返回值处理器
返回值处理器就是专门处理Handler方法的返回值的，比如执行完Controller中的一个具体方法，返回一个普通对象，如果此方法用@ResponseBody标注了，则返回的是一个JSON字符串，此时就是RequestResponseBodyMethodProcessor来处理的，我们看下返回值处理器的定义
public interface HandlerMethodReturnValueHandler {
/**
* Whether the given {@linkplain MethodParameter method return type} is
* supported by this handler.
* @param returnType the method return type to check
* @return {@code true} if this handler supports the supplied return type;
* {@code false} otherwise
* 根据指定的
*/
boolean supportsReturnType(MethodParameter returnType);
/**
* Handle the given return value by adding attributes to the model and
* setting a view or setting the
* {@link ModelAndViewContainer#setRequestHandled} flag to {@code true}
* to indicate the response has been handled directly.
* @param returnValue the value returned from the handler method
* @param returnType the type of the return value. This type must have
* previously been passed to {@link #supportsReturnType} which must
* have returned {@code true}.
* @param mavContainer the ModelAndViewContainer for the current request
* @param webRequest the current request
* @throws Exception if the return value handling results in an error
*/
void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;
}
RequestResponseBodyMethodProcessor 源码
//判断是否支持此类型
@Override
public boolean supportsReturnType(MethodParameter returnType) {
return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||
returnType.hasMethodAnnotation(ResponseBody.class));
}
//处理返回值
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);
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
初始化
因为后续一系列操作都是HandlerAdapter来完成的，所以参数解析器和返回值处理器都是在HandlerAdapter中初始化完成的
返回值处理器的初始化HandlerAdapter中完成，看下RequestMappingHandlerAdapter的源码，因为实现了InitializingBean接口，所以spring容器在加载bean时会执行void afterPropertiesSet() 方法
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);
}
}
getDefaultReturnValueHandlers()方法就是初始化返回值处理器的方法，会默认加载很多spring自定义的返回值处理器，基本满足绝大部分需求，其中的RequestResponseBodyMethodProcessor，也就是我们本章要学习的用来处理带有@ResponseBody注解的 返回值处理器
private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>();
// Single-purpose return value types
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));
// Annotation-based return value types
handlers.add(new ModelAttributeMethodProcessor(false));
handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),
this.contentNegotiationManager, this.requestResponseBodyAdvice));
// Multi-purpose return value types
handlers.add(new ViewNameMethodReturnValueHandler());
handlers.add(new MapMethodProcessor());
// Custom return value types
if (getCustomReturnValueHandlers() != null) {
handlers.addAll(getCustomReturnValueHandlers());
}
// Catch-all
if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
}
else {
handlers.add(new ModelAttributeMethodProcessor(true));
}
return handlers;
}
消息转换器
首先说明下HandlerAdapter的初始化是由标签mvc:annotation-driven来实现的，而标签的解析是由AnnotationDrivenBeanDefinitionParser来完成的，在加载过程中会初始化消息转换器messageConverters
如下代码可以解释为什么我们在使用JSON的时候，需要引用jackson相关的jar包，代码说明了如果类路径下存在某个类，则加载相应的消息转换器，针对注解ResponseBody，输出是JSON数据，需要MappingJackson2HttpMessageConverter这个消息转换器来处理的，所以当我们使用ResponseBody，也同时要引用jackson的jar包
class AnnotationDrivenBeanDefinitionParser implements BeanDefinitionParser {
public static final String HANDLER_MAPPING_BEAN_NAME = RequestMappingHandlerMapping.class.getName();
public static final String HANDLER_ADAPTER_BEAN_NAME = RequestMappingHandlerAdapter.class.getName();
public static final String CONTENT_NEGOTIATION_MANAGER_BEAN_NAME = "mvcContentNegotiationManager";
private static final boolean javaxValidationPresent =
ClassUtils.isPresent("javax.validation.Validator",
AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
private static boolean romePresent =
ClassUtils.isPresent("com.rometools.rome.feed.WireFeed",
AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
private static final boolean jaxb2Present =
ClassUtils.isPresent("javax.xml.bind.Binder",
AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
