Java如何在JPA中通过ID查找实体？
在此示例中，您将学习如何使用JPA通过实体ID查找实体对象。要通过ID查找实体，我们使用EntityManager.find()方法，并将实体ID作为参数传递给实体类。
在实例化DAO时，将从主程序传递EntityManager所需的以下代码段ArtistDaoImpl。查找Artist实体的过程findById()在DAO类的方法中定义。您必须将实体的ID传递给此方法。
该findById()方法调用该EntityManager.find()方法以查找实体。如果没有找到实体，其中artist==null一个javax.persistence.EntityNotFoundException将被抛出。
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();
}
@Override
@SuppressWarnings(value = "unchecked")
public List<Artist> getArtists() {
Query query = manager.createQuery("select a from Artist a", Artist.class);
return query.getResultList();
}
}要运行DAO类，我们在下面的代码片段中创建一个主程序。步骤EntityManagerFactory由您的persistence.xml文件创建配置。EntityManager使用工厂对象创建。创建DAO并将其传递EntityManager给它。最后调用findById()DAO类的方法。
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.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 ArtistDaoImpl(em);
//从数据库中找到ID=1的艺术家。该实体是
//存在于数据库中。
Artist artist = dao.findById(1L);
System.out.println("Artist = " + artist);
try {
//查找数据库中不存在的实体将
//引发异常。
artist = dao.findById(100L);
System.out.println("Artist = " + artist);
} catch (EntityNotFoundException e) {
System.out.println(e.getMessage());
}
}
}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>