`@Immutable`和`@NaturalId` – 特定于 Hiberate 的注解 - 《HowToDoInJava 中文系列教程》

1）@Immutable注解
2）@NaturalId注解
- 简单的自然 ID 示例
- 复合自然 ID 示例

原文： https://howtodoinjava.com/hibernate/immutable-and-naturalid-hibernate-specific-annotations/

在上一篇文章中，我们了解了Hiberate中最常用的 JPA 注解，还了解了与这些 JPA 注解互补的 Hiberate 注解。使用 JPA 注解使您的应用代码可移植到其他 JPA 实现中，这是一件好事。除了 JPA 注解之外，Hiberate 还具有一些自己的注解，您可以使用它们在应用代码中具有某些功能。但是您必须记住，将来的日期可能很难使您的代码可移植。

阅读更多： JPA2 持久化注解教程

在本文中，我们将学习两个特定于 Hibernate 的此类注解。

1）`@Immutable`注解

@Immutable注解将一个实体标记为不可变。这在您的实体表示参考数据的情况下非常有用，例如状态列表，性别或其他很少突变的数据。

由于状态之类的东西很少会更改，因此通常有人会通过 SQL 或管理应用手动更新数据。 Hibernate 可以积极地缓存此数据，需要将其考虑在内。如果参考数据发生变化，则需要确保已通知使用该数据的应用（可以使用refresh()方法）或以某种方式重新启动。

@Immutable注解告诉 Hibernate，对不可变实体的任何更新都不应传递给数据库而不会给出任何错误。 @Immutable也可以放在集合上；在这种情况下，对集合的更改（添加或删除）将引发HibernateException。

EmployeeEntity.java

import org.hibernate.annotations.Immutable;
@Immutable
@Entity 
@Table(name = "Employee")
public class EmployeeEntity implements Serializable
{
    private static final long serialVersionUID = -1798070786993154676L;
    @Id
    @Column(name = "ID", unique = true, nullable = false)
    private Integer           employeeId;
    @Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
    private String            firstName;
    @Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
    private String            lastName;
    //Setters and Getters
}

ImmutableAnnotationExample.java

public class ImmutableAnnotationExample
{
   public static void main(String[] args)
   {
      setupTestData();
      Session sessionOne = HibernateUtil.getSessionFactory().openSession();
      sessionOne.beginTransaction();
      //Load the employee in another session
      EmployeeEntity employee = (EmployeeEntity) sessionOne.load(EmployeeEntity.class, 1);
      //Update the first name
      employee.setFirstName("Alex");
      sessionOne.flush();
      sessionOne.close();
      Session sessionTwo = HibernateUtil.getSessionFactory().openSession();
      sessionTwo.beginTransaction();
      //Load the employee in another session
      EmployeeEntity employeeUpdated = (EmployeeEntity) sessionTwo.load(EmployeeEntity.class, 1);
      //Verify the first name
      System.out.println(employeeUpdated.getFirstName());
      sessionTwo.flush();
      sessionTwo.close();
      HibernateUtil.shutdown();
   }  
   private static void setupTestData(){
      Session session = HibernateUtil.getSessionFactory().openSession();
      session.beginTransaction();
      //Create Employee
      EmployeeEntity emp = new EmployeeEntity();
      emp.setEmployeeId(1);
      emp.setFirstName("Lokesh");
      emp.setLastName("Gupta");
      session.save(emp);
      session.getTransaction().commit();
      session.close();
   }
}
Output:
Hibernate: insert into Employee (FIRST_NAME, LAST_NAME, ID) values (?, ?, ?)
Hibernate: select employeeen0_.ID as ID1_1_0_, employeeen0_.FIRST_NAME as FIRST_NA2_1_0_, employeeen0_.LAST_NAME as LAST_NAM3_1_0_ 
from Employee employeeen0_ where employeeen0_.ID=?
Hibernate: select employeeen0_.ID as ID1_1_0_, employeeen0_.FIRST_NAME as FIRST_NA2_1_0_, employeeen0_.LAST_NAME as LAST_NAM3_1_0_ 
from Employee employeeen0_ where employeeen0_.ID=?
Lokesh //Value didn't updated in database i.e. immutable

2）`@NaturalId`注解

在过去的教程中，我们了解了很多有关@Id和@GeneratedValue注解以为数据库中的记录创建主键的知识。在大多数实际应用中，这些主键是“人工主键”，并且仅在应用运行时内部引用。但是，还存在“自然 ID”的概念，除了人工或复合主键之外，它还提供了另一种方便且合乎逻辑的方式来引用实体。

自然 ID 的示例可能是美国的社会安全号码或税号，而印度则是 PAN 号码。 实体（是个人或公司）可能具有由 Hibernate 生成的人为主键，但也可能具有唯一的税标识符。 Hibernate 还允许您基于这些自然 ID 搜索和加载实体。

对于自然 ID，有两种形式的加载机制：一种使用简单自然 ID（其中自然 ID 是一个且仅一个字段），另一种使用命名属性作为复合自然 ID 的一部分。

简单的自然 ID 示例

@Entity 
@Table(name = "Employee")
public class EmployeeEntity implements Serializable
{
   private static final long serialVersionUID = -1798070786993154676L;
   @Id
   @Column(name = "ID", unique = true, nullable = false)
   private Integer           employeeId;
   @Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
   private String            firstName;
   @Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
   private String            lastName;
   //Natural id can be SSN as well
   @NaturalId
   Integer SSN;
   //Setters and Getters
}

