The following code example show you how to delete or remove entity object from database using JPA. The first class that we are going to create is ArtistDao. This DAO class handles the delete process either by the entity Id or by the entity object itself. We define the delete process in deleteArtistById(Long artistId) and deleteArtist(Artist artist) methods.

In those methods we call the EntityManager.remove() method. This method of EntityManager will taking care of removing the entity object from our database. Let’s see the DAO code below:

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;
import javax.persistence.EntityNotFoundException;
import javax.persistence.Query;
import java.util.List;

public class ArtistDao {
    private EntityManager manager;

    public ArtistDao(EntityManager manager) {
        this.manager = manager;
    }

    /**
     * Find Artist based on the entity Id.
     *
     * @param artistId the artist Id.
     * @return Artist.
     * @throws EntityNotFoundException when no artist is found.
     */
    public Artist findById(Long artistId) {
        Artist artist = manager.find(Artist.class, artistId);
        if (artist == null) {
            throw new EntityNotFoundException("Can't find Artist for ID "
                    + artistId);
        }
        return artist;
    }

    /**
     * Create a new artist record in the database.
     *
     * @param artist Artist to be created.
     * @return the ID of saved Artist.
     */
    public void createArtist(Artist artist) {
        manager.getTransaction().begin();
        manager.persist(artist);
        manager.getTransaction().commit();
    }

    /**
     * Get Artists from database.
     *
     * @return a list of artist.
     */
    @SuppressWarnings("unchecked")
    public List getArtists() {
        Query query = manager.createQuery("select a from Artist a");
        return query.getResultList();
    }

    /**
     * Update Artist information.
     *
     * @param artist an Artist to be updated.
     */
    public void updateArtist(Artist artist) {
        manager.getTransaction().begin();
        manager.merge(artist);
        manager.getTransaction().commit();
    }

    /**
     * Delete artist by their Id.
     *
     * @param artistId the artist Id.
     */
    public void deleteArtistById(Long artistId) {
        Artist artist = manager.find(Artist.class, artistId);
        if (artist != null) {
            manager.getTransaction().begin();
            manager.remove(artist);
            manager.getTransaction().commit();
        }
    }

    /**
     * Delete artist entity.
     *
     * @param artist the object to be deleted.
     */
    public void deleteArtist(Artist artist) {
        manager.getTransaction().begin();
        manager.remove(artist);
        manager.getTransaction().commit();
    }
}

After defining the delete methods in the ArtistDao class we create a simple program to demonstrate both of them. In this program we start by create the EntityManagerFactory object from the defined persistence unit in the persistence.xml file. Then we create the EntityManager object and we pass it to our ArtistDao object. And then we call the delete methods to remove entity from the database.

To show you the result of the delete process we print out the artist data before and after the delete method is called.

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import java.util.List;

public class EntityRemoveDemo {
    public static final String PERSISTENCE_UNIT_NAME = "music";

    public static void main(String[] args) {
        EntityManagerFactory factory =
                Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
        EntityManager manager = factory.createEntityManager();

        ArtistDao dao = new ArtistDao(manager);
        System.out.println("Before Delete:");
        printArtists(dao.getArtists());

        //
        // Remove artist with Id = 1.
        //
        dao.deleteArtistById(1L);

        //
        // Remove artist with Id = 2.
        //
        Artist artist = dao.findById(2L);
        dao.deleteArtist(artist);

        System.out.println("After Delete:");
        printArtists(dao.getArtists());
    }

    private static void printArtists(List artists) {
        for (Artist artist : artists) {
            System.out.println("Artist = " + artist);
        }
    }
}

Here is the result of our code snippet. It shows the number of records before and after the delete process.

Before Delete:
Artist = org.kodejava.example.model.Artist@23cd88[id=1,name=Bryan Adams,records=[]]
Artist = org.kodejava.example.model.Artist@4dc5e1[id=3,name=Metallica,records=[]]
Artist = org.kodejava.example.model.Artist@1fa682c[id=2,name=Mr. Big,records=[]]
After Delete:
Artist = org.kodejava.example.model.Artist@4dc5e1[id=3,name=Metallica,records=[]]

If you want to get the primary key of any JPA entity object you can use PersistenceUnitUtil.getIdentifier() method. This method take a single parameter which is the entity object whose identifier to be read. The PersistenceUnitUtil instance can be accessed from the EntityManagerFactory object.

If the entity object contains an identifier the getIdentifier() method will return the identifier as a java.lang.Object. If the entity object doesn’t have an identifier ye it will return null.

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class GetEntityIdDemo {
    public static final String PERSISTENCE_UNIT_NAME = "music";

    public static void main(String[] args) {
        EntityManagerFactory factory =
                Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
        EntityManager manager = factory.createEntityManager();

        Artist artist = manager.find(Artist.class, 1L);
        if (artist != null) {
            Object identifier = 
                    factory.getPersistenceUnitUtil().getIdentifier(artist);
            
            System.out.println("Identifier = " + identifier);
        }
    }
}

In this example you will learn how to update an entity object in JPA. We use the EntityManager.merge() method to update an entity. This method takes the entity to be saved as the parameter and return the merged entity back as the result.

