spring mvc @ResponseBody为什么会返回json字符串
spring mvc 处理返回值要从RequestMappingHandlerAdapter 说起
RequestMappingHandlerAdapter 实现了接口InitializingBean
- RequestMappingHandlerAdapter bean 创建 的入口，spring boot 中是在
org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport配置类中
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(
@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
@Qualifier("mvcConversionService") FormattingConversionService conversionService,
@Qualifier("mvcValidator") Validator validator) {
RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();
adapter.setContentNegotiationManager(contentNegotiationManager);
// getMessageConverters 获取转换器
adapter.setMessageConverters(getMessageConverters());
adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer(conversionService, validator));
adapter.setCustomArgumentResolvers(getArgumentResolvers());
adapter.setCustomReturnValueHandlers(getReturnValueHandlers());
if (jackson2Present) {
adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));
adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));
}
AsyncSupportConfigurer configurer = new AsyncSupportConfigurer();
configureAsyncSupport(configurer);
if (configurer.getTaskExecutor() != null) {
adapter.setTaskExecutor(configurer.getTaskExecutor());
}
if (configurer.getTimeout() != null) {
adapter.setAsyncRequestTimeout(configurer.getTimeout());
}
adapter.setCallableInterceptors(configurer.getCallableInterceptors());
adapter.setDeferredResultInterceptors(configurer.getDeferredResultInterceptors());
return adapter;
}
- getMessageConverters 获取转换器
protected final List<HttpMessageConverter<?>> getMessageConverters() {
if (this.messageConverters == null) {
this.messageConverters = new ArrayList<>();
configureMessageConverters(this.messageConverters);
if (this.messageConverters.isEmpty()) {
// 添加默认的转换器
addDefaultHttpMessageConverters(this.messageConverters);
}
extendMessageConverters(this.messageConverters);
}
return this.messageConverters;
}
- addDefaultHttpMessageConverters添加默认的转换器，其中 就有MappingJackson2HttpMessageConverter 转换器可以转json
protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
messageConverters.add(new ByteArrayHttpMessageConverter());
messageConverters.add(new StringHttpMessageConverter());
messageConverters.add(new ResourceHttpMessageConverter());
messageConverters.add(new ResourceRegionHttpMessageConverter());
try {
messageConverters.add(new SourceHttpMessageConverter<>());
}
catch (Throwable ex) {
// Ignore when no TransformerFactory implementation is available...
}
messageConverters.add(new AllEncompassingFormHttpMessageConverter());
if (romePresent) {
messageConverters.add(new AtomFeedHttpMessageConverter());
messageConverters.add(new RssChannelHttpMessageConverter());
}
if (jackson2XmlPresent) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.xml();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2XmlHttpMessageConverter(builder.build()));
}
else if (jaxb2Present) {
messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
}
//这里就是转json 的转换器MappingJackson2HttpMessageConverter
if (jackson2Present) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
}
else if (gsonPresent) {
messageConverters.add(new GsonHttpMessageConverter());
}
else if (jsonbPresent) {
messageConverters.add(new JsonbHttpMessageConverter());
}
if (jackson2SmilePresent) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.smile();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2SmileHttpMessageConverter(builder.build()));
}
if (jackson2CborPresent) {
Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.cbor();
if (this.applicationContext != null) {
builder.applicationContext(this.applicationContext);
}
messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
}
}
- RequestMappingHandlerAdapter 初始化HandlerMethodReturnValueHandler
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) {
// 获取默认返回值处理器
List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
}
}
- 获取默认返回值处理器
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));
//
//
//
//
/**
* 我们经常用的返回json 就是由RequestResponseBodyMethodProcessor 这个处理器处理的
* 转成json 用到了getMessageConverters() 返回的转化器
* getMessageConverters 获取的就是在
* org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#requestMappingHandlerAdapter 方法中初始化的转换器
*/
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;
}
请求过来后按下面的路径获取json返回值
org.springframework.web.servlet.DispatcherServlet#doDispatch
org.springframework.web.servlet.DispatcherServlet#getHandlerAdapter
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter#handle
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#handleInternal
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#invokeHandlerMethod
org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite#handleReturnValue
org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor#handleReturnValue
org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#writeWithMessageConverters
org.springframework.http.converter.AbstractGenericHttpMessageConverter#write
参考博客