EntityManager.getReference

src/main/resources/META-INF/persistence.xml

<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence
             http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"
  version="2.1">
  <persistence-unit name="test">
    <properties>
      <property name="javax.persistence.jdbc.url" value="jdbc:p6spy:postgresql://localhost:5432/test" />
      <property name="javax.persistence.jdbc.user" value="postgres" />
      <property name="javax.persistence.jdbc.password" value="postgres" />

      <property name="javax.persistence.schema-generation.database.action" value="drop-and-create" />
    </properties>
  </persistence-unit>
</persistence>

package org.example.demo.hibernate;

import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Persistence;

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

        em.getTransaction().begin();
        Person person = new Person();
        person.name = "测试人";
        em.persist(person);
        em.getTransaction().commit();

        em.clear();

        Person p = em.getReference(Person.class, person.id);
        // class org.example.demo.hibernate.Test$Person_$$_jvst169_0
        System.out.println(p.getClass());
        // 如果没有下面这句，则将不会查询数据库
        // org.example.demo.hibernate.Test$Person@1f2d2181
        System.out.println(p);
        // null
        System.out.println(p.name);
        // 测试人
        System.out.println(p.getName());

        em.close();
        factory.close();
    }

    @Entity(name = "Person")
    public static class Person {
        @Id
        @GeneratedValue
        Integer id;
        String name;

        public Integer getId() {
            return id;
        }

        public void setId(Integer id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }
}

生成的 SQL

drop table if exists Person cascade
drop sequence if exists hibernate_sequence

create sequence hibernate_sequence start 1 increment 1
create table Person (id int4 not null, name varchar(255), primary key (id))
select nextval ('hibernate_sequence')
insert into Person (name, id) values ('测试人', 1)

select test_perso0_.id as id1_0_0_, test_perso0_.name as name2_0_0_ from Person test_perso0_ where test_perso0_.id=1

如果换个不存在的 id 即将第 23 行改为

        Person p = em.getReference(Person.class, 2);

则第 28 行将抛异常 javax.persistence.EntityNotFoundException: Unable to find org.example.demo.hibernate.Test$Person with id 2

注意第 30 行输出 null 这是因为 runtime proxy 只能使用 getter/setter 不能使用 fields 取值。注意这不是指 access strategy 要为 property-based ，而是因为 runtime proxy 只 override 了 getter/setter 方法，而不改变 field 的值。

Session.load

package org.example.demo.hibernate;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.model.naming.ImplicitNamingStrategyJpaCompliantImpl;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.service.ServiceRegistry;

public class Test {
    public static void main(String[] args) {
        ServiceRegistry standardRegistry = new StandardServiceRegistryBuilder()
                .applySetting("hibernate.connection.url", "jdbc:p6spy:postgresql://localhost:5432/test")
                .applySetting("hibernate.connection.username", "postgres")
                .applySetting("hibernate.connection.password", "postgres")
                .applySetting("hibernate.hbm2ddl.auto", "create").build();
        Metadata metadata = new MetadataSources(standardRegistry).addAnnotatedClass(Person.class).getMetadataBuilder()
                .applyImplicitNamingStrategy(ImplicitNamingStrategyJpaCompliantImpl.INSTANCE).build();
        SessionFactory sessionFactory = metadata.getSessionFactoryBuilder().build();
        Session session = sessionFactory.openSession();

        session.beginTransaction();
        Person person = new Person();
        person.name = "测试人";
        session.persist(person);
        session.getTransaction().commit();

        session.clear();

        Person p = session.load(Person.class, person.id);
        // class org.example.demo.hibernate.Test$Person_$$_jvsta0_0
        System.out.println(p.getClass());
        // 如果没有下面这句，则将不会查询数据库
        // org.example.demo.hibernate.Test$Person@22db8f4
        System.out.println(p);
        // null
        System.out.println(p.name);
        // 测试人
        System.out.println(p.getName());

        session.close();
        sessionFactory.close();
    }

    @Entity(name = "Person")
    public static class Person {
        @Id
        @GeneratedValue
        Integer id;
        String name;

        public Integer getId() {
            return id;
        }

        public void setId(Integer id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }
}

生成的 SQL

drop table if exists Person cascade
drop sequence if exists hibernate_sequence

create sequence hibernate_sequence start 1 increment 1
create table Person (id int4 not null, name varchar(255), primary key (id))
select nextval ('hibernate_sequence')
insert into Person (name, id) values ('测试人', 1)

select test_perso0_.id as id1_0_0_, test_perso0_.name as name2_0_0_ from Person test_perso0_ where test_perso0_.id=1

若将第 35 行改为

        Person p = session.load(Person.class, 2);

第 40 行将抛异常 org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [org.example.demo.hibernate.Test$Person#2] 。注意与 JPA 抛的异常类不一样。

Session.load 实际调用 Session.byId getReference ，参见 org.hibernate.internal.SessionImpl.load(Class, Serializable) 方法。