最后更新于：2018-10-09
SpringMVC(一) 初始化
SpringMVC(二) 请求调度
SpringMVC(三) 消息转换器
SpringMVC(四) 文件上传与下载
在大多数前后端分离的项目中，接口都是返回json类型的数据供前端使用，那么springmvc是怎么知道返回的是什么呢？一般我们会配置@ResponseBody来告诉springmvc返回数据而不是页面，将数据写入response body中。
源码分析
// RequestMappingHandlerAdapter
protected ModelAndView handleInternal(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ModelAndView mav;
// 如果有支持的方法，要求的session，检查request
checkRequest(request);
// Execute invokeHandlerMethod in synchronized block if required.
if (this.synchronizeOnSession) {
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
synchronized (mutex) {
mav = invokeHandlerMethod(request, response, handlerMethod);
}
}
else {
// No HttpSession available -> no mutex necessary
mav = invokeHandlerMethod(request, response, handlerMethod);
}
}
else {
// ----> 不需要同步 No synchronization on session demanded at all...
mav = invokeHandlerMethod(request, response, handlerMethod);
}
// 如果Header不存在Cache-Control
if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
// 通过SessionAttributes注解声明了session attributes
if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
}
else {
prepareResponse(response);
}
}
return mav;
}
1 . 调用HandlerMethod
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ServletWebRequest webRequest = new ServletWebRequest(request, response);
try {
WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
if (this.argumentResolvers != null) {
invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
}
if (this.returnValueHandlers != null) {
invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
}
invocableMethod.setDataBinderFactory(binderFactory);
invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
// 新建一个ModelAndViewContainer，里面有2个ModelMap(默认defaultModel、redirectModel)
ModelAndViewContainer mavContainer = new ModelAndViewContainer();
// 将所有的request中的attribute拷贝到mavContainer中
mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
// 初始化模型，将session域中的属性放入，调用标注了@ModelAttribute的方法并将结果放到mavContainer中，
// 找出@ModelAttribute修饰方法的参数，并确保放在模型中
modelFactory.initModel(webRequest, mavContainer, invocableMethod);
// 设置默认模型重定向是否忽略属性参数
mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);
AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
asyncWebRequest.setTimeout(this.asyncRequestTimeout);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.setTaskExecutor(this.taskExecutor);
asyncManager.setAsyncWebRequest(asyncWebRequest);
asyncManager.registerCallableInterceptors(this.callableInterceptors);
asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);
if (asyncManager.hasConcurrentResult()) {
Object result = asyncManager.getConcurrentResult();
mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
asyncManager.clearConcurrentResult();
if (logger.isDebugEnabled()) {
logger.debug("Found concurrent result value [" + result + "]");
}
invocableMethod = invocableMethod.wrapConcurrentResult(result);
}
// ----> 2. 调用handler方法
invocableMethod.invokeAndHandle(webRequest, mavContainer);
if (asyncManager.isConcurrentHandlingStarted()) {
return null;
}
return getModelAndView(mavContainer, modelFactory, webRequest);
}
finally {
webRequest.requestCompleted();
}
}
2 ，调用方法，处理返回结果
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 调用方法 -> 反射， 得到返回结果
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
// 根据@ResponseStatus注解设置状态
setResponseStatus(webRequest);
if (returnValue == null) {
if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
mavContainer.setRequestHandled(true);
return;
}
}
// 如果存在responseStatusReason
else if (StringUtils.hasText(getResponseStatusReason())) {
mavContainer.setRequestHandled(true);
return;
}
mavContainer.setRequestHandled(false);
Assert.state(this.returnValueHandlers != null, "No return value handlers");
try {
// ----> 3. 处理返回结果
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, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 数据绑定
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
"' with arguments " + Arrays.toString(args));
}
// 调用方法
Object returnValue = doInvoke(args);
if (logger.isTraceEnabled()) {
logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
"] returned [" + returnValue + "]");
}
return returnValue;
}
3 . 处理返回结果
// RequestResponseBodyMethodProcessor
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
// ----> 返回处理器: RequestResponseBodyMethodProcessor
HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
if (handler == null) {
throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
}
// ----> 处理返回结果
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}
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;
}
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
mavContainer.setRequestHandled(true);
// 根据NativeWebRequest创建ServletServerHttpRequest
ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
// 根据NativeWebRequest创建ServletServerHttpResponse
ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
// Try even with null return value. ResponseBodyAdvice could get involved.
