHeaders();
ByteArrayOutputStream outnew = new ByteArrayOutputStream();
int len = JSON.writeJSONString(outnew, //
fastJsonConfig.getCharset(), //
obj, //
fastJsonConfig.getSerializeConfig(), //
fastJsonConfig.getSerializeFilters(), //
fastJsonConfig.getDateFormat(), //
JSON.DEFAULT_GENERATE_FEATURE, //
fastJsonConfig.getSerializerFeatures());
headers.setContentLength(len);
OutputStream out = outputMessage.getBody();
outnew.writeTo(out);
outnew.close();
}
springMVC默认提供了很多参数和结果值处理器，包括如下：
（1）MappingJackson2HttpMessageConverter
（2）GsonHttpMessageConverter
（3）ByteArrayHttpMessageConverter
（4）ObjectToStringHttpMessageConverter
（5）ProtobufHttpMessageConverter
（6）ResourceHttpMessageConverter
（7）StringHttpMessageConverter
（8）AllEncompassingFormHttpMessageConverter