# `@Immutable`和`@NaturalId` – 特定于 Hiberate 的注解 > 原文： [https://howtodoinjava.com/hibernate/immutable-and-naturalid-hibernate-specific-annotations/](https://howtodoinjava.com/hibernate/immutable-and-naturalid-hibernate-specific-annotations/) 在上一篇文章中，我们了解了[Hiberate](//howtodoinjava.com/hibernate-tutorials/ "Hibernate Tutorials")中最常用的 [JPA 注解](//howtodoinjava.com/hibernate/hibernate-jpa-2-persistence-annotations-tutorial/ "Hibernate / JPA2 Persistence Annotations Tutorial")，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](//howtodoinjava.com/hibernate/hibernate-jpa-2-persistence-annotations-tutorial/ "Hibernate / JPA2 Persistence Annotations Tutorial")** 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`** ```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`** ```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 ```java @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` ```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 ```java @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` ```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！**