Fixing the “Could Not Determine Recommended JdbcType for Class” Error in JPA

1. Introduction

In this tutorial, we’ll explore the causes of the “could not determine recommended JdbcType for class” error in Hibernate and discuss how to resolve it.

2. Common Causes

When we encounter this error message, it’s usually because JPA (Java Persistence API) is throwing an exception. This happens when JPA can’t determine the recommended JDBC type for a class. Typically, this occurs during the startup of our Hibernate application, when Hibernate is trying to create the database schema or validate the mappings.

3. Complex Data Types

One common cause of the “could not determine recommended JdbcType for class” error in JPA is when we’re using complex data types or other non-primitive types in our entity classes. A typical example of this is using a java.util.Map to store dynamic key-value pairs.

Let’s consider the following entity class:

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    private Map<String, String> studentDetails;

    // getters and setters
}

In the above example, the studentDetails field is a Map<String, String>. JPA may throw an error because it can’t automatically determine the recommended JDBC type for this complex data type. To demonstrate this, let’s try to set up an EntityManagerFactory with this entity class:

PersistenceException exception = assertThrows(PersistenceException.class,
  () -> createEntityManagerFactory("jpa-undeterminejdbctype-complextype"));
assertThat(exception)
  .hasMessage("Could not determine recommended JdbcType for Java type 'java.util.Map<java.lang.String, java.lang.String>'");

To resolve this issue, we can use the @JdbcTypeCode annotation to specify the JDBC type that Hibernate should use for the studentDetails field. In this case, we can use the JSON data type to store the map as a JSON object.

First, we need to add the Jackson dependency to our pom.xml file:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.17.0</version>
</dependency>

Jackson is a popular library for JSON processing in Java, and it provides the necessary tools for mapping JSON data types. Next, we use the @JdbcTypeCode annotation to tell Hibernate to use the JSON data type for the studentDetails field. Here’s the updated entity class:

@Entity
public class StudentWithJson {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @JdbcTypeCode(SqlTypes.JSON)
    private Map<String, String> studentDetails;

    // getters and setters
}

By adding the @JdbcTypeCode annotation, we explicitly instruct Hibernate on handling the complex data type. The SqlTypes.JSON constant is used to indicate the JDBC type as JSON. Hibernate now stores the studentDetails field as a JSON object in the database, allowing it to properly map the Java Map to a JSON column in the database:

EntityManagerFactory factory = createEntityManagerFactory("jpa-undeterminejdbctype-complextypewithJSON");
EntityManager entityManager = factory.createEntityManager();

entityManager.getTransaction().begin();

StudentWithJson student = new StudentWithJson();
Map<String, String> details = new HashMap<>();
details.put("course", "Computer Science");
details.put("year", "2024");
student.setStudentDetails(details);

entityManager.persist(student);
entityManager.getTransaction().commit();

StudentWithJson retrievedStudent = entityManager.find(StudentWithJson.class, student.getId());
assertEquals(student.getStudentDetails(), retrievedStudent.getStudentDetails());

4. JPA Relationship Mapping

JPA relies on annotations to understand the relationships between entities and their field types. Omitting these annotations can lead to Hibernate being unable to determine the recommended JDBC type for the fields involved, causing errors.

Let’s explore two common relationship mappings: @OneToOne and @OneToMany, and see how missing annotations can cause issues.

4.1. One-to-One Mapping

We’ll demonstrate a scenario where an Employee entity has a one-to-one relationship with an Address entity. If we forget to annotate this relationship, Hibernate may throw a PersistenceException because it can’t determine the appropriate JDBC type for the Address field:

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    private Address address;

    // setter and getter
}

In this case, when we try to initialize the EntityManagerFactory, we’ll encounter the following exception:

PersistenceException exception = assertThrows(PersistenceException.class,
  () -> createEntityManagerFactory("jpa-undeterminejdbctype-relationship"));
assertThat(exception)
  .hasMessage("Could not determine recommended JdbcType for Java type 'com.baeldung.jpa.undeterminejdbctype.Address'");

The error occurs because Hibernate doesn’t know how to map the address field to the database. To fix this, we need to properly annotate the one-to-one relationship. Let’s add the necessary annotations to the Employee entity:

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToOne
    @JoinColumn(name = "address_id", referencedColumnName = "id")
    private Address address;

    // setters and getters
}

By adding the @OneToOne annotations, we’re instructing Hibernate on handling the relationship between Employee and Address. Now, Hibernate knows that the address field in the Employee entity is mapped to the Address entity, and it uses the address_id column in the Employee table to establish this relationship.

4.2. One-to-Many Mapping

Similar to one-to-one mapping, when dealing with one-to-many relationships in JPA, proper annotations are essential to avoid exceptions. Let’s explore this with a one-to-many mapping example between the Department and Employee entities.

In this scenario, a Department can have multiple Employees. If we forget to annotate this relationship correctly, Hibernate may throw a PersistenceException because it can’t determine the appropriate JDBC type for the employees field.

Here is the initial setup without proper annotations:

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    private List<Employee> employees = new ArrayList<>();

    // setters and getters
}

This setup results in an exception when initializing the EntityManagerFactory:

PersistenceException exception = assertThrows(PersistenceException.class,
  () -> createEntityManagerFactory("jpa-undeterminejdbctype-relationship"));
assertThat(exception)
  .hasMessage("Could not determine recommended JdbcType for Java type 'java.util.List<com.baeldung.jpa.undeterminejdbctype.Employee>'");

The error occurs because Hibernate doesn’t know how to map the employees field to the database. To fix this, we need to annotate the one-to-many relationship in the Department entity properly.

Let’s add the necessary annotations to the Department entity:

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToMany(mappedBy = "department", cascade = CascadeType.ALL)
    private List<Employee> employees = new ArrayList<>();

    // setters and getters
}

We often add the @OneToMany relationship in the Department class but forget to annotate the Employee class with the @ManyToOne relationship, leading to such errors. Here’s how to properly annotate both sides of the relationship:

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @ManyToOne
    @JoinColumn(name = "department_id")
    private Department department;

    // setters and getters
}

This setup ensures that Hibernate can correctly map the employees field in the Department entity and the department field in the Employee entity, avoiding the PersistenceException and enabling proper database interactions.

5. Conclusion

In this article, we’ve explored the causes of the “could not determine recommended JdbcType for class” error in Hibernate and discussed how to resolve it. We’ve seen how complex data types and relationship mappings can lead to this error and looked at solutions to prevent it.