Java如何在JPA中删除实体对象？
以下代码示例向您展示如何使用JPA从数据库中删除或删除实体对象。我们要创建的第一个类是ArtistDaoImpl实现ArtistDao。此DAO类通过实体ID或实体对象本身处理删除过程。我们在deleteById(Longid)和delete(Artistartist)方法中定义删除过程。
在那些方法中，我们称该EntityManager.remove()方法。这种方法EntityManager将负责从数据库中删除实体对象。让我们看看下面的DAO代码：
package org.nhooo.example.jpa.dao;
import org.nhooo.example.jpa.entity.Artist;
import java.util.List;
public interface ArtistDao {
Artist findById(Long id);
void save(Artist artist);
void update(Artist artist);
List<Artist> getArtists();
void deleteById(Long id);
void delete(Artist artist);
}package org.nhooo.example.jpa.dao.impl;
import org.nhooo.example.jpa.dao.ArtistDao;
import org.nhooo.example.jpa.entity.Artist;
import javax.persistence.EntityManager;
import javax.persistence.EntityNotFoundException;
import javax.persistence.Query;
import java.util.List;
public class ArtistDaoImpl implements ArtistDao {
private EntityManager manager;
public ArtistDaoImpl(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;
}
@Override
public void save(Artist artist) {
manager.getTransaction().begin();
manager.persist(artist);
manager.getTransaction().commit();
}
/**
* Update Artist information.
*
* @param artist an Artist to be updated.
*/
@Override
public void update(Artist artist) {
manager.getTransaction().begin();
manager.merge(artist);
manager.getTransaction().commit();
}
@Override
@SuppressWarnings(value = "unchecked")
public List<Artist> getArtists() {
Query query = manager.createQuery("select a from Artist a", Artist.class);
return query.getResultList();
}
/**
* Delete artist by their Id.
*
* @param id the artist Id.
*/
@Override
public void deleteById(Long id) {
Artist artist = manager.find(Artist.class, id);
if (artist != null) {
manager.getTransaction().begin();
manager.remove(artist);
manager.getTransaction().commit();
}
}
/**
* Delete artist entity.
*
* @param artist the object to be deleted.
*/
@Override
public void delete(Artist artist) {
manager.getTransaction().begin();
manager.remove(artist);
manager.getTransaction().commit();
}
}在类中定义了delete方法之后，ArtistDao我们创建一个简单的程序来演示这两个方法。在此程序中，我们首先EntityManagerFactory从persistence.xml文件中定义的持久性单元创建对象。然后，我们创建EntityManager对象并将其传递给我们的ArtistDaoImpl对象。然后，我们调用delete方法从数据库中删除实体。
为了向您显示删除过程的结果，我们在调用delete方法之前和之后打印出艺术家数据。
package org.nhooo.example.jpa;
import org.nhooo.example.jpa.dao.ArtistDao;
import org.nhooo.example.jpa.dao.impl.ArtistDaoImpl;
import org.nhooo.example.jpa.entity.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 ArtistDaoImpl(manager);
System.out.println("Before Delete:");
printArtists(dao.getArtists());
//移除ID=1的演出者。
dao.deleteById(1L);
//移除ID=2的演出者。
Artist artist = dao.findById(2L);
dao.delete(artist);
System.out.println("After Delete:");
printArtists(dao.getArtists());
}
private static void printArtists(List<Artist> artists) {
for (Artist artist : artists) {
System.out.println("Artist = " + artist);
}
}
}这是我们的代码片段的结果。它显示删除过程之前和之后的记录数。
Before Delete:
Artist = Artist{id=1, name='Bon Jovi'}
Artist = Artist{id=2, name='Mr. Big'}
Artist = Artist{id=3, name='Metallica'}
After Delete:
Artist = Artist{id=3, name='Metallica'}Maven依赖
<dependencies>
<!-- https://search.maven.org/remotecontent?filepath=javax/persistence/javax.persistence-api/2.2/javax.persistence-api-2.2.jar -->
<dependency>
<groupId>javax.persistence</groupId>
<artifactId>javax.persistence-api</artifactId>
<version>2.2</version>
</dependency>
<!-- https://search.maven.org/remotecontent?filepath=org/hibernate/hibernate-core/5.4.1.Final/hibernate-core-5.4.1.Final.jar -->
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>5.4.1.Final</version>
</dependency>
<!-- https://search.maven.org/remotecontent?filepath=mysql/mysql-connector-java/5.1.47/mysql-connector-java-5.1.47.jar -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.47</version>
</dependency>
</dependencies>