SimpleNaturalIdExample.java

public class SimpleNaturalIdExample
{
   public static void main(String[] args)
   {
      setupTestData();
      Session sessionOne = HibernateUtil.getSessionFactory().openSession();
      sessionOne.beginTransaction();
      //Load the employee
      EmployeeEntity employee1 = (EmployeeEntity) sessionOne.load(EmployeeEntity.class, 1);
      //Just to ensure that employee is loasded from DB
      System.out.println(employee1.getFirstName());
      //Get the employee for natural id i.e. SSN; This does not execute another SQL SELECT as entity is already present in session
      EmployeeEntity employee2 = (EmployeeEntity) sessionOne.bySimpleNaturalId(EmployeeEntity.class).load(12345);
      //Verify that employee1 and employee2 refer to same object
      assert(employee1 == employee2);
      sessionOne.flush();
      sessionOne.close();
      System.out.println("====================================");
      Session sessionTwo = HibernateUtil.getSessionFactory().openSession();
      sessionTwo.beginTransaction();
      //Get the employee for natural id i.e. SSN; entity is not present in this session
      EmployeeEntity employee = (EmployeeEntity) sessionTwo.bySimpleNaturalId(EmployeeEntity.class).load(12345);
      sessionTwo.flush();
      sessionTwo.close();
      HibernateUtil.shutdown();
   }  
   private static void setupTestData(){
      Session session = HibernateUtil.getSessionFactory().openSession();
      session.beginTransaction();
      //Create Employee
      EmployeeEntity emp = new EmployeeEntity();
      emp.setEmployeeId(1);
      emp.setFirstName("Lokesh");
      emp.setLastName("Gupta");
      emp.setSSN(12345);
      session.save(emp);
      session.getTransaction().commit();
      session.close();
   }
}
Output:
Hibernate: insert into Employee (SSN, FIRST_NAME, LAST_NAME, ID) values (?, ?, ?, ?)
Hibernate: select employeeen0_.ID as ID1_1_0_, employeeen0_.SSN as SSN2_1_0_, employeeen0_.FIRST_NAME as FIRST_NA3_1_0_,
 employeeen0_.LAST_NAME as LAST_NAM4_1_0_ from Employee employeeen0_ where employeeen0_.ID=?
Lokesh
====================================
Hibernate: select employeeen_.ID as ID1_1_ from Employee employeeen_ where employeeen_.SSN=?
Hibernate: select employeeen0_.ID as ID1_1_0_, employeeen0_.SSN as SSN2_1_0_, employeeen0_.FIRST_NAME as FIRST_NA3_1_0_, 
employeeen0_.LAST_NAME as LAST_NAM4_1_0_ from Employee employeeen0_ where employeeen0_.ID=?

请密切注意如果会话中不存在实体，并且如果您使用实体的自然 ID 获取实体，则使用自然 ID 提取第一个主要 ID；然后使用此主要 ID 提取实体。 如果会话中已经存在实体，则返回相同实体的引用，而无需在数据库中执行其他SELECT语句。

复合自然 ID 示例

@Entity 
@Table(name = "Employee")
public class EmployeeEntity implements Serializable
{
   private static final long serialVersionUID = -1798070786993154676L;
   @Id
   @Column(name = "ID", unique = true, nullable = false)
   private Integer           employeeId;
   @Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
   private String            firstName;
   @Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
   private String            lastName;
   //Natural id part 1
   @NaturalId
   Integer seatNumber;
   //Natural id part 2
   @NaturalId
   String departmentName;
   //Setters and Getters
}

CompositeNaturalIdExample.java

public class CompositeNaturalIdExample
{
   public static void main(String[] args)
   {
      setupTestData();
      Session sessionOne = HibernateUtil.getSessionFactory().openSession();
      sessionOne.beginTransaction();
      //Get the employee for natural id i.e. SSN; entity is not present in this session
      EmployeeEntity employee = (EmployeeEntity) sessionOne.byNaturalId(EmployeeEntity.class)
                                                            .using("seatNumber", 12345)
                                                            .using("departmentName", "IT")
                                                            .load();
      System.out.println(employee.getFirstName());
      sessionOne.flush();
      sessionOne.close();
      HibernateUtil.shutdown();
   }  
   private static void setupTestData(){
      Session session = HibernateUtil.getSessionFactory().openSession();
      session.beginTransaction();
      //Create Employee
      EmployeeEntity emp = new EmployeeEntity();
      emp.setEmployeeId(1);
      emp.setFirstName("Lokesh");
      emp.setLastName("Gupta");
      emp.setSeatNumber(12345);
      emp.setDepartmentName("IT");
      session.save(emp);
      session.getTransaction().commit();
      session.close();
   }
}
Output:
Hibernate: insert into Employee (departmentName, FIRST_NAME, LAST_NAME, seatNumber, ID) values (?, ?, ?, ?, ?)
Hibernate: select employeeen_.ID as ID1_1_ from Employee employeeen_ where employeeen_.departmentName=? and employeeen_.seatNumber=?
Hibernate: select employeeen0_.ID as ID1_1_0_, employeeen0_.departmentName as departme2_1_0_, employeeen0_.FIRST_NAME as FIRST_NA3_1_0_, 
employeeen0_.LAST_NAME as LAST_NAM4_1_0_, employeeen0_.seatNumber as seatNumb5_1_0_ from Employee employeeen0_ where employeeen0_.ID=?
Lokesh

复合自然 ID 的实体获取逻辑与简单自然 ID 相同。除了使用多个自然键而不是一个以外，没有区别。

这些就是所有这些众所周知的注解。继续在评论中发表您的想法。

祝您学习愉快！

`@Immutable`和`@NaturalId` – 特定于 Hiberate 的注解

1）@Immutable注解

2）@NaturalId注解

简单的自然 ID 示例

复合自然 ID 示例

1）`@Immutable`注解

2）`@NaturalId`注解