JPA - Part 3

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Callback Methods

When insert, update and delete occurs, we can listen to those events and call some methods when such events occur. Callback methods should be prefixed by following annotations:
  • @PrePersist - called before persist()
  • @PostPersist - called after persist()
  • @PreRemove - called before remove()
  • @PostRemove - called after commit()
  • @PreUpdate - called before commit()
  • @PostUpdate - called after commit()
 More than one annotation can applied to a single method. There should be only one method with a particular callback annotation(example, there shouldn’t be more than one @Prepersist methods). The callback methods shouldn’t take any arguments.

Entity Listeners

Mixing callback methods into Entity class is not a good practice. So we write it in another class which is an Entity Listener.

Annotations Used:

  • @EntityListeners(EntityListener_ClassName.class)
    • Used before the Entity class
- Entity Listener should have public default constructor - An Entity class can have some callback methods defined in it and some in an Entity Listener - An Entity Listener class can listen to more than one Entity classes. - The callback methods defined in Entity Listener class must take an object of Entity class as argument

Example:

@Entity
@EntityListeners(EmployeeListener.class)
public class Employee {
    .....
}

public class EmployeeListener {
    @PrePersist
    public void beforePersist(Employee e) {
        System.out.println("Before persist");
    }
}

Merge Method

- Used to synchronise the changes between objects and database tables.
- Adds a new row if the entity is not present in db table.
- Updates the existing row if it is present in table.

Example:

et.begin();
    ....
    ....
    em.merge(empObject); //merges the changes in empObject with table
et.commit();
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