Could not write JSON NullPointerException

报错：

Could not write JSON: (was java.lang.NullPointerException); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: entity["key1"]->entity2["key2"])

解决办法：json转换失败，可以去key2的get方法，看看是不是报异常了。例如报空指针