Hibernate JPA 级联类型
原文： https://howtodoinjava.com/hibernate/hibernate-jpa-cascade-types/
我们已经在之前的教程中了解了 Hiberate 中的映射关联实体，例如一对一映射和一对多映射。 每当要保存关系所有者实体时，我们都想保存映射的实体。 为此，我们使用了“CascadeType”属性。 在此 JPA 级联类型教程中，我们将通过CascadeType了解各种可用的级联选项。
JPA 级联类型如何工作？
在继续之前，让我们看一下如何在代码中定义此级联类型属性。 让我们举个例子来更清楚地了解。 假设一个雇员可以有多个帐户； 但一个帐户只能与一名员工关联。 为了清楚起见，让我们以最少的信息创建实体。
EmployeeEntity.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;
@OneToMany(cascade=CascadeType.ALL, fetch = FetchType.LAZY)
@JoinColumn(name="EMPLOYEE_ID")
private Set<AccountEntity> accounts;
//Getters and Setters Ommited
}
AccountEntity.java
@Entity
@Table(name = "Account")
public class AccountEntity implements Serializable
{
private static final long serialVersionUID = 1L;
@Id
@Column(name = "ID", unique = true, nullable = false)
@GeneratedValue(strategy = GenerationType.SEQUENCE)
private Integer accountId;
@Column(name = "ACC_NO", unique = false, nullable = false, length = 100)
private String accountNumber;
@OneToOne (mappedBy="accounts", fetch = FetchType.LAZY)
private EmployeeEntity employee;
}
查看上面EmployeeEntity.java源代码中的粗体行。 它定义了“cascade=CascadeType.ALL”，从本质上讲意味着EmployeeEntity上发生的任何更改也必须级联到AccountEntity。 如果您保存员工，则所有关联的帐户也将保存到数据库中。 如果删除雇员，则与该雇员关联的所有帐户也将被删除。 很简单。
但是，如果我们只希望级联仅保存操作而不删除级联，该怎么办。 然后，我们需要使用以下代码明确指定它。
@OneToMany(cascade=CascadeType.PERSIST, fetch = FetchType.LAZY)
@JoinColumn(name="EMPLOYEE_ID")
private Set<AccountEntity> accounts;
现在，仅当使用员工实例调用save()或persist()方法时，才会保留帐户。 如果在会话中调用任何其他方法，则该方法的效果不会影响/级联到帐户。
JPA 级联类型
Java 持久化架构支持的级联类型如下：
CascadeType.PERSIST：级联类型presist表示save()或persist()操作级联到相关实体。CascadeType.MERGE：级联类型merge表示在合并所有者实体时会合并相关实体。CascadeType.REFRESH：级联类型refresh对refresh()操作执行相同的操作。CascadeType.REMOVE：级联类型remove在删除所有者实体时会删除与此设置关联的所有相关实体。CascadeType.DETACH：如果发生“手动分离”，则级联类型detach分离所有相关实体。CascadeType.ALL：级联类型all是上述所有级联操作的简写。
JPA 中没有默认级联类型。 默认情况下，没有操作级联。
级联配置选项接受一个CascadeTypes数组。 因此，如我们的示例所示，为了在一对多关系的级联操作中仅包括刷新和合并，您可能会看到以下内容：
@OneToMany(cascade={CascadeType.REFRESH, CascadeType.MERGE}, fetch = FetchType.LAZY)
@JoinColumn(name="EMPLOYEE_ID")
private Set<AccountEntity> accounts;
上述级联将导致仅合并和刷新帐户集合。
Hiberate 级联类型
现在，让我们了解在哪种情况下使用 Hiberate 的 Hiberate 方式。
除了 JPA 提供的级联类型之外，Hiberate 中还有一个级联操作，它不是上面讨论的正常设置的一部分，称为“孤例删除”。 将拥有的对象从其拥有的关系中删除后，该对象将从数据库中删除。
让我们看一个例子。 在我们的“雇员和帐户”实体示例中，我对它们进行了如下更新，并在帐户上提到“orphanRemoval=true”。 从本质上讲，这意味着每当我要删除“帐户组中的帐户”时（即我要删除该帐户与Employee之间的关系）； 与数据库上任何其他雇员（即孤例）没有关联的帐户实体也应删除。
EmployeeEntity.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;
@OneToMany(orphanRemoval = true, mappedBy = "employee")
private Set<AccountEntity> accounts;
}
AccountEntity.java
@Entity (name = "Account")
@Table(name = "Account")
public class AccountEntity implements Serializable
{
private static final long serialVersionUID = 1L;
@Id
@Column(name = "ID", unique = true, nullable = false)
@GeneratedValue(strategy = GenerationType.SEQUENCE)
private Integer accountId;
@Column(name = "ACC_NO", unique = false, nullable = false, length = 100)
private String accountNumber;
@ManyToOne
private EmployeeEntity employee;
}
TestOrphanRemovalCascade.java
public class TestOrphanRemovalCascade
{
public static void main(String[] args)
{
