Custom Types | Advanced Topics | Hibernate Tutorial

Introduction to Custom Types

Hibernate is a powerful ORM (Object-Relational Mapping) framework for Java applications. One of the advanced features of Hibernate is the ability to create custom types. Custom types allow developers to map complex data types to database columns in a way that is not supported by the default Hibernate types.

Why Use Custom Types?

Using custom types can be beneficial when:

You need to store complex data structures in a single database column.
You want to encapsulate business logic related to a specific type.
You need to optimize the storage or retrieval of specific data types.

Creating a Custom Type

To create a custom type in Hibernate, you need to implement the org.hibernate.usertype.UserType interface. This interface provides methods that you need to implement to define how your custom type is persisted to and loaded from the database.

Example: Creating a Custom Type for a List of Strings

In this example, we will create a custom type that allows us to store a list of strings in a single column in the database.

Step 1: Implement the UserType Interface

import org.hibernate.engine.spi.SharedSessionContractImplementor;
import org.hibernate.usertype.UserType;
import java.io.Serializable;
import java.sql.*;
import java.util.Arrays;
import java.util.List;

public class StringListType implements UserType {
    @Override
    public int[] sqlTypes() {
        return new int[]{Types.VARCHAR};
    }

    @Override
    public Class returnedClass() {
        return List.class;
    }

    @Override
    public boolean equals(Object x, Object y) {
        return x == y || (x != null && x.equals(y));
    }

    @Override
    public int hashCode(Object x) {
        return x.hashCode();
    }

    @Override
    public Object nullSafeGet(ResultSet rs, String[] names, SharedSessionContractImplementor session, Object owner) throws SQLException {
        String value = rs.getString(names[0]);
        return value != null ? Arrays.asList(value.split(",")) : null;
    }

    @Override
    public void nullSafeSet(PreparedStatement st, Object value, int index, SharedSessionContractImplementor session) throws SQLException {
        if (value == null) {
            st.setNull(index, Types.VARCHAR);
        } else {
            st.setString(index, String.join(",", (List) value));
        }
    }

    // Other required methods omitted for brevity...
}

Step 2: Mapping the Custom Type

Next, you can use the custom type in your entity class:

import org.hibernate.annotations.Type;

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

    @Type(type = "com.example.StringListType")
    private List myList;

    // Getters and Setters...
}

Using the Custom Type

Once you have defined your custom type and mapped it to an entity, you can use it like any other property:

MyEntity entity = new MyEntity();
entity.setMyList(Arrays.asList("Item1", "Item2", "Item3"));
session.save(entity);

This will store the list as a comma-separated string in the database column.

Conclusion

Custom types in Hibernate provide a powerful way to handle complex data structures and encapsulate business logic related to data storage. By implementing the UserType interface, you can effectively manage how your data is persisted and retrieved, making your application more efficient and flexible.

Custom Types in Hibernate