Spring-data-mongo无法使用构造函数实例化java.util.List

Using spring-data-mongodb-1.5.4 and mongodb-driver-3.4.2

我有课Hotel

    public class Hotel {

        private String name;
        private int pricePerNight;
        private Address address;
        private List<Review> reviews;
//getter, setter, default constructor, parameterized constructor 

Review班级 ：

public class Review {

    private int rating;
    private String description;
    private User user;
    private boolean isApproved;
 //getter, setter, default constructor, parameterized constructor 

当我打电话时Aggregation.unwind("reviews");它抛出

org.springframework.data.mapping.model.MappingInstantiationException： 无法使用构造函数 NO_CONSTRUCTOR 实例化 java.util.List 有参数

UnwindOperation unwindOperation = Aggregation.unwind("reviews");
Aggregation aggregation = Aggregation.newAggregation(unwindOperation);
AggregationResults<Hotel> results=mongoOperations.aggregate(aggregation,"hotel", Hotel.class);

I see 这个问题 https://stackoverflow.com/questions/26361297/why-is-spring-data-mongodb-unable-to-instantiate-this-nested-type-structure但对我没有帮助。

如何解决这个问题？

当你$unwind reviews字段，查询的返回 json 结构与您的不匹配Hotel不再上课了。因为$unwind操作使reviews子对象而不是列表。如果您在 robomongo 或其他一些工具中尝试查询，您可以看到您的返回对象是这样的

{
  "_id" : ObjectId("59b519d72f9e340bcc830cb3"),
  "id" : "59b23c39c70ff63135f76b14",
  "name" : "Signature",
  "reviews" : {
    "id" : 1,
    "userName" : "Salman",
    "rating" : 8,
    "approved" : true
  }
}

所以你应该使用另一个类而不是Hotellike UnwindedHotel

public class UnwindedHotel {

    private String name;
    private int pricePerNight;
    private Address address;
    private Review reviews;
}

UnwindOperation unwindOperation = Aggregation.unwind("reviews");
Aggregation aggregation = Aggregation.newAggregation(unwindOperation);
AggregationResults<UnwindedHotel> results=mongoOperations.aggregate(aggregation,"hotel", UnwindedHotel.class);