Question

How does a client get a remote reference to an EJB instance?

Short answer

A client gets a remote reference by performing a JNDI lookup for the EJB using the InitialContext.

Step-by-step solution

Understanding EJB Remote Reference

First, understand that EJB (Enterprise JavaBeans) allows applications to communicate over a network. To communicate with an EJB instance, you need a remote reference that serves as a pointer or access point to the EJB object located on a server.

Locate Initial Context

To get a reference to an EJB component, the client must first retrieve an InitialContext, which is used to access the server-side components. This is typically done by creating a new instance of `InitialContext` with necessary configurations.

Lookup EJB Reference

Using the InitialContext instance, the client performs a lookup operation. This involves calling the `lookup` method with the name that maps to the EJB component. This name is usually defined in the application server's deployment descriptor or via annotation in the EJB class.

Narrowing to EJB Interface

The object returned from the `lookup` call is usually an instance of `javax.rmi.PortableRemoteObject`. The client should narrow this general object to the specific EJB remote interface expected, using `PortableRemoteObject.narrow()` method.

Key concepts

InitialContext

In Java EE applications, the InitialContext is your starting point for accessing various resources on a server, such as EJBs (Enterprise JavaBeans). It's akin to a directory that provides a way to locate server-side components.
To access an EJB, a client application begins by creating an instance of `InitialContext`. This instance acts as a gateway for obtaining the necessary remote references to EJB components. The `InitialContext` involves configuration properties, typically provided as a hash table, which include details like the server's URL, authentication information, and other environment-specific settings.
This process is crucial because without correctly setting up the `InitialContext`, your client won't be able to reach or communicate with the EJB instances on the server.

• It helps locate resources registered on the server.
• Requires correct configuration and properties for successful connection.
• Acts as a standard interface for Java Naming and Directory Interface (JNDI) operations.

EJB Lookup

Once the InitialContext is prepared, the next task is to perform a lookup for the specific EJB component you wish to access. This is done using the `lookup` method provided by the InitialContext, which requires the JNDI name of the desired EJB.
The JNDI name serves as an identifier for the EJB within the application server, ensuring you can fetch the exact EJB instance you need from the many possible located on the server.
A successful lookup will return a reference object that points to your EJB, but this object might not immediately appear as the specific EJB type you expect. Instead, it offers a generic reference that still requires some narrowing down.

• Requires knowing the JNDI name of the EJB component.
• Returns a reference that is not yet type-specific.
• Essential for discovering and connecting to the EJB instance.

Remote Interface

EJBs often operate under a client-server architecture where the Remote Interface serves as a contract. It defines the methods that a client can invoke on the EJB component running on a server.
After successfully performing an EJB lookup, the client will receive a proxy reference that needs to be narrowed to the Remote Interface. This narrowing is done using `javax.rmi.PortableRemoteObject.narrow()`. This method ensures that the generic object returned by the lookup can be treated as the specific EJB type defined by the Remote Interface.
The Remote Interface is crucial because it allows separate systems to communicate through well-defined boundaries, making sure that changes in the server's internal logic do not affect the client's ability to interact with the EJB.

• Acts as a mediator between client and server-side logic.
• Ensures type safety and correct method invocation.
• Defines operations that are permissible from remote locations.