why-bean-doesnt-implement-interface

The component interface (Remote/Local) defines the business methods the client can call. It seems natural for the bean class to implement this interface directly—this would give compile-time checking that method signatures match. So why doesn’t HelloBean implements Hello?

There are two good reasons NOT to have the bean class implement the component interface:

1. Pollution: Component interfaces extend EJBObject or EJBLocalObject, which have extra methods (remove(), getHandle(), etc.) intended for clients. If the bean implements the interface, you’d need to provide no-op implementations of these methods—they don’t belong in the bean.

2. The “this” danger: If the bean implements the same interface as the EJB object, you could accidentally pass this (the bean instance) to another bean instead of the EJB object (proxy). All clients must go through the proxy, never the bean directly.

The Solution — Business Interface Pattern:

// Business interface (no EJB stuff)
public interface HelloBusinessMethods {
    String hello() throws RemoteException;
}
 
// Remote interface extends both EJBObject and business interface
public interface HelloRemote extends EJBObject, HelloBusinessMethods {}
 
// Bean implements business interface only
public class HelloBean implements SessionBean, HelloBusinessMethods {
    public String hello() { return "Hello, World!"; }
}

• Separation of concerns: Bean handles business logic, EJB Object handles middleware
• Compile-time safety: Business interface ensures bean has required methods
• No pollution: Bean doesn’t contain client-specific methods (remove(), etc.)
• Avoids this danger: Bean can’t accidentally pass itself (only EJB object gets passed)

• Built from: Remote Interface, EJB Object
• Builds into: Session Bean, Entity Bean
• Related: Business Interface Pattern
• Contrasts with: Regular Java (where impl implements the interface directly)

• Local interface problem: The business interface pattern still causes local interfaces to throw RemoteException—annoying but tolerable
• Not mandatory: You CAN implement the component interface—it’s just not recommended
• Modern EJB (3.x+): Uses annotations (@Remote, @Local)—this problem is largely solved