/kotlin-tooling-java-to-kotlin

Hibernate / JPA Conversion Guide

When This Applies

Detected when imports match any of:

• javax.persistence.*
• jakarta.persistence.*
• org.hibernate.*

Critical Rules

1. Do NOT use data classes for JPA entities. Data classes generate equals/hashCode based on all properties, which breaks Hibernate's identity semantics and proxy creation.

2. Keep entity classes open. Hibernate creates proxies via subclassing. Kotlin classes are final by default, so you must use open explicitly (or use the allopen compiler plugin with JPA annotation support).

3. Provide a no-argument constructor if Hibernate requires one for proxy creation. Use a secondary constructor or default values for all primary constructor parameters.

4. Annotation site targets matter:

   • @Id, @Column, @GeneratedValue on fields → use @field:Id, @field:Column, etc. in Kotlin, OR place annotations on constructor parameters with @field: site target.
   • @ManyToOne, @OneToMany, @JoinColumn → same @field: targeting.

5. Lazy loading considerations: @ManyToOne(fetch = FetchType.LAZY) requires the entity class to be open for proxy creation. @OneToMany with lazy collections work with Kotlin's MutableList.

6. @Embeddable classes: Can be data classes (they don't need proxies).

7. @MappedSuperclass: Must be open abstract class in Kotlin.

Examples

Example 1: JPA Entity with @Id, @Column, and Relationships

Java:

@Entity
@Table(name = "users")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "username", nullable = false, unique = true)
    private String username;

    @Column(name = "email")
    private String email;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "department_id")
    private Department department;

    protected User() {}

    public User(String username, String email, Department department) {
        this.username = username;
        this.email = email;
        this.department = department;
    }

    public Long getId() { return id; }
    public String getUsername() { return username; }
    public String getEmail() { return email; }
    public void setEmail(String email) { this.email = email; }
    public Department getDepartment() { return department; }
    public void setDepartment(Department department) { this.department = department; }
}

Kotlin:

@Entity
@Table(name = "users")
open class User(

    @field:Column(name = "username", nullable = false, unique = true)
    open val username: String,

    @field:Column(name = "email")
    open var email: String? = null,

    @field:ManyToOne(fetch = FetchType.LAZY)
    @field:JoinColumn(name = "department_id")
    open var department: Department? = null

) {
    @field:Id
    @field:GeneratedValue(strategy = GenerationType.IDENTITY)
    open var id: Long? = null
        protected set

    protected constructor() : this(username = "")
}

Example 2: @Embeddable Value Object

Java:

@Embeddable
public class Address {

    @Column(name = "street")
    private String street;

    @Column(name = "city")
    private String city;

    @Column(name = "zip_code")
    private String zipCode;

    protected Address() {}

    public Address(String street, String city, String zipCode) {
        this.street = street;
        this.city = city;
        this.zipCode = zipCode;
    }

    public String getStreet() { return street; }
    public String getCity() { return city; }
    public String getZipCode() { return zipCode; }
}

Kotlin:

@Embeddable
data class Address(

    @field:Column(name = "street")
    val street: String = "",

    @field:Column(name = "city")
    val city: String = "",

    @field:Column(name = "zip_code")
    val zipCode: String = ""
)

@Embeddable classes can safely be data classes because Hibernate does not proxy them. Default values satisfy the no-arg constructor requirement.

Example 3: Entity with @ManyToOne and @OneToMany

Java:

@Entity
@Table(name = "departments")
public class Department {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name", nullable = false)
    private String name;

    @OneToMany(mappedBy = "department", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<User> users = new ArrayList<>();

    protected Department() {}

    public Department(String name) {
        this.name = name;
    }

    public Long getId() { return id; }
    public String getName() { return name; }
    public List<User> getUsers() { return users; }

    public void addUser(User user) {
        users.add(user);
        user.setDepartment(this);
    }

    public void removeUser(User user) {
        users.remove(user);
        user.setDepartment(null);
    }
}

Kotlin:

@Entity
@Table(name = "departments")
open class Department(

    @field:Column(name = "name", nullable = false)
    open val name: String = ""

) {
    @field:Id
    @field:GeneratedValue(strategy = GenerationType.IDENTITY)
    open var id: Long? = null
        protected set

    @field:OneToMany(mappedBy = "department", cascade = [CascadeType.ALL], orphanRemoval = true)
    open val users: MutableList<User> = mutableListOf()

    protected constructor() : this(name = "")

    fun addUser(user: User) {
        users.add(user)
        user.department = this
    }

    fun removeUser(user: User) {
        users.remove(user)
        user.department = null
    }
}

Key points in this example:

• cascade array syntax uses Kotlin's [CascadeType.ALL] instead of Java's {CascadeType.ALL}.
• The collection is typed as MutableList to allow Hibernate to manage the relationship.
• The class and its properties are open so Hibernate can create proxies.
• The no-arg constructor delegates to the primary constructor with default values.