setupTestData();
Session sessionOne = HibernateUtil.getSessionFactory().openSession();
org.hibernate.Transaction tx = sessionOne.beginTransaction();
//Load the employee in another session
EmployeeEntity employee = (EmployeeEntity) sessionOne.load(EmployeeEntity.class, 1);
//Verify there are 3 accounts
System.out.println("Step 1 : " + employee.getAccounts().size());
//Remove an account from first position of collection
employee.getAccounts().remove(employee.getAccounts().iterator().next());
//Verify there are 2 accounts in collection
System.out.println("Step 2 : " + employee.getAccounts().size());
tx.commit();
sessionOne.close();
//In another session check the actual data in database
Session sessionTwo = HibernateUtil.getSessionFactory().openSession();
sessionTwo.beginTransaction();
EmployeeEntity employee1 = (EmployeeEntity) sessionTwo.load(EmployeeEntity.class, 1);
//Verify there are 2 accounts now associated with Employee
System.out.println("Step 3 : " + employee1.getAccounts().size());
//Verify there are 2 accounts in Account table
Query query = sessionTwo.createQuery("from Account a");
@SuppressWarnings("unchecked")
List<AccountEntity> accounts = query.list();
System.out.println("Step 4 : " + accounts.size());
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);
//Create Account 1
AccountEntity acc1 = new AccountEntity();
acc1.setAccountId(1);
acc1.setAccountNumber("11111111");
acc1.setEmployee(emp);
session.save(acc1);
//Create Account 2
AccountEntity acc2 = new AccountEntity();
acc2.setAccountId(2);
acc2.setAccountNumber("2222222");
acc2.setEmployee(emp);
session.save(acc2);
//Create Account 3
AccountEntity acc3 = new AccountEntity();
acc3.setAccountId(3);
acc3.setAccountNumber("33333333");
acc3.setEmployee(emp);
session.save(acc3);
session.getTransaction().commit();
session.close();
}
}
Output:
Hibernate: insert into Employee (FIRST_NAME, LAST_NAME, ID) values (?, ?, ?)
Hibernate: insert into Account (ACC_NO, employee_ID, ID) values (?, ?, ?)
Hibernate: insert into Account (ACC_NO, employee_ID, ID) values (?, ?, ?)
Hibernate: insert into Account (ACC_NO, employee_ID, 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 accounts0_.employee_ID as employee3_1_0_, accounts0_.ID as ID1_0_0_, accounts0_.ID as ID1_0_1_,
accounts0_.ACC_NO as ACC_NO2_0_1_, accounts0_.employee_ID as employee3_0_1_ from Account accounts0_ where accounts0_.employee_ID=?
Step 1 : 3
Step 2 : 2
Hibernate: delete from Account where 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=?
Hibernate: select accounts0_.employee_ID as employee3_1_0_, accounts0_.ID as ID1_0_0_, accounts0_.ID as ID1_0_1_,
accounts0_.ACC_NO as ACC_NO2_0_1_, accounts0_.employee_ID as employee3_0_1_ from Account accounts0_ where accounts0_.employee_ID=?
Step 3 : 2
Hibernate: select accountent0_.ID as ID1_0_, accountent0_.ACC_NO as ACC_NO2_0_, accountent0_.employee_ID as employee3_0_
from Account accountent0_
Step 4 : 2
这是从集合中删除匹配项/不匹配项的好方法（即多对一或一对多关系）。 您只需从集合中删除该项目，然后 Hiberate 即可为您处理其余所有事情。 它将检查是否从任何地方引用了实体； 如果不是，它将从数据库本身中删除该实体。
让我知道您对Hiberate 5 级联类型或 JPA 级联类型的想法和问题。
学习愉快！
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