EntityManager

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 = "john doe";
        em.persist(person);
        em.getTransaction().commit();

        em.clear();

        Person p = em.find(Person.class, person.id);
        em.getTransaction().begin();
        em.createQuery("update Person set name = UPPER(name)").executeUpdate();
        em.getTransaction().commit();

        //        em.clear();
        em.refresh(p);
        // JOHN DOE
        System.out.println(p.name);

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

    @Entity(name = "Person")
    public static class Person {
        @Id
        @GeneratedValue
        Integer id;
        String 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 ('john doe', 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
update Person set name=upper(name)

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

如果将第 28 行的 em.clear(); 打开，则第 29 行将抛异常 java.lang.IllegalArgumentException: Entity not managed

Session

package org.example.demo.hibernate;

import java.sql.Statement;

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 = "john doe";
        session.persist(person);
        session.getTransaction().commit();

        session.clear();

        Person p = session.get(Person.class, person.id);
        session.doWork(connection -> {
            try (Statement statement = connection.createStatement()) {
                statement.executeUpdate("UPDATE Person SET name = UPPER(name)");
            }
        });
        session.clear();
        session.refresh(p);
        // JOHN DOE
        System.out.println(p.name);

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

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

执行的 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 ('john doe', 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
UPDATE Person SET name = UPPER(name)
select test_perso0_.id as id1_0_0_, test_perso0_.name as name2_0_0_ from Person test_perso0_ where test_perso0_.id=1

Refresh gotchas

package org.example.demo.hibernate;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Persistence;

import org.hibernate.annotations.NaturalId;

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 = "john doe";
        em.persist(person);
        em.getTransaction().commit();

        em.clear();

        Person p = em.find(Person.class, person.id);

        Book book = new Book();
        book.id = 100;
        book.title = "Hibernate User Guide";
        book.author = p;
        p.books.add(book);

        // javax.persistence.EntityNotFoundException: No row with the given identifier exists: [org.example.demo.hibernate.Test$Book#100]
        em.refresh(p);

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

    @Entity(name = "Person")
    public static class Person {
        @Id
        @GeneratedValue
        Integer id;
        String name;
        @OneToMany(mappedBy = "author", cascade = CascadeType.REFRESH)
        List<Book> books = new ArrayList<>();
    }

    @Entity(name = "Book")
    public static class Book {
        @Id
        Integer id;
        String title;
        @NaturalId
        String isbn;
        @ManyToOne
        Person author;
    }
}

注意要有 cascade = CascadeType.REFRESH 才会出错，否则只会 refresh person

另外假如 Book 的 id 是自动生成的，则将抛出 org.hibernate.AssertionFailure: null identifier