现象

[{"delete": false,"index": 0,"list": [{"duration": 71000,"id": 0,"mediumId": 9231114101362,"path": "","point": 100,"title": "垃圾"},{"duration": 66000,"id": 0,"mediumId": 9231114112346,"path": "","point": 100,"title": "切糕"},{"$ref": "$[0].list[1]"},{"$ref": "$[0].list[0]"}],"tagId": "23080713857"}
]

解决方法

在list集合中出现"$ref":"$.list"表示在该list中存在重复引用的情况。为了解决这个问题，可以使用FastJson提供的关闭引用检测的功能。
JSON.toJSONString(list, SerializerFeature.DisableCircularReferenceDetect);

        List<Object> list = new ArrayList<>();Object obj1 = new Object();Object obj2 = new Object();list.add(obj1);list.add(obj2);list.add(list.get(1));String jsonString = JSON.toJSONString(list, SerializerFeature.DisableCircularReferenceDetect);System.out.println(jsonString);