Failed to instantiate * using constructor public * with arguments **

今天在从MongoDB中读取数据到包含有内部类的结构中去的时候一直抛出异常，跟了半天也不知道是什么原因导致的。后来偶然看到
Stackoverflow上面一哥们也遇到同样的问题，然后才解决了，做一下大自然的搬运工，记录一篇中文博客。
保存在MongoDB中的数据结构如下：

{    _id: "A",    groups:[{        groupId: "someId",        groupName: "someName",        params: {            type1: ["a", "b"],            type2: ["c", d]        }    }],    config: {        person: {}        dataDetails: {            dataTypeDetails: {},            dataList: ["dt1", "dt2"]        }    }}

Java类定义如下：

// Imports etc.@Documentpublic class Entity {    @Id    private String _id;    private List<Group> groups;    private Config config;    // setters and getters    public class Group {        private String groupId;        private String groupName;        private ParamData params;        // setter and getters    }    public class ParamData {        private List<String> type1;        private List<String> type2;    }    public class Config {        private Map person;        private DataConfig dataDetails;    }    public class DataConfig {        private Map dataTypeDetails;        private List<String> dataList;    }}

报错信息如下：

org.springframework.data.mapping.model.MappingInstantiationException: Failed to instantiate com.****.common.models.Entity$ParamData using constructor public com.****.common.models.Entity$ParamData(com.****.common.models.Entity) with arguments com.****.common.models.Entity$Group@2eb61a7b    at org.springframework.data.convert.ReflectionEntityInstantiator.createInstance(ReflectionEntityInstantiator.java:78)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:257)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:237)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.readValue(MappingMongoConverter.java:1136)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.access$100(MappingMongoConverter.java:78)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter$MongoDbPropertyValueProvider.getPropertyValue(MappingMongoConverter.java:1085)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.getValueInternal(MappingMongoConverter.java:816)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter$1.doWithPersistentProperty(MappingMongoConverter.java:270)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter$1.doWithPersistentProperty(MappingMongoConverter.java:263)    at org.springframework.data.mapping.model.BasicPersistentEntity.doWithProperties(BasicPersistentEntity.java:261)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:263)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:237)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.readCollectionOrArray(MappingMongoConverter.java:861)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.readValue(MappingMongoConverter.java:1134)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.access$100(MappingMongoConverter.java:78)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter$MongoDbPropertyValueProvider.getPropertyValue(MappingMongoConverter.java:1085)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.getValueInternal(MappingMongoConverter.java:816)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter$1.doWithPersistentProperty(MappingMongoConverter.java:270)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter$1.doWithPersistentProperty(MappingMongoConverter.java:263)    at org.springframework.data.mapping.model.BasicPersistentEntity.doWithProperties(BasicPersistentEntity.java:261)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:263)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:237)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:201)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:197)    at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:78)    at org.springframework.data.mongodb.core.MongoTemplate$ReadDbObjectCallback.doWith(MongoTemplate.java:2016)    at org.springframework.data.mongodb.core.MongoTemplate.executeFindMultiInternal(MongoTemplate.java:1700)    at org.springframework.data.mongodb.core.MongoTemplate.doFind(MongoTemplate.java:1523)    at org.springframework.data.mongodb.core.MongoTemplate.doFind(MongoTemplate.java:1507)    at org.springframework.data.mongodb.core.MongoTemplate.find(MongoTemplate.java:532)    at org.springframework.data.mongodb.core.MongoTemplate.findOne(MongoTemplate.java:497)    at org.springframework.data.mongodb.core.MongoTemplate.findOne(MongoTemplate.java:489)

解决办法如下：
将ParamData移到Group类里面，将DataConfig移到Config类里面。人家的原版回答如下：
This seems to be an issue with doubly nested inner classes and the synthetically generated constructors created by the compiler. I could reproduce that issue locally and see if we can provide a fix. In the meantime you have two options:
- Turn the inner class into static ones as this will remove the synthetic constructors and instantiation will work correctly.
- Nest the type declarations in the same way you nest the properties. I.e. move the ParamData class into the Group class, DataConfig into Config as that will cause the synthetic constructors created in a way they match instantiation order Spring Data currently relies on.

I’d suggest the former approach as it doesn’t artificially bind the classes to instances of the outer class.