[Hibernate] 注解映射例子

Hibernate 注解(Hibernate Annotation) 是一种比较新的方式，通过在 java 简单类增加注解，来声明 java 类和数据库表的映射，作用和 xml 文件相似。hibernate 注解可以用来增强，或者替换 xml 的映射声明方式。

hibernate 注解功能需要使用下面三个 jar 文件

hibernate-annotations.jar

ejb3-persistence.jar

hibernate-commons-annotations.jar

代码的目录结构如下：

hibernate.cfg.xml 存储数据库信息，如数据库类型，账号密码，数据库名称。

Employee.java Employee 实体类，包含注解，声明 java 类和数据库表结构的映射关系。

ManageEmployee.java 管理 Employee，并对外提供操作 Employee 对象数据的接口。

App.java，演示本例子。

代码详情

在数据库中，创建 EMPLOYEE 表结构

create table EMPLOYEE (
   id INT NOT NULL auto_increment,
   first_name VARCHAR(20) default NULL,
   last_name  VARCHAR(20) default NULL,
   salary     INT  default NULL,
   PRIMARY KEY (id)
);

创建 java 简单类，含有注解。

@Entity 表明 Employee 类似一个实体类( Entity Bean )，并且有一个无参构造函数

@Table 声明用于持久化当前类( Employee ) 的数据库表名。

@Id，每一个实体类都有一个主键，用 @Id 声明

@Column，表明当前变量所映射的数据库表的字段

package tony.hibernateAnnotation;

import javax.persistence.*;

@Entity
@Table(name="EMPLOYEE")
public class Employee {
   @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
   @Column(name = "id")
   private int id;

   @Column(name = "first_name")
   private String firstName;

   @Column(name = "last_name")
   private String lastName;

   @Column(name = "salary")
   private int salary;  

   public Employee() {}
   public int getId() {
      return id;
   }
   public void setId( int id ) {
      this.id = id;
   }
   public String getFirstName() {
      return firstName;
   }
   public void setFirstName( String first_name ) {
      this.firstName = first_name;
   }
   public String getLastName() {
      return lastName;
   }
   public void setLastName( String last_name ) {
      this.lastName = last_name;
   }
   public int getSalary() {
      return salary;
   }
   public void setSalary( int salary ) {
      this.salary = salary;
   }
}

ManageEmployee.java Employee 的操作工具类。对业务层提供操作 Employee 的接口。

package tony.hibernateAnnotation;

import java.util.List;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public class ManageEmployee {

    private static SessionFactory factory;
    {
        factory = new Configuration().configure()
                .addPackage("tony.hibernateAnnotation")
                .addAnnotatedClass(Employee.class)
                .buildSessionFactory();
    }

    public int addEmployee(String fname, String lname, int salary){
          Session session = factory.openSession();
          Transaction tx = null;
          Integer employeeID = null;
          try{
             tx = session.beginTransaction();
             Employee employee = new Employee();
             employee.setFirstName(fname);
             employee.setLastName(lname);
             employee.setSalary(salary);

             employeeID = (Integer) session.save(employee);
             tx.commit();
          }catch (HibernateException e) {
             if (tx!=null) tx.rollback();
             e.printStackTrace();
          }finally {
             session.close();
          }
          return employeeID;
    }

    public void listEmployees(){
        Session session = factory.openSession();
        Transaction tx = null;
        tx = session.beginTransaction();
        List<Employee> employees = session.createQuery("FROM Employee").list();
        for(Employee emp : employees){
            System.out.println(emp.getId() + ", " + emp.getFirstName() + ", " + emp.getLastName() + ", " + emp.getSalary());;
        }
        tx.commit();
        session.close();
    }

    public List<Employee> getAllEmployees(){
        Session session = factory.openSession();
        Transaction tx = null;
        tx = session.beginTransaction();
        List<Employee> employees = session.createQuery("FROM Employee").list();
        tx.commit();
        session.close();
        return employees;
    }

    public void updateEmployee(int employeeId, int salary){
        Session session = factory.openSession();
        Transaction tx = null;

        tx = session.beginTransaction();
        Employee emp = session.get(Employee.class, employeeId);
        emp.setSalary(salary);
        session.update(emp);
        tx.commit();
        session.close();
    }

    public void deleteEmployee(int employeeId){
        Session session = factory.openSession();
        Transaction tx = null;

        tx = session.beginTransaction();
        Employee emp = session.get(Employee.class, employeeId);
        session.delete(emp);
        tx.commit();
        session.close();
    }
}

pom.xml 本例子所依赖的包

        <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>5.2.3.Final</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.6</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-annotations -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-annotations</artifactId>
            <version>3.5.6-Final</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.hibernate/ejb3-persistence -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>ejb3-persistence</artifactId>
            <version>3.3.2.Beta1</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-commons-annotations -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-commons-annotations</artifactId>
            <version>3.2.0.Final</version>
        </dependency>

说明

根据 tutorialspoint 例子的代码，无法运行成功，可能与 hibernate 使用版本有关，修正下面错误后，运作正常。

运行错误 1

java.lang.NoClassDefFoundError: org/hibernate/cfg/Mappings

在ManageEmployee.java 中，创建 SessionFactory 对象时，应该使用 org.hibernate.cfg.Configuration，而不是用 org.hibernate.cfg.AnnotationConfiguration。参考 java.lang.NoClassDefFoundError, stackOverflow

运行错误2

com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Table 'hibernatetest.hibernate_sequence' doesn't exist

这应该和 hibernate 版本有关，在 hibernate 5 及以上版本，在 java 文件中定义主键字段时，使用 @GeneratedValue(strategy = GenerationType.IDENTITY) 代替 @GeneratedValue。参考 Hibernate-sequence doesn't exist, stackOverflow

参考资料

Hibernate - Annotations, tutorialspoint