The last blog post about the JPA EntityManager didn’t cover the JPA entity lifecycle. This post builds on the original one by covering the JPA Entity lifecycle and associated lifecycle events.
JPA Entity Lifecycle
As a reminder, the purpose of the EntityManager is to the relationship between the JPA entity and the underlying datasource.
The above diagram shows the 5 key stages of JPA entity management:
- Object Doesn't Exist – This is a null object
MyObject myObject = null;
- New Object – Not associated with the EntityManager, and doesn't exist on database
MyObject myObject = new MyObject();
- Managed – This is the stage where the object becomes persisted and managed by the EntityManager. To do this, we need to call the persist method from within a transaction. The object is then persisted to the database when the commit method is called
entityManager.getTransaction().begin(); MyObject myObject = new MyObject(); entityManager.persist(myObject); entityManager.getTransaction().commit();
- Detached – This state removes the object from the EntityManager, but the object still exists on the database. Some EntityManager methods on a detached object will result in an IllegalArgumentException. The object can be reattached to the EntityManager through the merge method
- Removed – Deletes the object from the database. Like persist, this also needs to take place inside a transaction.
entityManager.getTransaction().begin(); entityManager.removed(myObject); entityManager.getTransaction().commit();
Callback Methods on JPA Entities
The final part of the JPA lifecycle event is the optional callback methods, and their associated annotations:
The pre-methods are executed before the associated method action is executed (i.e. @PrePersist is executed before em.persist).
The post-methods are executed after the associated method action is executed (i.e. @PostPerist is executed after em.persist)
If either the pre- or post- methods/annotations throw an exception then the transaction will be rolled back.
What Can You Use These Callback Events For?
The pre- and post-persist methods are useful for setting timestamps for auditing or setting default values.