EntityManager.find

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.find(Person.class, person.id);
        // class org.example.demo.hibernate.Test$Person
        System.out.println(p.getClass());
        // org.example.demo.hibernate.Test$Person@59221b97
        System.out.println(p);

        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 ('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

如果将第 23 行改为

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

则第 25 行将抛 java.lang.NullPointerException

Session.get

Session.get 方式

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.get(Person.class, person.id);
        // class org.example.demo.hibernate.Test$Person
        System.out.println(p.getClass());
        // org.example.demo.hibernate.Test$Person@72bca894
        System.out.println(p);

        session.close();
        sessionFactory.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 ('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

如果将第 35 行改为

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

则第 37 行将抛 java.lang.NullPointerException

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

Session.byId load

Session.byId 方式

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.byId(Person.class).load(person.id);
        // class org.example.demo.hibernate.Test$Person
        System.out.println(p.getClass());
        // org.example.demo.hibernate.Test$Person@72bca894
        System.out.println(p);

        session.close();
        sessionFactory.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 ('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

如果将第 35 行改为

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

则第 37 行将抛 java.lang.NullPointerException

Session.byId loadOptional

package org.example.demo.hibernate;

import java.util.Optional;

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();

        Optional<Person> optionalPerson = session.byId(Person.class).loadOptional(person.id);
        // class java.util.Optional
        System.out.println(optionalPerson.getClass());
        // org.example.demo.hibernate.Test$Person@72bca894
        System.out.println(optionalPerson.get());

        session.close();
        sessionFactory.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 ('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

如果将第 37 行改为

        Optional<Person> optionalPerson = session.byId(Person.class).loadOptional(2);

则第 41 行抛 java.util.NoSuchElementException: No value present