Enum: JPA and Enums via @Enumerated

JPA and Enums via @Enumerated

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It can sometimes be desirable to have a Java enum type to represent a particular column in a database. JPA supports converting database data to and from Java enum types via the @javax.persistence.Enumerated annotation.

This example will show basic @Enumerated usage in a field of an @Entity as well as enums as the parameter of a Query. We'll also see that the actual database representation can be effectively String or int.

Enum

For our example we will leverage the familiar Movie entity and add a new field to represent the MPAA.org rating of the movie. This is defined via a simple enumthat requires no JPA specific annotations.

public enum Rating {
    UNRATED,
    G,
    PG,
    PG13,
    R,
    NC17
}

@Enumerated

In our Movie entity, we add a rating field of the enum type Rating and annotate it with @Enumerated(EnumType.STRING) to declare that its value should be converted from what is effectively a String in the database to the Rating type.

@Entity
public class Movie {

    @Id
    @GeneratedValue
    private int id;
    private String director;
    private String title;
    private int year;

    @Enumerated(EnumType.STRING)
    private Rating rating;

    public Movie() {
    }

    public Movie(String director, String title, int year, Rating rating) {
        this.director = director;
        this.title = title;
        this.year = year;
        this.rating = rating;
    }

    public String getDirector() {
        return director;
    }

    public void setDirector(String director) {
        this.director = director;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public Rating getRating() {
        return rating;
    }

    public void setRating(Rating rating) {
        this.rating = rating;
    }
}

The above is enough and we are effectively done. For the sake of completeness we'll show a sample Query

Enum in JPQL Query

Note the findByRating method which creates a Query with a rating named parameter. The key thing to notice is that the rating enum instance itself is passed into the query.setParameter method, not rating.name() or rating.ordinal().

Regardless if you use EnumType.STRING or EnumType.ORDINAL, you still always pass the enum itself in calls to query.setParameter.

@Stateful
public class Movies {

    @PersistenceContext(unitName = "movie-unit", type = PersistenceContextType.EXTENDED)
    private EntityManager entityManager;

    public void addMovie(Movie movie) {
        entityManager.persist(movie);
    }

    public void deleteMovie(Movie movie) {
        entityManager.remove(movie);
    }

    public List<Movie> findByRating(Rating rating) {
        final Query query = entityManager.createQuery("SELECT m FROM Movie as m WHERE m.rating = :rating");
        query.setParameter("rating", rating);
        return query.getResultList();
    }

    public List<Movie> getMovies() throws Exception {
        Query query = entityManager.createQuery("SELECT m from Movie as m");
        return query.getResultList();
    }

}

EnumType.STRING vs EnumType.ORDINAL

It is a matter of style how you would like your enum data represented in the database. Either name() or ordinal() are supported:

• @Enumerated(EnumType.STRING) Rating rating the value of rating.name() is written and read from the corresponding database column; e.g. G, PG,PG13
• @Enumerated(EnumType.ORDINAL) Rating rating the value of rating.ordinal() is written and read from the corresponding database column; e.g. 0,1, 2

The default is EnumType.ORDINAL

There are advantages and disadvantages to each.

Disadvantage of EnumType.ORDINAL

A disadvantage of EnumType.ORDINAL is the effect of time and the desire to keep enums in a logical order. With EnumType.ORDINAL any new enum elements must be added to the end of the list or you will accidentally change the meaning of all your records.

Let's use our Rating enum and see how it would have had to evolve over time to keep up with changes in the MPAA.org ratings system.

1980

public enum Rating {
    G,
    PG,
    R,
    UNRATED
}

1984 PG-13 is added

public enum Rating {
    G,
    PG,
    R,
    UNRATED,
    PG13
}

1990 NC-17 is added

public enum Rating {
    G,
    PG,
    R,
    UNRATED,
    PG13,
    NC17
}

If EnumType.STRING was used, then the enum could be reordered at anytime and would instead look as we have defined it originally with ratings starting at Gand increasing in severity to NC17 and eventually UNRATED. With EnumType.ORDINAL the logical ordering would not have withstood the test of time as new values were added.

