Hibernate入门之创建数据库表

前言

Hibernate 5.1和更早版本至少需要Java 1.6和JDBC 4.0，Hibernate 5.2和更高版本至少需要Java 1.8和JDBC 4.2，从本节开始我们正式进入Hibernate学习，所采用版本为5.4.12.Final稳定版本。

Hibernate创建数据库表

首先我们通过idea常见mven项目，并下载JDBC和Hibernate包以及添加java 8语法支持，包和添加和项目创建如下：

<dependencies>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.13</version>
    </dependency>
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-core</artifactId>
        <version>5.4.12.Final</version>
    </dependency>
</dependencies>
<build>
    <sourceDirectory>src/main/java</sourceDirectory>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <source>8</source>
                <target>8</target>
            </configuration>
        </plugin>
    </plugins>
</build>

接下来我们创建如上Student实体

package hibernate.entity;

import javax.persistence.*;

@Entity
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column
    private int id;

    @Column
    private String firstName;

    @Column
    private String lastName;

    @Column
    private String email;

    public Student() {

    }

    public Student(String firstName, String lastName, String email) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.email = email;
    }

    public int getId() {
        return id;
    }

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

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Override
    public String toString() {
        return "Student [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + ", email=" + email + "]";
    }
}

然后在上述资源文件夹resources下创建Hibernate配置文件，如下：

<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- JDBC Database connection settings -->
        <property name="connection.driver_class">com.mysql.cj.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/hibernate_db?serverTimezone=UTC</property>
        <property name="connection.username">root</property>
        <property name="connection.password">root</property>
        <!-- Select our SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQL5Dialect</property>
        <!-- Echo the SQL to stdout -->
        <property name="show_sql">true</property>
        <!-- Set the current session context -->
        <property name="current_session_context_class">thread</property>
        <!-- Drop and re-create the database schema on startup -->
        <property name="hibernate.hbm2ddl.auto">create-drop</property>
        <!-- dbcp connection pool configuration -->
        <property name="hibernate.dbcp.initialSize">5</property>
        <property name="hibernate.dbcp.maxTotal">20</property>
        <property name="hibernate.dbcp.maxIdle">10</property>
        <property name="hibernate.dbcp.minIdle">5</property>
        <property name="hibernate.dbcp.maxWaitMillis">-1</property>
        <mapping class="hibernate.entity.Student" />
    </session-factory>
</hibernate-configuration>

接下来我们创建Hibernate的SessionFactory，以此来进行对象的会话管理，如下：

package hibernate.util;

import org.hibernate.SessionFactory;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

public class HibernateUtil {
    private static StandardServiceRegistry registry;
    private static SessionFactory sessionFactory;

    public static SessionFactory getSessionFactory() {
        if (sessionFactory == null) {
            try {

                // Create registry
                 registry = new StandardServiceRegistryBuilder().configure().build();

                // Create MetadataSources
                MetadataSources sources = new MetadataSources(registry);

                // Create Metadata
                 Metadata metadata = sources.getMetadataBuilder().build();

                // Create SessionFactory
                sessionFactory = metadata.getSessionFactoryBuilder().build();

            } catch (Exception e) {
                e.printStackTrace();
                if (registry != null) {
                    StandardServiceRegistryBuilder.destroy(registry);
                }
            }
        }
        return sessionFactory;
    }

    public static void shutdown() {
        if (registry != null) {
            StandardServiceRegistryBuilder.destroy(registry);
        }
    }
}

最后则是通过上述工厂打开会话，初始化数据库表，然后保存对象实例，如下：

package hibernate;

import hibernate.entity.Student;
import hibernate.util.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.Transaction;

import java.util.List;

public class App {
    public static void main(String[] args) {

        Student student = new Student("Jeffcky1", "Wang1", "2752154844@qq.com");
        Student student1 = new Student("Jeffcky2", "Wang2", "2752154844@qq.com");

        Transaction transaction = null;

        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            // start a transaction
            transaction = session.beginTransaction();
            // save the student objects
            session.save(student);
            session.save(student1);
            // commit transaction
            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        }

        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            List<Student> students = session.createQuery("from Student", Student.class).list();
            students.forEach(s -> System.out.println(s.getFirstName()));
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        }
    }
}

 在上述Hibernate配置文件中，我们开启了在控制台输出SQL日志，如下：

Hibernate: create table Student (id integer not null auto_increment, email varchar(255), firstName varchar(255), lastName varchar(255), primary key (id)) engine=MyISAM

总结 

本节我们学习了如何通过Hibernate来创建数据库表的基本过程，我们需要通过JPA注解POJO对象，然后通过配置文件配置数据库连接和映射POJO对象，最后创建Hibernate会话工厂开启管理对象会话。本节我们到此结束，还是比较简单，下一节我们开始讲解命名策略。

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