# Com / Optage / Testdata / Di

## Overview

The `com.optage.testdata.di` package is a minimal demonstration of Java EE / Jakarta EE dependency injection (DI) patterns. It showcases the interface-based design commonly used in enterprise Java applications: a service interface (`[BillingService]`), a concrete implementation (`[BillingServiceImpl]`), and a `@Stateless` session bean (`[StatelessBeanExample]`) that consumes the service through CDI injection. This package appears to serve as test data or a reference implementation for understanding DI wiring rather than production business logic.

## Key Classes and Interfaces

### [BillingService]

**Type:** Interface

The `BillingService` interface declares a single contract method:

- **`bill()`** — performs a billing operation. Returns `void`, indicating it may trigger side effects (e.g., charging a customer, writing to a database) rather than producing a value.

This interface follows the dependency-inversion principle: consumers depend on the abstraction (`BillingService`) rather than a concrete class, making the system easier to test and extend.

### [BillingServiceImpl]

**Type:** Class (implements `[BillingService]`)

`BillingServiceImpl` is the concrete implementation of `BillingService`. At present, the `bill()` method has an empty body, suggesting this is either a stub awaiting future implementation or a placeholder used to demonstrate injection wiring.

Key method:

- **`bill()`** — no-op implementation of the interface contract. Returns `void`. In a production scenario, this would contain the actual billing logic (e.g., calculating charges, processing payments, persisting records).

### [StatelessBeanExample]

**Type:** Class (CDI `@Stateless` EJB)

`StatelessBeanExample` is a Jakarta EE `@Stateless` session bean — a server-managed, thread-safe component that handles business logic. It serves as the entry point demonstrating how DI works in practice.

Key members:

- **`@Inject private BillingService billingService`** — the billing service is injected by the container at runtime. The `@Inject` annotation tells the CDI (Contexts and Dependency Injection) framework to locate and provide a `BillingService` implementation (here, `BillingServiceImpl`) automatically.

- **`business()`** — demonstrates the injected service in action. It first checks that `billingService` is not null (a defensive check, since the container should always inject it), then delegates to `billingService.bill()`. In a real application, this method would represent a business operation that depends on billing functionality.

- **`toString()`** — returns the simple string `"StatelessBeanExample"`. A standard convenience method for identification and debugging.

## How It Works

The typical flow when using these classes looks like this:

```mermaid
flowchart LR
    A["StatelessBeanExample"] -->|"injects"| B["BillingService"]
    B -->|"implemented by"| C["BillingServiceImpl"]
```

1. **Injection setup:** The CDI container creates a `StatelessBeanExample` instance and satisfies its `@Inject` dependency by locating a `BillingService` bean — which is provided by `BillingServiceImpl`. No explicit wiring code is needed; the container resolves it automatically.

2. **Business call:** A caller invokes `StatelessBeanExample.business()`.

3. **Service delegation:** Inside `business()`, the injected `billingService` reference is used to call `bill()`. The null-check ensures robustness if injection fails unexpectedly.

4. **Execution:** `BillingServiceImpl.bill()` executes (currently a no-op). In production, this would perform the actual billing logic.

This pattern — interface + implementation + `@Stateless` consumer — is a standard Java EE architecture for separating concerns and enabling testability (e.g., substituting a mock `BillingService` in unit tests).

## Dependencies and Integration

- **Jakarta EE APIs:** Uses `javax.ejb.Stateless` (EJB container management) and `javax.inject.Inject` (CDI injection). This places the module in a Jakarta EE / Java EE environment.
- **CDI Container:** Relies on the container to wire `[BillingService]` into `[StatelessBeanExample]` at deployment or runtime.
- No external libraries or cross-module dependencies are present in this package.

## Notes for Developers

- **Stub implementations:** Both `BillingServiceImpl.bill()` and `BillingService.bill()` are empty. If this is a test-data package, those stubs may be intentional. If this is meant to be production code, the billing logic needs to be implemented.
- **Extensibility:** To add new billing behavior, implement a different class that implements `[BillingService]` and annotate it as a CDI bean (e.g., `@Named` or `@Singleton`). The container will automatically wire it to `[StatelessBeanExample]` since there is currently a single concrete implementation.
- **Testing:** Because `[StatelessBeanExample]` depends on an interface rather than a concrete class, you can easily unit test it by injecting a mock `BillingService` (e.g., using Mockito).
- **Thread safety:** The `@Stateless` annotation on `StatelessBeanExample` means the container manages a pool of instances and guarantees thread safety — no instance fields should hold per-request state. The only field (`billingService`) is a container-injected reference, which is safe.