您正在将StringBuilder类的实例传递给JSONObject构造函数.

You are passing into the JSONObject constructor an instance of a StringBuilder class.

这是使用JSONObject(Object)构造函数，而不是JSONObject(String)构造函数.

This is using the JSONObject(Object) constructor, not the JSONObject(String) one.

您的代码应为:

JSONObject jsonObj = new JSONObject(jsonString.toString());