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I am starting to learn about EJB, despite I know they handle the business logic, I don't understand why an EJB has to implement an interface.

I know that the interface is a list of the methods and is used by the client to access them, but what if I don't use an interface?

I know that the no-interface view exist but when should I use an interface then?

could you please explain it using a no IT example? I am taking a course about Java EE 7 and I am stuck in this part, I have read the Oracle tutorial but I've got problems understanding this.

I apologize for my wording mistakes.

thanks in advance

Seeker
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user3281382
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2 Answers2

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The reason for an interface is because you need to invoke a method in one JVM that transparently can invoke your EJB in another JVM. All the complexity of Java EE come from that it is designed to work across multiple JVMs.

This can be handled in many ways. The approach chosen here is that an interface can make this almost transparent in your code (just compare with invocation through reflection) but the object in the first JVM contains not your code, but instead code that knows how to reach the other JVM and ask it to invoke your method and return the result.

In other words, an interface allows the compiler to help you doing it right in your code, and the application server then provides the magic glue in the object called to reach your EJB.

Thorbjørn Ravn Andersen
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An EJB doesn't have to implement an interface (anymore). Well, this is only true, if you haven't different VMs accessing the same EJB container. You could host your JBoss in the cloud (e.g.) and have another JEE-Server (e.g. Tomcat) at your company site (or anywhere else) and let the TomEE retrieve its EJB instances from the JBoss Application Server. Then you have to use an interface to program against, since you don't know what the implementation will be.

Since EJB 3 no-interface views are possible. You're free to use interfaces, but you shall be happy if you don't... as long as you dont need distributed EJB-services.

An interface is always a great choice, when you design a big system though. You can easily change you underlying logic if you program against the interface and not the concrete class. So if specifications change, code is much easier to maintain.

Zhedar
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