浅析：

众所周知，前后端分离项目的前端我们是利用Jquery实现ajax异步请求与后端进行数据交互：意思就是从前端页面中取出数据，请求到对应的controller层方法上，然后后端对前端请求的数据进行业务逻辑处理，最后将对应的状态返回给前端；

HTTPMessageConverter原理：

将你项目中的实体类数据转为JSON数据或者倒转，利用读写方法；

设计restful风格的API，通过json数据进行前后端交互：数据如何解析的？SpringMVC中启动时会自动配置一些HttpMessageConverter，那么什么作用呢？能够支持json数据类型，当后端接收到请求时会判断是否能读，能读则读，返回结果时判断是否能写，能写则写；

public interface HttpMessageConverter<T> {boolean canRead(Class<?> var1, @Nullable MediaType var2);boolean canWrite(Class<?> var1, @Nullable MediaType var2);List<MediaType> getSupportedMediaTypes();T read(Class<? extends T> var1, HttpInputMessage var2) throws IOException, HttpMessageNotReadableException;void write(T var1, @Nullable MediaType var2, HttpOutputMessage var3) throws IOException, HttpMessageNotWritableException;
}

1、那么问题来了读归读写归写，但是你的数据得转换，那么转换数据是不是得需要Converter，那么在WebMvcConfigurationSupport类中，里面有一个非常关键的方法：addDefaultHttpMessageConverters()；

protected 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());}}

由上述可以知道这是用来判断的，找到数据需要的Converter转换器，如果需要的话，就添加到messageConverter中；这里需要注意当我们配置了自己的MessageConverter时(专门储存Converter的地方)，addDefaultHttpMessageConverters()方法就不会被SpringMVC调用；

2、getMessageConverter()：用来判断converters转换器是用自定义的还是springMVC默认配置的（这里有点迷，isEmpty是判断转换器中的内容吗？）：我的理解是：如果转换器为空(意思就是没东西取了)，我们就new一个集合(转换器的)，然后配置当前的转换器，（这里应该是我们某个数据需要某个转换器时，如果这个转换器没有，就new一个转换器集合，并配置一个对应这种数据的转换器，将数据转换）——也就是自己的转换器；然后后面判断转换器中内容是否为空，若为空，springMVC就会加载addDefaultHttpMessageConverters(),去获取属性对应的转换器；

protected final List<HttpMessageConverter<?>> getMessageConverters() {if (this.messageConverters == null) {this.messageConverters = new ArrayList<HttpMessageConverter<?>>();configureMessageConverters(this.messageConverters);if (this.messageConverters.isEmpty()) {addDefaultHttpMessageConverters(this.messageConverters);}extendMessageConverters(this.messageConverters);}return this.messageConverters;}

 

3、对于MessageConverters的处理：

遍历的每一个converter去比较，直至找到Jackson的处理类型；

// 遍历 messageConverters
for (HttpMessageConverter<?> converter : this.messageConverters) {Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();// 上文类关系图处要重点记住的地方，主要判断 MappingJackson2HttpMessageConverter 是否是 GenericHttpMessageConverter 类型if (converter instanceof GenericHttpMessageConverter) {GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;if (genericConverter.canRead(targetType, contextClass, contentType)) {if (logger.isDebugEnabled()) {logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");}if (inputMessage.getBody() != null) {inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType);body = genericConverter.read(targetType, contextClass, inputMessage);body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType);}else {body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType);}break;}}else if (targetClass != null) {if (converter.canRead(targetClass, contentType)) {if (logger.isDebugEnabled()) {logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");}if (inputMessage.getBody() != null) {inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType);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;}}
}

4、然后判断读取操作：能读就读，

protected Object _readMapAndClose(JsonParser p0, JavaType valueType)throws IOException{try (JsonParser p = p0) {Object result;JsonToken t = _initForReading(p);if (t == JsonToken.VALUE_NULL) {// Ask JsonDeserializer what 'null value' to use:DeserializationContext ctxt = createDeserializationContext(p,getDeserializationConfig());result = _findRootDeserializer(ctxt, valueType).getNullValue(ctxt);} else if (t == JsonToken.END_ARRAY || t == JsonToken.END_OBJECT) {result = null;} else {DeserializationConfig cfg = getDeserializationConfig();DeserializationContext ctxt = createDeserializationContext(p, cfg);JsonDeserializer<Object> deser = _findRootDeserializer(ctxt, valueType);if (cfg.useRootWrapping()) {result = _unwrapAndDeserialize(p, ctxt, cfg, valueType, deser);} else {result = deser.deserialize(p, ctxt);}ctxt.checkUnresolvedObjectId();}// Need to consume the token toop.clearCurrentToken();return result;}}

此时，解析已经全部完成，那么还有后端的结果（也就是返回给前端的状态）的过程没有完成；