Spring JPA simplifies working with relational databases using the Java Persistence API (JPA), making mapping Java objects to database tables easier. However, issues can arise when entity field names do not align with database column names, causing mapping failures. This article examines a common case where mismatched naming conventions lead to errors, explains Hibernate’s implicit naming strategy and provides a practical solution to resolve the issue.
1. Understanding the Problem
Spring JPA maps Java entity attributes to database columns. By default, it assumes the names of the attributes match the column names. If they don’t align, runtime errors occur. Consider a database schema where column names use snake_case, while the Java code follows PascalCase. Without explicit mappings, JPA fails to resolve these differences.
Let’s consider a database table employee with the following schema:
CREATE TABLE employee (
emp_id BIGINT PRIMARY KEY,
first_name VARCHAR(50),
last_name VARCHAR(50),
email_address VARCHAR(250)
);
INSERT INTO employee (emp_id, first_name, last_name, email_address)
VALUES (1, 'Jane', 'Fish', 'jane_fish@jcg.com');
INSERT INTO employee (emp_id, first_name, last_name, email_address)
VALUES (2, 'Thomas', 'Brown', 't_brown@jcg.com');
The mismatch arises when the Java entity does not correctly map these column names to its attributes. Let’s look at the problematic entity and solutions to address it.
2. Incorrect Entity Definition
Now, we define a corresponding JPA entity:
@Entity
public class Employee {
@Id
@Column(name = "emp_id")
private Long empId;
@Column(name = "first_name")
private String FirstName; // JPA assumes this maps to 'firstName'
@Column(name = "last_name")
private String LastName; // JPA assumes this maps to 'lastName'
@Column(name = "email_address")
private String EmailAddress; // JPA assumes this maps to 'emailAddress'
public Employee() {
}
public Long getEmpId() {
return empId;
}
public void setEmpId(Long empId) {
this.empId = empId;
}
public String getFirstName() {
return FirstName;
}
public void setFirstName(String FirstName) {
this.FirstName = FirstName;
}
public String getLastName() {
return LastName;
}
public void setLastName(String LastName) {
this.LastName = LastName;
}
public String getEmailAddress() {
return EmailAddress;
}
public void setEmailAddress(String EmailAddress) {
this.EmailAddress = EmailAddress;
}
}
Repository
import org.springframework.data.jpa.repository.JpaRepository;
import java.util.List;
public interface EmployeeRepository extends JpaRepository<Employee, Long> {
List<Employee> findAllByOrderByEmailAddressAscLastNameAscEmpIdDesc();
}
Controller
@RestController
public class EmployeeController {
@Autowired
private EmployeeRepository employeeRepository;
@GetMapping("/employees/sorted")
public List<Employee> getSortedEmployees() {
return employeeRepository.findAllByOrderByEmailAddressAscLastNameAscEmpIdDesc();
}
}
When running the application, queries will fail with an error because JPA expects column names LastName and emailAddress in the database, which does not exist.
Caused by: java.lang.IllegalArgumentException: Failed to create query for method public abstract java.util.List com.jcg.EmployeeRepository.findAllByOrderByEmailAddressAscLastNameAscEmpIdDesc(); Could not resolve attribute 'emailAddress' of 'com.jcg.Employee'
3. Solution to Attribute Naming Issues
The @Entity class Employee fails to map correctly to the database schema because Hibernate, by default, uses naming conventions aligned with Java field names. Specifically, Hibernate expects fields to follow the Java camelCase naming convention (e.g., firstName, lastName, emailAddress). However, in our Employee class, the fields are named with an initial uppercase letter (FirstName, LastName, EmailAddress), deviating from camelCase. Consequently, Hibernate attempts to map these fields to columns named FirstName, LastName, and EmailAddress, which do not exist in the table.
Although we have used @Column annotations to specify the database column names, Hibernate’s implicit naming strategy still applies to the field names. This strategy converts field names into column names unless the annotation explicitly overrides the mapping. In this case, the database columns first_name, last_name, and email_address do not match the improperly cased field names in the Employee class, resulting in a mapping failure.
To fix this issue, we must rename the fields in the Employee class to adhere to the standard Java camelCase naming convention. For example, change FirstName to firstName, LastName to lastName, and EmailAddress to emailAddress. This adjustment ensures that Hibernate recognizes the fields correctly and maps them to the corresponding database columns as specified by the @Column annotations.
3.1 Updated Entity Definition
@Entity
public class Employee {
@Id
@Column(name = "emp_id")
private Long empId;
@Column(name = "first_name")
private String firstName;
@Column(name = "last_name")
private String lastName;
@Column(name = "email_address")
private String emailAddress;
public Employee() {
}
public Long getEmpId() {
return empId;
}
public void setEmpId(Long empId) {
this.empId = empId;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getEmailAddress() {
return emailAddress;
}
public void setEmailAddress(String emailAddress) {
this.emailAddress = emailAddress;
}
}
3.2 Verifying SQL Queries
To verify the queries generated by Hibernate and troubleshoot attribute mapping issues, we can enable Hibernate SQL logging. This is done by adding specific logging configurations in the application.properties file, allowing us to monitor SQL statements executed by Hibernate and identify potential issues in the mapping or query execution.
application.properties
spring.jpa.hibernate.ddl-auto=update spring.jpa.show-sql=true spring.jpa.properties.hibernate.format_sql=true logging.level.org.hibernate.SQL=DEBUG logging.level.org.hibernate.type.descriptor.sql=TRACE
This helps to confirm the SQL generated and identify incorrect mappings.
With the updated and correct entity mapping, running the application ensures that Hibernate successfully maps the Employee entity fields to their corresponding database columns. The application retrieves and persists data without errors, confirming that the database columns (first_name, last_name, email_address) align with the camelCase field names (firstName, lastName, emailAddress).
Here is an example of the Hibernate console output after running the application with the corrected entity mapping:
select
e1_0.emp_id,
e1_0.email_address,
e1_0.first_name,
e1_0.last_name
from
employee e1_0
order by
e1_0.email_address,
e1_0.last_name,
e1_0.emp_id desc
Hibernate:
select
e1_0.emp_id,
e1_0.email_address,
e1_0.first_name,
e1_0.last_name
from
employee e1_0
order by
e1_0.email_address,
e1_0.last_name,
e1_0.emp_id desc
4. Conclusion
In this article, we explored how Hibernate’s default naming conventions can lead to mapping issues when the field names in the entity class do not align with the column names in the database. By examining a common case with mismatched naming conventions, we demonstrated how Hibernate’s implicit naming strategy attempts to map fields based on Java’s camelCase convention, causing failures when the entity fields use a different casing.
A solution was provided by renaming the fields to follow the standard camelCase convention, ensuring proper mapping. With this fix, the application works properly, carrying out database operations without issues and ensuring that the entity and database schema match correctly.
5. Download the Source Code
This article explored troubleshooting attribute naming issues in Spring JPA.
You can download the full source code of this example here: spring jpa troubleshooting attribute naming issues