private static final boolean jackson2Present =
ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper",
AnnotationDrivenBeanDefinitionParser.class.getClassLoader()) &&
ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator",
AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
private static final boolean jackson2XmlPresent =
ClassUtils.isPresent("com.fasterxml.jackson.dataformat.xml.XmlMapper",
AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
private static final boolean jackson2SmilePresent =
ClassUtils.isPresent("com.fasterxml.jackson.dataformat.smile.SmileFactory",
AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
private static final boolean jackson2CborPresent =
ClassUtils.isPresent("com.fasterxml.jackson.dataformat.cbor.CBORFactory",
AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
private static final boolean gsonPresent =
ClassUtils.isPresent("com.google.gson.Gson", AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
}
private ManagedList<?> getMessageConverters(Element element, @Nullable Object source, ParserContext parserContext) {
Element convertersElement = DomUtils.getChildElementByTagName(element, "message-converters");
ManagedList<? super Object> messageConverters = new ManagedList<>();
if (convertersElement != null) {
messageConverters.setSource(source);
for (Element beanElement : DomUtils.getChildElementsByTagName(convertersElement, "bean", "ref")) {
Object object = parserContext.getDelegate().parsePropertySubElement(beanElement, null);
messageConverters.add(object);
}
}
if (convertersElement == null || Boolean.valueOf(convertersElement.getAttribute("register-defaults"))) {
messageConverters.setSource(source);
messageConverters.add(createConverterDefinition(ByteArrayHttpMessageConverter.class, source));
RootBeanDefinition stringConverterDef = createConverterDefinition(StringHttpMessageConverter.class, source);
stringConverterDef.getPropertyValues().add("writeAcceptCharset", false);
messageConverters.add(stringConverterDef);
messageConverters.add(createConverterDefinition(ResourceHttpMessageConverter.class, source));
messageConverters.add(createConverterDefinition(ResourceRegionHttpMessageConverter.class, source));
messageConverters.add(createConverterDefinition(SourceHttpMessageConverter.class, source));
messageConverters.add(createConverterDefinition(AllEncompassingFormHttpMessageConverter.class, source));
if (romePresent) {
messageConverters.add(createConverterDefinition(AtomFeedHttpMessageConverter.class, source));
messageConverters.add(createConverterDefinition(RssChannelHttpMessageConverter.class, source));
}
if (jackson2XmlPresent) {
Class<?> type = MappingJackson2XmlHttpMessageConverter.class;
RootBeanDefinition jacksonConverterDef = createConverterDefinition(type, source);
GenericBeanDefinition jacksonFactoryDef = createObjectMapperFactoryDefinition(source);
jacksonFactoryDef.getPropertyValues().add("createXmlMapper", true);
jacksonConverterDef.getConstructorArgumentValues().addIndexedArgumentValue(0, jacksonFactoryDef);
messageConverters.add(jacksonConverterDef);
}
else if (jaxb2Present) {
messageConverters.add(createConverterDefinition(Jaxb2RootElementHttpMessageConverter.class, source));
}
if (jackson2Present) {
Class<?> type = MappingJackson2HttpMessageConverter.class;
RootBeanDefinition jacksonConverterDef = createConverterDefinition(type, source);
GenericBeanDefinition jacksonFactoryDef = createObjectMapperFactoryDefinition(source);
jacksonConverterDef.getConstructorArgumentValues().addIndexedArgumentValue(0, jacksonFactoryDef);
messageConverters.add(jacksonConverterDef);
}
else if (gsonPresent) {
messageConverters.add(createConverterDefinition(GsonHttpMessageConverter.class, source));
}
if (jackson2SmilePresent) {
Class<?> type = MappingJackson2SmileHttpMessageConverter.class;
RootBeanDefinition jacksonConverterDef = createConverterDefinition(type, source);
GenericBeanDefinition jacksonFactoryDef = createObjectMapperFactoryDefinition(source);