If the order of the enum values is significant to your code, avoid EnumType.ORDINAL

Unit Testing the JPA @Enumerated

public class MoviesTest extends TestCase {

    public void test() throws Exception {

        final Properties p = new Properties();
        p.put("movieDatabase", "new://Resource?type=DataSource");
        p.put("movieDatabase.JdbcDriver", "org.hsqldb.jdbcDriver");
        p.put("movieDatabase.JdbcUrl", "jdbc:hsqldb:mem:moviedb");

        EJBContainer container = EJBContainer.createEJBContainer(p);
        final Context context = container.getContext();

        final Movies movies = (Movies) context.lookup("java:global/jpa-scratch/Movies");

        movies.addMovie(new Movie("James Frawley", "The Muppet Movie", 1979, Rating.G));
        movies.addMovie(new Movie("Jim Henson", "The Great Muppet Caper", 1981, Rating.G));
        movies.addMovie(new Movie("Frank Oz", "The Muppets Take Manhattan", 1984, Rating.G));
        movies.addMovie(new Movie("James Bobin", "The Muppets", 2011, Rating.PG));

        assertEquals("List.size()", 4, movies.getMovies().size());

        assertEquals("List.size()", 3, movies.findByRating(Rating.G).size());

        assertEquals("List.size()", 1, movies.findByRating(Rating.PG).size());

        assertEquals("List.size()", 0, movies.findByRating(Rating.R).size());

        container.close();
    }
}

Running

To run the example via maven:

cd jpa-enumerated
mvn clean install

Which will generate output similar to the following:

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running org.superbiz.jpa.enums.MoviesTest
Apache OpenEJB 4.0.0-beta-2    build: 20120115-08:26
http://tomee.apache.org/
INFO - openejb.home = /Users/dblevins/openejb/examples/jpa-enumerated
INFO - openejb.base = /Users/dblevins/openejb/examples/jpa-enumerated
INFO - Using 'javax.ejb.embeddable.EJBContainer=true'
INFO - Configuring Service(id=Default Security Service, type=SecurityService, provider-id=Default Security Service)
INFO - Configuring Service(id=Default Transaction Manager, type=TransactionManager, provider-id=Default Transaction Manager)
INFO - Configuring Service(id=movieDatabase, type=Resource, provider-id=Default JDBC Database)
INFO - Found EjbModule in classpath: /Users/dblevins/openejb/examples/jpa-enumerated/target/classes
INFO - Beginning load: /Users/dblevins/openejb/examples/jpa-enumerated/target/classes
INFO - Configuring enterprise application: /Users/dblevins/openejb/examples/jpa-enumerated
INFO - Configuring Service(id=Default Stateful Container, type=Container, provider-id=Default Stateful Container)
INFO - Auto-creating a container for bean Movies: Container(type=STATEFUL, id=Default Stateful Container)
INFO - Configuring Service(id=Default Managed Container, type=Container, provider-id=Default Managed Container)
INFO - Auto-creating a container for bean org.superbiz.jpa.enums.MoviesTest: Container(type=MANAGED, id=Default Managed Container)
INFO - Configuring PersistenceUnit(name=movie-unit)
INFO - Auto-creating a Resource with id 'movieDatabaseNonJta' of type 'DataSource for 'movie-unit'.
INFO - Configuring Service(id=movieDatabaseNonJta, type=Resource, provider-id=movieDatabase)
INFO - Adjusting PersistenceUnit movie-unit  to Resource ID 'movieDatabaseNonJta' from 'movieDatabaseUnmanaged'
INFO - Enterprise application "/Users/dblevins/openejb/examples/jpa-enumerated" loaded.
INFO - Assembling app: /Users/dblevins/openejb/examples/jpa-enumerated
INFO - PersistenceUnit(name=movie-unit, provider=org.apache.openjpa.persistence.PersistenceProviderImpl) - provider time 406ms
INFO - Jndi(name="java:global/jpa-enumerated/Movies!org.superbiz.jpa.enums.Movies")
INFO - Jndi(name="java:global/jpa-enumerated/Movies")
INFO - Created Ejb(deployment-id=Movies, ejb-name=Movies, container=Default Stateful Container)
INFO - Started Ejb(deployment-id=Movies, ejb-name=Movies, container=Default Stateful Container)
INFO - Deployed Application(path=/Users/dblevins/openejb/examples/jpa-enumerated)
INFO - Undeploying app: /Users/dblevins/openejb/examples/jpa-enumerated
INFO - Closing DataSource: movieDatabase
INFO - Closing DataSource: movieDatabaseNonJta
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 2.831 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0