开始 controller 方法写的是   

    @RequestMapping( value = "/add", method = RequestMethod.POST )

    public String add( @RequestBody Map<String, Object> params ) {  

报错 Content type ‘application/x-www-form-urlencoded;charset=UTF-8‘ not supported

改成

    @RequestMapping( value = "/add", method = RequestMethod.POST )

    public String add( @RequestParam Map<String, Object> params ) {

就不报错了。
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作者：guile
来源：CSDN
原文：https://blog.csdn.net/beguile/article/details/80460957
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springboot 报错 Content type 'application/x-www-form-urlencoded;charset=UTF-8' not supported