Why Do I Get a Null EJB When Injecting It into a JAX-RS Webservice?

EJB Injection into JAX-RS Webservice: A Comprehensive Guide

In the realm of Java Enterprise Edition (JEE) development, it's often necessary to inject a Stateless EJB into a JAX-RS webservice for efficient data access. However, encountering a null EJB and the dreaded NullPointerException can be frustrating. This article delves into the causes and provides practical solutions to resolve this issue.

Initial attempts to inject the EJB via annotations may not always succeed due to underlying platform limitations. To address this, there are several approaches you can take:

Option 1: Utilize the Injection Provider SPI

The Injection Provider SPI provides a hook to customize the injection process. By implementing an InjectableProvider, you can instruct the container how to resolve and inject the EJB into your JAX-RS resource. Below is an example for the Jersey server:

@Provider
public class EJBProvider implements InjectableProvider<EJB, Type> {
    // ... Implementation Details ...
}

Option 2: Elevate the BookResource to an EJB

By making the BookResource an EJB itself, you ensure that the EJB container manages the lifecycle and injections within the class. This eliminates the need for external annotations:

@Stateless
@Path("book")
public class BookResource {
    @EJB
    private BookEJB bookEJB;
    // ...
}

Option 3: Harness the Power of CDI

CDI (Contexts and Dependency Injection) offers a convenient and declaratively configured approach to inject EJBs. By utilizing the @Inject annotation, you can seamlessly access the injected EJB within your JAX-RS resource:

@Path("book")
@RequestScoped
public class BookResource {
    @Inject
    private BookEJB bookEJB;
    // ...
}

It's worth noting that selecting the appropriate approach depends on the specific requirements and design considerations of your application. Option 1 provides fine-grained control over the injection process, while Options 2 and 3 simplify it by leveraging standard EJB or CDI mechanisms.

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