jpa-attribute-converters
JPA Attribute Converters
1. Introduction
In this quick article, we’ll cover the usage of the Attribute Converters available in JPA 2.1 – which, simply put, allow us to map JDBC types to Java classes.
We’ll use Hibernate 5 as our JPA implementation here.
2. Creating a Converter
First, let’s create a PersonName class – that will be converted later:
public class PersonName implements Serializable {
private String name;
private String surname;
// getters and setters
}
Then, we’ll add an attribute of type PersonName to an @Entity class:
@Entity(name = "PersonTable")
public class Person {
private PersonName personName;
//...
}
Now we need to create a converter that transforms the PersonName attribute to a database column and vice-versa. In our case, we’ll convert the attribute to a String value that contains both name and surname fields.
To do so we have to annotate our converter class with @Converter and implement the AttributeConverter interface. We’ll parametrize the interface with the types of the class and the database column, in that order:
@Converter
public class PersonNameConverter implements
AttributeConverter<PersonName, String> {
private static final String SEPARATOR = ", ";
@Override
public String convertToDatabaseColumn(PersonName personName) {
if (personName == null) {
return null;
}
StringBuilder sb = new StringBuilder();
if (personName.getSurname() != null && !personName.getSurname()
.isEmpty()) {
sb.append(personName.getSurname());
sb.append(SEPARATOR);
}
if (personName.getName() != null
&& !personName.getName().isEmpty()) {
sb.append(personName.getName());
}
return sb.toString();
}
@Override
public PersonName convertToEntityAttribute(String dbPersonName) {
if (dbPersonName == null || dbPersonName.isEmpty()) {
return null;
}
String[] pieces = dbPersonName.split(SEPARATOR);
if (pieces == null || pieces.length == 0) {
return null;
}
PersonName personName = new PersonName();
String firstPiece = !pieces[0].isEmpty() ? pieces[0] : null;
if (dbPersonName.contains(SEPARATOR)) {
personName.setSurname(firstPiece);
if (pieces.length >= 2 && pieces[1] != null
&& !pieces[1].isEmpty()) {
personName.setName(pieces[1]);
}
} else {
personName.setName(firstPiece);
}
return personName;
}
}
Notice that we had to implement 2 methods: convertToDatabaseColumn() and convertToEntityAttribute().
The two methods are used to convert from the attribute to a database column and vice-versa.
3. Using the Converter
To use our converter, we just need to add the @Convert annotation to the attribute and specify the converter class we want to use:
@Entity(name = "PersonTable")
public class Person {
@Convert(converter = PersonNameConverter.class)
private PersonName personName;
// ...
}
Finally, let’s create a unit test to see that it really works.
To do so, we’ll first store a Person object in our database:
@Test
public void givenPersonName_whenSaving_thenNameAndSurnameConcat() {
String name = "name";
String surname = "surname";
PersonName personName = new PersonName();
personName.setName(name);
personName.setSurname(surname);
Person person = new Person();
person.setPersonName(personName);
Long id = (Long) session.save(person);
session.flush();
session.clear();
}
Next, we’re going to test that the PersonName was stored as we defined it in the converter – by retrieving that field from the database table:
@Test
public void givenPersonName_whenSaving_thenNameAndSurnameConcat() {
// ...
String dbPersonName = (String) session.createNativeQuery(
"select p.personName from PersonTable p where p.id = :id")
.setParameter("id", id)
.getSingleResult();
assertEquals(surname + ", " + name, dbPersonName);
}
Let’s also test that the conversion from the value stored in the database to the PersonName class works as defined in the converter by writing a query that retrieves the whole Person class:
@Test
public void givenPersonName_whenSaving_thenNameAndSurnameConcat() {
// ...
Person dbPerson = session.createNativeQuery(
"select * from PersonTable p where p.id = :id", Person.class)
.setParameter("id", id)
.getSingleResult();
assertEquals(dbPerson.getPersonName()
.getName(), name);
assertEquals(dbPerson.getPersonName()
.getSurname(), surname);
}
4. Conclusion
As always, the full source code for the examples is available over on GitHub.