entity-bean-instance-vs-data

The term “Entity Bean” is used loosely—sometimes it means the in-memory Java object, sometimes it means the database record. This confusion makes discussions about entity beans unclear. What’s the difference?

The EJB spec clarifies two distinct concepts:

1. Entity Bean Instance: The in-memory Java object (an instance of your entity bean class). It’s the “view” into the database.
2. Entity Bean Data: The actual persistent data stored in the database (the record/row).

The instance loads data from the database (in ejbLoad()), modifies it in memory, and saves it back (in ejbStore()).

Database Record (Entity Bean Data)
    ↓ ejbLoad()
In-Memory Object (Entity Bean Instance)
    ↓ business methods modify fields
In-Memory Object (with changes)
    ↓ ejbStore()
Database Record (updated)

• Instance is temporary: Created when needed, may be pooled/reused for different data
• Data is permanent: Survives server crashes, lives in database
• One-to-one mapping: At any moment, one instance represents one data record
• Container manages sync: ejbLoad() and ejbStore() keep instance and data in sync

• Built from: Entity Bean, ejbLoad(), ejbStore()
• Builds into: BMP, CMP
• Related: Persistence Concepts
• Contrasts with: Session beans (no persistent data, only in-memory instances)

• Stale data: If another client modifies the DB directly, in-memory instance has old data (until next ejbLoad())
• Instance pooling: The same Java instance might represent Ray’s account now, Bob’s account later (container reuses instances)
• Transparent to client: Client doesn’t know or care about instance vs data distinction—they just call methods