An Entity itself can have several methods defined to receive events when any instances of that class pass through lifecycles changes. Let's take an example

@Entity
public class Account
{
    @Id
    Long accountId;

    Integer balance;
    boolean preferred;

    public Integer getBalance() { ... }

    @PrePersist
    protected void validateCreate()
    {
        if (getBalance() < MIN_REQUIRED_BALANCE)
        {
            throw new AccountException("Insufficient balance to open an account");
        }
    }

    @PostLoad
    protected void adjustPreferredStatus()
    {
        preferred = (getBalance() >= AccountManager.getPreferredStatusLevel());
    }
}

So in this example just before any "Account" object is persisted the validateCreate method will be called. In the same way, just after the fields of any "Account" object are loaded the adjustPreferredStatus method is called. Very simple.

You can register callbacks for the following lifecycle events

• PrePersist
• PostPersist
• PreRemove
• PostRemove
• PreUpdate
• PostUpdate
• PostLoad

The only other rule is that any method marked to be a callback method has to take no arguments as input, and have void return.

As an alternative to having the actual callback methods in the Entity class itself you can define a separate class as an EntityListener . So lets take the example shown before and do it for an EntityListener.

@Entity
@EntityListeners(org.datanucleus.MyEntityListener.class)
public class Account
{
    @Id
    Long accountId;

    Integer balance;
    boolean preferred;

    public Integer getBalance() { ... }
}

public class MyEntityListener
{
    @PostPersist
    public void newAccountAlert(Account acct)
    {
         ... do something when we get a new Account
    }
}

So we define our "Account" entity as normal but mark it with an EntityListener , and then in the EntityListener we define the callbacks we require. As before we can define any of the 7 callbacks as we require. The only difference is that the callback method has to take an argument of type "Object" that it will be called for, and have void return.