jacksonFactoryDef.getPropertyValues().add("factory", new SmileFactory());
jacksonConverterDef.getConstructorArgumentValues().addIndexedArgumentValue(0, jacksonFactoryDef);
messageConverters.add(jacksonConverterDef);
}
if (jackson2CborPresent) {
Class<?> type = MappingJackson2CborHttpMessageConverter.class;
RootBeanDefinition jacksonConverterDef = createConverterDefinition(type, source);
GenericBeanDefinition jacksonFactoryDef = createObjectMapperFactoryDefinition(source);
jacksonFactoryDef.getPropertyValues().add("factory", new CBORFactory());
jacksonConverterDef.getConstructorArgumentValues().addIndexedArgumentValue(0, jacksonFactoryDef);
messageConverters.add(jacksonConverterDef);
}
}
return messageConverters;
}
找到对应的HttpMessageConverter，调用write方法，可以直接将Controller中的Action返回的结果写入到输出流中，调用flush方法，把数据输入到客户端，其实也就是直接操作HttpServletResponse这个输出流，然后向客户端输入数据
protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
Object outputValue;
Class<?> valueType;
Type declaredType;
if (value instanceof CharSequence) {
outputValue = value.toString();
valueType = String.class;
declaredType = String.class;
}
else {
outputValue = value;
valueType = getReturnValueType(outputValue, returnType);
declaredType = getGenericType(returnType);
}
if (isResourceType(value, returnType)) {
outputMessage.getHeaders().set(HttpHeaders.ACCEPT_RANGES, "bytes");
if (value != null && inputMessage.getHeaders().getFirst(HttpHeaders.RANGE) != null) {
Resource resource = (Resource) value;
try {
List<HttpRange> httpRanges = inputMessage.getHeaders().getRange();
outputMessage.getServletResponse().setStatus(HttpStatus.PARTIAL_CONTENT.value());
outputValue = HttpRange.toResourceRegions(httpRanges, resource);
valueType = outputValue.getClass();
declaredType = 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());
}
}
}
List<MediaType> mediaTypesToUse;
MediaType contentType = outputMessage.getHeaders().getContentType();
if (contentType != null && contentType.isConcrete()) {
mediaTypesToUse = Collections.singletonList(contentType);
}
else {
HttpServletRequest request = inputMessage.getServletRequest();
List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request, valueType, declaredType);
if (outputValue != null && producibleMediaTypes.isEmpty()) {
throw new HttpMessageNotWritableException(
"No converter found for return value of type: " + valueType);
}
mediaTypesToUse = new ArrayList<>();
for (MediaType requestedType : requestedMediaTypes) {
for (MediaType producibleType : producibleMediaTypes) {
if (requestedType.isCompatibleWith(producibleType)) {
mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
}
}
}
if (mediaTypesToUse.isEmpty()) {
if (outputValue != null) {
throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
}
return;
}
MediaType.sortBySpecificityAndQuality(mediaTypesToUse);
}
MediaType selectedMediaType = null;
for (MediaType mediaType : mediaTypesToUse) {
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();
for (HttpMessageConverter<?> converter : this.messageConverters) {
GenericHttpMessageConverter genericConverter =
(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
if (genericConverter != null ?
((GenericHttpMessageConverter) converter).canWrite(declaredType, valueType, selectedMediaType) :
converter.canWrite(valueType, selectedMediaType)) {
outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) converter.getClass(),
inputMessage, outputMessage);
if (outputValue != null) {
addContentDispositionHeader(inputMessage, outputMessage);
if (genericConverter != null) {
genericConverter.write(outputValue, declaredType, selectedMediaType, outputMessage);
}
else {
((HttpMessageConverter) converter).write(outputValue, selectedMediaType, outputMessage);
}
if (logger.isDebugEnabled()) {
logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
"\" using [" + converter + "]");
}
}
return;
}
}
}
if (outputValue != null) {
throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
}
}
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