How to change the Hibernate CharacterTypeDescriptor to handle empty column values

Question

Our problem is that we can't get data (which includes empty strings with length 0) from a legacy database due to a StringIndexOutOfBoundsExceptiion originating from Hibernate's CharacterTypeDescriptor. We would like to change Hibernate's behavior to properly resolve empty strings.

Example data:

1, 'Berlin', 17277, '', 'aUser'
2, 'London', 17277, '', 'anotherUser'

We use hibernate with javax.persistence.Query.

String sql = "SELECT * FROM table";
Query query = entityManager.createNativeQuery(sql);
List resultList = query.getResultList();

This leads to a StringIndexOutOfBoundsException with its root being the following code from Hibernate:

if ( String.class.isInstance( value ) ) {
   final String str = (String) value;
   return Character.valueOf( str.charAt(0) );  // this fails, as there is no char at position 0
}

This was confirmed by a post on the hibernate forums.

We have no option of upgrading hibernate from this buggy version and look for a way to change Hibernate's mapping.

We can not use PreparedStatements or plain JDBC-Connections nor JPA-Entities.

Altering the legacy database is not possible either. The SQL statement works flawlessly using DBVisualizer.

Is there anway to change Hibernate's way of mapping strings?

Answer 1

The CharacterType presented in this answer is available via the hibernate-types project, so there is no need to write it yourself.

First, you need to define an ImmutableType:

public abstract class ImmutableType<T> implements UserType {

    private final Class<T> clazz;

    protected ImmutableType(Class<T> clazz) {
        this.clazz = clazz;
    }

    @Override
    public Object nullSafeGet(
        ResultSet rs, 
        String[] names,
        SharedSessionContractImplementor session, 
        Object owner) 
        throws SQLException {
        return get(rs, names, session, owner);
    }

    @Override
    public void nullSafeSet(
        PreparedStatement st, 
        Object value, 
        int index,
        SharedSessionContractImplementor session) 
        throws SQLException {
        set(st, clazz.cast(value), index, session);
    }

    protected abstract T get(
        ResultSet rs, 
        String[] names,
        SharedSessionContractImplementor session, 
        Object owner) throws SQLException;

    protected abstract void set(
        PreparedStatement st, 
        T value, 
        int index,
        SharedSessionContractImplementor session) 
        throws SQLException;


    @Override
    public Class<T> returnedClass() {
        return clazz;
    }

    @Override
    public boolean equals(Object x, Object y) {
        return Objects.equals(x, y);
    }

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

    @Override
    public Object deepCopy(Object value) {
        return value;
    }

    @Override
    public boolean isMutable() {
        return false;
    }

    @Override
    public Serializable disassemble(Object o) {
        return (Serializable) o;
    }

    @Override
    public Object assemble(
        Serializable cached, 
        Object owner) {
        return cached;
    }

    @Override
    public Object replace(
        Object o, 
        Object target, 
        Object owner) {
        return o;
    }
}

Now, we can move to defining the actual CharacterType:

public class CharacterType 
    extends ImmutableType<Character> {

    public CharacterType() {
        super(Character.class);
    }

    @Override
    public int[] sqlTypes() { 
        return new int[]{Types.CHAR}; 
    }

    @Override
    public Character get(
        ResultSet rs, 
        String[] names,
        SharedSessionContractImplementor session, 
        Object owner) 
        throws SQLException {
        String value = rs.getString(names[0]);
        return (value != null && value.length() > 0) ? 
            value.charAt(0) : null;
    }

    @Override
    public void set(
        PreparedStatement st, 
        Character value, 
        int index,
        SharedSessionContractImplementor session) 
        throws SQLException {
        if (value == null) {
            st.setNull(index, Types.CHAR);
        } else {
            st.setString(index, String.valueOf(value));
        }
    }
}

The entity mapping looks like this:

@Entity(name = "Event")
@Table(name = "event")
public class Event {

    @Id
    @GeneratedValue
    private Long id;

    @Type(type = "com.vladmihalcea.book.hpjp.hibernate.type.CharacterType")
    @Column(name = "event_type")
    private Character type;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public Character getType() {
        return type;
    }

    public void setType(Character type) {
        this.type = type;
    }
}

And let's say we have these table rows:

INSERT INTO event (id, event_type) VALUES (1, 'abc');    
INSERT INTO event (id, event_type) VALUES (2, '');
INSERT INTO event (id, event_type) VALUES (3, 'b');

When reading all entities:

doInJPA(entityManager -> {
    List<Event> events = entityManager.createQuery(
        "select e from Event e", Event.class)
    .getResultList();
    for(Event event : events) {
        LOGGER.info("Event type: {}", event.getType());
    }
});

You'll get the expected output:

Event type: a
Event type:  
Event type: b

Check out the source code on GitHub.