MongoDB中试图实例化抽象类时发生异常

Question

我们正在将Spring Data用于Mongo DB 1.3.3，并且在检索其中嵌套对象之一为抽象类型的文档时遇到异常。 如果我们将User类更改为具体的，则不会抛出异常并且代码可以正常工作。

BSON结构：

{ "_id" : { "$oid" : "52d99dec88c6bd5da9c05a86"},
     "_class": "com.xyz.Quote" , 
     "name" : "sssds" ,
     "quoteNumber" : 1006 ,
      "account" : { "accountNumber" : "060926" ,
                     "phone" : [ ] , "fax" : [ ] ,
                 "leadDealerAccount" : "066418" , 
................................................
"quoteCreatedBy" : { "_class" : "com.xyz.DealerUser" , 
                     "lastName" : "TILLMAN" ,
                    "firstName" : "TIMOTHY" ,
                   "middleName" : "M" 
}
}

POJO类：

public abstract class User {
   private String lastName;
   private String firstName; 
} 

public class DealerUser extends User {
   private Account primaryAccount;
}

public class AnotherUser extends User{
   private List<Address> availableAddresses;
}

public class Quote {
    private User quoteCreatedBy;
}

代码：

Query searchQuoteQuery = new Query(Criteria.where("account.leadDealerAccount"). is(066418));
List<Quote> mongoTemplate.find(searchQuoteQuery , Quote.class);

Exception最后一行引发以下异常：

[2/7/14 12:54:14:019 CST] 00000032 SystemErr R org.springframework.data.mapping.model.MappingInst antiationException: Failed to instantiate com.deere.mp.common.domain.User using constructor public com.deere.mp.common.domain.User() with arguments
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.convert.ReflectionEntityI nstantiator.createInstance(ReflectionEntityInstantiator.java:78)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.read(MappingMongoConverter.java:243)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.read(MappingMongoConverter.java:223)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.readValue(MappingMongoConverter.java:1050)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.access$100(MappingMongoConverter.java:77)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter$MongoDbPropertyValueProvider.get PropertyValue(MappingMongoConverter.java:999)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.getValueInternal(MappingMongoConverter.java:755)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter$1.doWithPersistentProperty(MappingMongoConverter.java:256)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter$1.doWithPersistentProperty(MappingMongoConverter.java:249)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mapping.model.BasicPersis tentEntity.doWithProperties(BasicPersistentEntity.java:257)
[2/7/14 12:54:14:020 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.read(MappingMongoConverter.java:249)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.read(MappingMongoConverter.java:223)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.read(MappingMongoConverter.java:187)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.read(MappingMongoConverter.java:183)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.convert.Mapp ingMongoConverter.read(MappingMongoConverter.java:77)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.MongoTemplat e$ReadDbObjectCallback.doWith(MongoTemplate.java:1975)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.MongoTemplat e.executeFindMultiInternal(MongoTemplate.java:1671)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.MongoTemplat e.doFind(MongoTemplate.java:1496)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.MongoTemplat e.doFind(MongoTemplate.java:1480)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.MongoTemplat e.find(MongoTemplate.java:527)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at org.springframework.data.mongodb.core.MongoTemplat e.find(MongoTemplate.java:518)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at com.deere.mp.quote.dao.impl.QuoteDAOImpl.findByUse rId(QuoteDAOImpl.java:137)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:60)
[2/7/14 12:54:14:021 CST] 00000032 SystemErr R at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)

Answer 1

用@ BsonDiscriminator注释类：

@org.bson.codecs.pojo.annotations.BsonDiscriminator()
public class DealerUser {}

同样，如果User类是其他类的属性/字段，则必须使用@@ BsonProperty（useDiscriminator = true）注释该字段：

public class Some class {

    @org.bson.codecs.pojo.annotations.BsonProperty(useDiscriminator = true)
    private User user;

}

Answer 2

从stacktrace（以及Java基本逻辑）非常明显，您无法实例化Java中的抽象类。那么，您如何期望Spring Data API将JSON数据填充到无法实例化的类中？ Spring将从mongodb中获取数据，并尝试将相同的数据推送到指定类的POJO中（在您的情况下，该类是抽象的且无法实例化）。