You can see a simple example to the code snippet below:

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;

public class ArtistDao {
    private EntityManager manager;

    public ArtistDao(EntityManager manager) {
        this.manager = manager;
    }

    /**
     * Update Artist information.
     *
     * @param artist an Artist to be updated.
     */
    public void updateArtist(Artist artist) {
        manager.getTransaction().begin();
        manager.merge(artist);
        manager.getTransaction().commit();
    }
}

And here a main program for running the ArtistDao class to update the artist data in the database.

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class EntityUpdateDemo {
    public static final String PERSISTENCE_UNIT_NAME = "music";

    public static void main(String[] args) {
        EntityManagerFactory factory =
                Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
        EntityManager em = factory.createEntityManager();

        ArtistDao dao = new ArtistDao(em);
        Artist artist = dao.findById(1L);
        System.out.println("Artist = " + artist);

        artist.setName("The New Artist Name");
        dao.updateArtist(artist);

        artist = dao.findById(artist.getId());
        System.out.println("Artist = " + artist);
    }
}

In this example you will learn how to find an entity object by its ID using JPA. To find entity by ID we use the EntityManager.find() method and pass the entity class the the entity ID as the parameters.

In the code snippet below the EntityManager required by the ArtistDao will be passed from the main program when the DAO is instantiated. The process of finding the Artist entity is defined in the findById() method in the DAO class. You must pass the ID of the entity to this method.

The findById() method call the EntityManager.find() method to find the entity. If no entity is found, where the artist == null a javax.persistence.EntityNotFoundException will be thrown.

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;
import javax.persistence.EntityNotFoundException;

public class ArtistDao {
    private EntityManager manager;

    public ArtistDao(EntityManager manager) {
        this.manager = manager;
    }

    /**
     * Find Artist based on the entity Id.
     *
     * @param artistId the artist Id.
     * @return Artist.
     * @throws EntityNotFoundException when no artist is found.
     */
    public Artist findById(Long artistId) {
        Artist artist = manager.find(Artist.class, artistId);
        if (artist == null) {
            throw new EntityNotFoundException("Can't find Artist for ID "
                    + artistId);
        }
        return artist;
    }
}

To run the DAO class we create a main program in the code snippet below. The steps is create the EntityManagerFactory configured by your persistence.xml file. Create the EntityManager using the factory object. Create the DAO and pass the EntityManager to it. And finally call the findById() method of the DAO class.

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityNotFoundException;
import javax.persistence.Persistence;

public class FindEntityByIdDemo {
    public static final String PERSISTENCE_UNIT_NAME = "music";

    public static void main(String[] args) {
        EntityManagerFactory factory =
                Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
        EntityManager em = factory.createEntityManager();

        ArtistDao dao = new ArtistDao(em);

        //
        // Find an artist with ID = 1 from the database. The entity is
        // exists in the database.
        //
        Artist artist = dao.findById(1L);
        System.out.println("Artist = " + artist);

        try {
            //
            // Find an entity that is not exists in the database will
            // throw an exception.
            //
            artist = dao.findById(100L);
            System.out.println("Artist = " + artist);
        } catch (EntityNotFoundException e) {
            System.out.println("Can't find artist in the database.");
        }
    }
}

In this example you are going to learn how to persist or save an entity object to database table using JPA. We are going to create a data access object (DAO) for persisting an Artist entity.

We create a class called ArtistDao with a constructor that accept an EntityManager parameter. We provide a couple methods in this DAO such as the createArtist() and getArtist() methods. This method is for persisting entity and retrieve a collection of entities from the database.

To persist object to database we call the EntityManager.persist() method with the entity object to be saved as the parameter. We also have to begin and commit the transaction before and after we call the persist() method. Here is the code for our DAO.

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;
import javax.persistence.Query;
import java.util.List;

public class ArtistDao {
    private EntityManager manager;

    public ArtistDao(EntityManager manager) {
        this.manager = manager;
    }

    public Artist findById(Long artistId) {
        return manager.find(Artist.class, artistId);
    }

    public void createArtist(Artist artist) {
        manager.getTransaction().begin();
        manager.persist(artist);
        manager.getTransaction().commit();
    }

    @SuppressWarnings("unchecked")
    public List getArtists() {
        Query query = manager.createQuery("select a from Artist a");
        return query.getResultList();
    }
}

To demonstrate the DAO we create a simple program as you can see below. The program start by creating the EntityManagerFactory configured by the persistence unit defined in the persistence.xml file. From the factory we create the EntityManager object which will be passed to the ArtistDao.

After create an instance of the ArtistDao we insert some artist record to database by calling the dao.createArtist() method. To check that the data successfully stored in the database we call the dao.getArtists() to read the data back from the database and print it out to the screen.

package org.kodejava.example.jpa;

import org.kodejava.example.model.Artist;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import java.util.List;

public class EntityPersistDemo {
    public static final String PERSISTENCE_UNIT_NAME = "music";

    public static void main(String[] args) {
        EntityManagerFactory factory =
                Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
        EntityManager manager = factory.createEntityManager();

        String[] artistNames = {"Bryan Adams", "Mr. Big", "Metallica"};

        ArtistDao dao = new ArtistDao(manager);

        for (String name : artistNames) {
            Artist artist = new Artist();
            artist.setName(name);
            dao.createArtist(artist);
        }

        List artistList = dao.getArtists();
        for (Artist artist : artistList) {
            System.out.println("artist = " + artist);
        }
    }
}