spring mvc 下，ajax调用后台controller方法时报415 (Unsupported Media Type)错误

错误：ajax的post方法调用后台controller方法时报错：415 (Unsupported Media Type)。下面是错误时的代码

前端：

var url ="xxx/xxQuery.action";
var params={
			year:'2017'
		}				    					    	
	ajax(url,params,function(result){
			console.log(result);	
		
	})


function ajax(url,params,callback,contentType){
	  var ajaxoptions={
	        url:url,
	        data:params,
	        type:"POST",
	        dataType:"json",
	         		// contentType:",//application/x-www-form-urlencoded application/json"
	        success:function(res){
	         	callback(res);        			
	        },
	         error:function(){
	         	callback({success:false,description:"失败"});	         		 }
	   }
	if(contentType!=undefined){
	   			 ajaxoptions=$.extend(ajaxoptions,{contentType:contentType})
	   }
	   		 
	   $.ajax(ajaxoptions);
}

　　

后台：

@RequestMapping(value = { "/xxx/xxQuery.action" })
    @ResponseBody
    public Map<String, Object> xxQuery(Model model, HttpServletRequest request,
            @RequestBody ORParameter parameter) {

        ........
        return result;
    }

我的思路：对后台spring mvc不熟，只是有个概念服务器应该接收到了http请求，只是因为某种原因没有进入controller，而提前被拦截处理了。然后就开始找http处理的源头，笨办法,从HttpServlet.service开始找 -> DispatcherServlet.doDispatch,找到关键的处理方法handle

抛出错误的位置

package org.springframework.web.servlet.mvc.method.annotation. RequestResponseBodyMethodProcessor;

@Override
	protected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter methodParam,
			Type paramType) throws IOException, HttpMediaTypeNotSupportedException {

		HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
		HttpInputMessage inputMessage = new ServletServerHttpRequest(servletRequest);

		RequestBody ann = methodParam.getParameterAnnotation(RequestBody.class);
		if (!ann.required()) { //由于controller调用的这个方法参数加了@RequestBody，ann.required()这个为true就不走这里，

			InputStream inputStream = inputMessage.getBody();
			if (inputStream == null) {
				return null;
			}
			else if (inputStream.markSupported()) {
				inputStream.mark(1);
				if (inputStream.read() == -1) {
					return null;
				}
				inputStream.reset();
			}
			else {
				final PushbackInputStream pushbackInputStream = new PushbackInputStream(inputStream);
				int b = pushbackInputStream.read();
				if (b == -1) {
					return null;
				}
				else {
					pushbackInputStream.unread(b);
				}
				inputMessage = new ServletServerHttpRequest(servletRequest) {
					@Override
					public InputStream getBody() {
						// Form POST should not get here
						return pushbackInputStream;
					}
				};
			}
		}

		return super.readWithMessageConverters(inputMessage, methodParam, paramType);
	}

package org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver(类);


@SuppressWarnings("unchecked")
	protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage,
			MethodParameter methodParam, Type targetType) throws IOException, HttpMediaTypeNotSupportedException {

		MediaType contentType;
		try {
			contentType = inputMessage.getHeaders().getContentType();
		}
		catch (InvalidMediaTypeException ex) {
			throw new HttpMediaTypeNotSupportedException(ex.getMessage());
		}
		if (contentType == null) {
			contentType = MediaType.APPLICATION_OCTET_STREAM;
		}

		Class<?> contextClass = methodParam.getDeclaringClass();
		Map<TypeVariable, Type> map = GenericTypeResolver.getTypeVariableMap(contextClass);
		Class<T> targetClass = (Class<T>) GenericTypeResolver.resolveType(targetType, map);

		for (HttpMessageConverter<?> converter : this.messageConverters) {
			if (converter instanceof GenericHttpMessageConverter) {
				GenericHttpMessageConverter genericConverter = (GenericHttpMessageConverter) converter;
				if (genericConverter.canRead(targetType, contextClass, contentType)) {
					if (logger.isDebugEnabled()) {
						logger.debug("Reading [" + targetType + "] as "" +
								contentType + "" using [" + converter + "]");
					}
					return genericConverter.read(targetType, contextClass, inputMessage);
				}
			}
			if (targetClass != null) {
				if (converter.canRead(targetClass, contentType)) {
					if (logger.isDebugEnabled()) {
						logger.debug("Reading [" + targetClass.getName() + "] as "" +
								contentType + "" using [" + converter + "]");
					}
					return ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);
				}
			}
		}
　　　　　　　　　　//这里是报错位置
		throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);//没有找到参数对应的转换器就报错
	}

　　

 原因：@RequestBody

  　a) 该注解用于读取Request请求的body部分数据，根据request的header部分的Content-Type类型，匹配HttpMessageConverter进行解析，然后把相应的数据绑定到要返回的对象上；

　　b) 再把HttpMessageConverter返回的对象数据绑定到 controller中方法的参数上。

　　前台没有注明后台解析参考的content-type，默认为application/x-www-form-urlencoded ，同时参数是对象也是不是json字符串，后台找不到解析方法。

解决：content-type为application/json格式的数据可以被jackson解读(这里在canread判断时用到)，json对应jackson处理请求中的参数，jackson的参数应该是json字符串（enum JsonToken 这里有关于解析的字符串格式token的枚举内容： START_OBJECT("{").....）， 而不是application/x-www-form-urlencoded编码的字符串“year=2017“。

所以前端请求需改为： 

        ajax(url,JSON.stringify(params),function(result){
                console.log(result); 
                },"application/json")

　　

正确解析时，调用的转换器是jackson。

备注：调用到参数解析的调用栈

可参考：https://blog.csdn.net/mingtianhaiyouwo/article/details/51445345

1.浏览器请求时没有设置Content-Type为Json，对于浏览器来说就是要把POST的内容放到jsonData属性中，而不是params。

2.第三方工具没有设置Content-Type的功能，默认的是Content-Type: application/x-www-form-urlencoded