// ----> 4. 根据给定返回类型写入输出消息
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
4 . 根据给定返回类型写入输出消息
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;
// 值类型
valueType = getReturnValueType(outputValue, returnType);
declaredType = getGenericType(returnType);
}
// 是否是Resource
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;
// 获取消息头里的contentType
MediaType contentType = outputMessage.getHeaders().getContentType();
if (contentType != null && contentType.isConcrete()) {
mediaTypesToUse = Collections.singletonList(contentType);
}
else {
HttpServletRequest request = inputMessage.getServletRequest();
// 获得可接受的媒体类型
List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
// 获取可生产的媒体类型: 如果在requestMapping定义了produces，直接获取: application/json
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) {
// 如果兼容
if (requestedType.isCompatibleWith(producibleType)) {
mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
}
}
}
if (mediaTypesToUse.isEmpty()) {
if (outputValue != null) {
throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
}
return;
}
// 排序
MediaType.sortBySpecificityAndQuality(mediaTypesToUse);
}
MediaType selectedMediaType = null;
for (MediaType mediaType : mediaTypesToUse) {
// 此MIME类型是否是具体的
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) {
// 返回副本(不包含quality value)
selectedMediaType = selectedMediaType.removeQualityValue();
// 消息转换器
for (HttpMessageConverter<?> converter : this.messageConverters) {
GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
(GenericHttpMessageConverter<?>) converter : null);
// 判断给定的类是否能被该转换器转换
if (genericConverter != null ?
((GenericHttpMessageConverter) converter).canWrite(declaredType, valueType, selectedMediaType) :
converter.canWrite(valueType, selectedMediaType)) {
// 获取增强执行前置操作
outputValue = getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) converter.getClass(),
inputMessage, outputMessage);
if (outputValue != null) {
// 添加Content-Disposition
addContentDispositionHeader(inputMessage, outputMessage);
if (genericConverter != null) {
// 转换
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);
}
}
问题
1 . returnValueHandlers是哪里配置的？
2 . messageConverters是哪里配置的？
初始化的时候，initServletBean方法中执行initWebApplicationContext初始化WebApplicationContext，执行applicationContext的refresh方法。在finishBeanFactoryInitialization(beanFactory)实例化时，会实例化RequestMappingHandlerAdapter。由于RequestMappingHandlerAdapter实现了InitializingBean接口。所以初始化时调用到afterPropertiesSet方法
// RequestMappingHandlerAdapter
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) {
// ----> 获取默认的handlers
List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
}
}
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;
}
那么转换器是哪里配置的呢？在构造器里只设置了4个，显然还有其他地方定义了
public RequestMappingHandlerAdapter() {
StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
stringHttpMessageConverter.setWriteAcceptCharset(false); // see SPR-7316
this.messageConverters = new ArrayList<>(4);
this.messageConverters.add(new ByteArrayHttpMessageConverter());
this.messageConverters.add(stringHttpMessageConverter);
this.messageConverters.add(new SourceHttpMessageConverter<>());
this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
}
发现在设置RequestMappingHandlerAdapter属性的时候加入了另外的全部转换器，那这些是哪里定义的呢？annotation-driven标签会注册AnnotationDrivenBeanDefinitionParser，在其parse方法中，定义了消息转换器
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);
}
// ClassUtils.isPresent("javax.xml.bind.Binder", AnnotationDrivenBeanDefinitionParser.class.getClassLoader())
else if (gsonPresent) {
messageConverters.add(createConverterDefinition(GsonHttpMessageConverter.class, source));
}
// ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", AnnotationDrivenBeanDefinitionParser.class.getClassLoader()) && ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
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;
}
所以，如果要加入jacksonConverter，还需要maven依赖
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.7</version>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-core</artifactId>
<version>2.9.7</version>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-annotations</artifactId>
<version>2.9.7</version>
</dependency>
自定义消息转换器
1 . 写一个类继承AbstractHttpMessageConverter类
2 . 在mvc:annotation-driven标签中添加子标签mvc:message-converters，里面配置定义好的bean
扩展：
浅谈Convert/Format机制与HttpMessageConverter的关系