问题描述：
在SpringBoot中测试Elasticsearch的时候，当测试ElasticsearchRepository的文档操作的时候，在调用find()查询方法时报错，错误如下：
com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.zmx.elasticsearch.pojo.Item` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator) at [Source: (String); line: 1, column: 2] at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67) at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1452) at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1028) at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1297) at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326) at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159) at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013) at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3004) at org.springframework.data.elasticsearch.core.DefaultEntityMapper.mapToObject(DefaultEntityMapper.java:82) at org.springframework.data.elasticsearch.core.AbstractResultMapper.mapEntity(AbstractResultMapper.java:43) ... 56 more
原因：
这是因为在实体类Item中为了方便实例化添加了一个有参构造函数，导致JVM不能添加默认的无参构造函数了，但是jackson的反序列化需要使用无参构造函数，所以报错！
解决：
在实体类Item中添加一个无参构造函数就可以了！