HTTP 400 error – invalid request (Bad request) is sometimes reported when ajax requests background data. The invalid request indicates that the request did not enter the background service;
Reasons: 1) The field name or field type of the data submitted by the front end is inconsistent with the entity class in the background, resulting in the failure of encapsulation;
2) The data submitted from the front end to the background should be of the JSON string type, while the front end did not convert the object to the string type;
Solutions:
1) Ensure consistency by comparing field names and types
2) Use Stringify to convert the object passed in front into string data: jSON.Stringify (param);

https://www.cnblogs.com/beppezhang/p/5824986.html