JSON格式化异常：JsonMappingException

使用RestTemplate请求post json接口报错：Caused by: org.codehaus.jackson.map.JsonMappingException: Can not instantiate value of type [simple type, class com.xx.xxxbean] from JSON String; no single-String constructor/factory method (through reference chain: com.xxxx.xxxx["body"])

修改为使用map，而不是json字符串。

具体现象

刚开始请求时HttpEntity设置的参数为json字符串，HttpEntity<String> request = new HttpEntity<>(jsonParam, headers);请求时报如上错误

Map<String, String> param = new HashMap();
param.put("userName", "tom");
param.put("password", encrypt);
String jsonParam = JSON.toJSONString(param);
HttpHeaders headers = new HttpHeader