JPA 属性转换器

1. Introduction

In this quick article, we’ll cover the usage of the Attribute Converters available in JPA 3.0 – which, simply put, allow us to map JDBC types to Java classes.

We’ll use Hibernate 6 as our JPA implementation here.

2. Creating a Converter

We’re going to show how to implement an attribute converter for a custom Java class.

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

In this brief tutorial, we showed how to use the newly introduced Attribute Converters in JPA 3.0.

As always, the full source code for the examples is available over on GitHub.