Java如何获取任何JPA实体的主键？

如果要获取任何JPA实体对象的主键，则可以使用PersistenceUnitUtil.getIdentifier()方法。此方法采用单个参数，该参数是要读取其标识符的实体对象。该PersistenceUnitUtil实例可以从被访问EntityManagerFactory的对象。

如果实体对象包含标识符，则该getIdentifier()方法将以形式返回该标识符java.lang.Object。如果实体对象没有标识符，它将返回null。

package org.nhooo.example.jpa;

import org.nhooo.example.jpa.entity.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();

        // 获取存在实体的对象标识符。
        Artist artist = manager.find(Artist.class, 1L);
        if (artist != null) {
            Object identifier =
                factory.getPersistenceUnitUtil().getIdentifier(artist);

            System.out.println("Identifier = " + identifier);
        }

        // 获取新插入实体的对象标识符。
        Artist newArtist = new Artist();
        newArtist.setName("Bon Jovi");

        manager.getTransaction().begin();
        manager.persist(newArtist);
        manager.getTransaction().commit();

        Object identifier =
            factory.getPersistenceUnitUtil().getIdentifier(newArtist);
        System.out.println("Identifier = " + identifier);
    }
}

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>