Hibernate Entity Lifecycle Explained

In Hibernate (and JPA), an entity lifecycle describes the different states an entity instance goes through during its interaction with the Persistence Context (Session / EntityManager) and the database.

🧩 Entity Lifecycle States

| State             | Description
|-------------------|----------------------------------------------------------------------------------
| **Transient**     | Newly created object, not yet associated with a Hibernate session. Not saved in DB.
| **Persistent**    | Associated with a Hibernate session and has a corresponding row in DB.
| **Detached**      | Was persistent, but session is closed or entity was evicted. Changes are not tracked.
| **Removed**       | Marked for deletion from DB, will be deleted upon commit.

🔄 Typical Lifecycle Flow

// Transient
User user = new User("Alice");

// Persistent
session.save(user);

// Detached
session.close(); // or session.evict(user);

// Removed
session.delete(user);

⚙️ Lifecycle Callbacks (JPA Annotations)

You can hook into these state transitions using annotations:

| Annotation     |              When it’s Called         |
|----------------|---------------------------------------|
| `@PrePersist`  | Before entity is persisted (inserted) |
| `@PostPersist` | After entity is persisted             |
| `@PreUpdate`   | Before entity is updated              |
| `@PostUpdate`  | After entity is updated               |
| `@PreRemove`   | Before entity is removed              |
| `@PostRemove`  | After entity is removed               |
| `@PostLoad`    | After entity is loaded from DB        |

Example:

@Entity
public class User {
    @Id
    private Long id;

    @PrePersist
    public void beforeSave() {
        System.out.println("About to persist: " + this);
    }

    @PostLoad
    public void afterLoad() {
        System.out.println("Entity loaded: " + this);
    }
}

🧠 Summary Diagram

   new (Transient)
        │
        ▼
  save()/persist()
        │
        ▼
   Persistent ──► remove() ──► Removed
        │
  session.close()/evict()
        ▼
     Detached