Table of Contents
org.hibernate.QueryException: could not instantiate class [com.ak47.cms.cms.dto.TechArticleDto] from tuple
Abnormal background
solution
in conclusion
org.hibernate.QueryException: could not instantiate class [com.ak47.cms.cms.dto.TechArticleDto] from tuple
When performing Hibernate-based data query, you may encounter an exception similar to ??org.hibernate.QueryException: could not instantiate class??, especially when using DTO (Data Transfer Object) from When mapping data in query results. This technical blog will help solve this problem and provide solutions.
Abnormal background
In Hibernate, we use HQL (Hibernate Query Language) for data query. Sometimes, we want to map query results to a custom DTO class in order to get a specified data structure. However, when the defined DTO class does not match the query result, an ??org.hibernate.QueryException: could not instantiate class?? exception occurs. Typically, the cause of this exception is that the constructor of the DTO class cannot be instantiated with the query results. Therefore, the constructor of the DTO class needs to be modified so that it can adapt to the structure of the query results.
Solution
The following are the steps to solve the ??org.hibernate.QueryException?? exception: Step 1: Check the query statement. First, we need to check whether the query statement is correct and ensure that the returned fields match the attribute names of the DTO class. match. The query statement should explicitly specify the alias for each field to match when mapping to a DTO class. Step 2: Update the constructor of the DTO class
javaCopy codepublic class TechArticleDto {
private Long id;
private String title;
private String content;
public TechArticleDto(Long id, String title, String content) {
this.id = id;
this.title = title;
this.content = content;
}
// Getters and setters
}
To resolve the ??org.hibernate.QueryException?? exception, you need to check the constructor of the DTO class. In the above example, we can see that the ??TechArticleDto?? class has a class that receives ??Long??, ??String?? and ??String?? constructor of type parameters, respectively corresponding to the fields in the query results. Make sure that the constructor parameters of the DTO class are consistent with the field order and data types selected in the query statement. If a field in the query results does not match the attribute name of the DTO class, you can use an alias to rename the field so that the correct mapping occurs. Step 3: Use the mapping method to specify the DTO class. If the above steps cannot solve the problem, you can try to use the mapping method in Hibernate to specify the data mapping relationship of the DTO class. This can be achieved through the ??@SqlResultSetMapping?? and ??@ConstructorResult?? annotations provided by Hibernate. First, add the ??@SqlResultSetMapping?? annotation to the DTO class to specify the returned result set mapping relationship.
javaCopy code@SqlResultSetMapping(
name = "TechArticleDtoMapping",
classes = @ConstructorResult(
targetClass = TechArticleDto.class,
columns = {
@ColumnResult(name = "id", type = Long.class),
@ColumnResult(name = "title", type = String.class),
@ColumnResult(name = "content", type = String.class)
}
)
)
Then, use the ??@NamedNativeQuery?? annotation on the query statement to specify the mapping relationship of the returned result set.
javaCopy code@Entity
@NamedNativeQuery(
name = "getTechArticles",
query = "SELECT id, title, content FROM tech_articles",
resultSetMapping = "TechArticleDtoMapping"
)
public class TechArticle {
// Entity fields and annotations
}
Finally, use ??EntityManager?? to query and specify the mapping relationship to be used.
javaCopy codeQuery query = entityManager.createNamedQuery("getTechArticles");
List<TechArticleDto> techArticleDtos = query.getResultList();
By using the ??@SqlResultSetMapping?? and ??@ConstructorResult?? annotations, we can correctly construct instances of the DTO class from the query results and resolve the ??org.hibernate.QueryException?? Exception.
Conclusion
When making Hibernate-based queries, if you encounter the ??org.hibernate.QueryException: could not instantiate class?? exception, it is usually because the constructor of the DTO class cannot be instantiated correctly. This technical blog provides some solutions, including updating the constructor of the DTO class and using mapping to specify the data mapping relationship of the DTO class.
DTO (Data Transfer Object) is a design pattern used to transfer data between various layers of the system. It mainly solves the problem of avoiding exposing too many internal implementation details and data fields when transmitting data between different layers. The core idea of the DTO pattern is to encapsulate data into a simple object, which only contains data and does not contain business logic. The characteristics of DTO are as follows:
- Simplified interface: DTO is often used to encapsulate raw data obtained from a database, external API, or other sources. It can combine multiple fields and objects into a simpler structure, exposing only the required fields and methods in the interface, simplifying the complexity of the interface.
- Reduce network overhead: In a distributed system, large amounts of data may need to be passed between different layers. Using DTO can reduce network overhead because DTO only transmits the required data and not redundant data fields or business logic.
- Prevent data leakage: By using DTO, you can avoid exposing database entity classes directly to the outside, thereby preventing data leakage. DTO allows you to selectively expose fields in entity classes to protect data security.
- Compatible with different data sources: Since the data structures used by different data sources (such as databases and external APIs) may be different, DTO can convert data source-specific structures into common structures, making it more convenient and flexible to use data in the system. Here is an example showing how to use the DTO pattern: Suppose there is an online store system where product information needs to be transferred between different layers. First, define a Product class to represent the entity of the product:
javaCopy codepublic class Product {
private Long id;
private String name;
private String description;
private double price;
// Getters and setters
}
Then, define a ProductDTO class to represent the product information transmitted to the client:
javaCopy codepublic class ProductDTO {
private Long id;
private String name;
private double price;
// Getters and setters
}
In the service layer, the Product object is obtained by querying the database and converted into a ProductDTO object:
javaCopy codepublic ProductDTO getProductById(Long id) {
Product product = productRepository.findById(id);
ProductDTO productDTO = new ProductDTO();
productDTO.setId(product.getId());
productDTO.setName(product.getName());
productDTO.setPrice(product.getPrice());
return productDTO;
}
In this example, the Product class is the domain model and represents all the attributes of the product, while the ProductDTO class is the data transfer object and only exposes the attributes (id, name and price) that need to be displayed to the client. This keeps data secure and simplifies the process of transferring data between different tiers. To summarize, the DTO pattern is a design pattern for transmitting data between different layers. It simplifies the interface, reduces network overhead, prevents data leakage, and is compatible with different data sources by encapsulating data into simple objects. When using the DTO mode, you need to decide when and how to use DTO based on specific scenarios and needs.
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