Avaje Inject: Resolving Package-Protected Type Injection Issues

Hey everyone! Today, we're diving deep into a specific issue you might encounter while using Avaje Inject: package-protected types not being found for injection. This can be a bit of a head-scratcher, but don't worry, we'll break it down and explore how to solve it. We'll discuss why this happens and how Avaje can be improved to handle these scenarios more gracefully. Let's get started!

Understanding the Problem: Package-Protected Types and Avaje Inject

So, what's the deal with package-protected types? In Java, package-level access (or package-private) is a visibility modifier that restricts the access of a class, method, or field to only other classes within the same package. This is a common technique used to improve encapsulation and guide developers towards using a well-defined public API. By keeping implementation details hidden within a package, we can reduce coupling and make our code more maintainable. Encapsulation is key to robust software design, and package-private access plays a vital role in it.

Now, imagine you have a scenario where you want Avaje Inject to construct these package-protected types and inject them into other classes within the same package. This is a perfectly valid use case, as it allows you to manage dependencies within your internal implementation details. However, Avaje might throw an error in this situation. This is because, by default, Avaje might not be configured to recognize and inject package-protected types. This can be frustrating, especially when you're trying to keep your internal implementation hidden while still leveraging the power of dependency injection. The core issue revolves around how Avaje discovers and manages beans for injection. When a class is package-protected, it's visibility is limited, and Avaje's default scanning mechanisms might not pick it up. This limitation can lead to the dreaded "No dependency provided" error, leaving you scratching your head. To truly understand the problem, it's helpful to look at a concrete example.

A Concrete Example: Demonstrating the Issue

Let's walk through a practical example that highlights this issue. Consider the following Java code:

package com.foo;

interface Adder {
 int add(int a, int b);
}

package com.foo;

import jakarta.inject.Singleton;

@Singleton
class AdderImpl implements Adder {

 @Override
 public int add(int a, int b) {
 return a + b;
 }
}

package com.foo;

import jakarta.inject.Singleton;

@Singleton
public class Calculator {
 private final Adder adder;

 public Calculator(Adder adder) {
 this.adder = adder;
 }

 public int sum(int a, int b) {
 return adder.add(a, b);
 }
}

package com.foo;

import static org.assertj.core.api.Assertions.*;

import io.avaje.inject.test.InjectTest;
import jakarta.inject.Inject;
import org.junit.jupiter.api.Test;

@InjectTest
final class CalculatorTest {

 @Inject Calculator calculator;

 @Test
 void verify() {
 assertThat(calculator.sum(40, 2)).isEqualTo(42);
 }
}

In this scenario, we have an Adder interface and a package-protected implementation AdderImpl. The Calculator class depends on the Adder interface. We're using Avaje Inject with @Singleton annotations and a JUnit test with @InjectTest. You might expect this to work seamlessly, but you'll likely encounter a compilation error similar to this:

/project-path/src/main/java/com/foo/Calculator.java:[6,8] No dependency provided for com.foo.Adder on com.foo.Calculator
[ERROR] Dependencies [com.foo.Adder] are not provided - there are no @Singleton, @Component, @Factory/@Bean that currently provide this type. If this is an external dependency consider specifying via @External

This error message clearly indicates that Avaje can't find a suitable dependency for com.foo.Adder. The reason? AdderImpl is package-protected, and Avaje's default component scanning isn't picking it up. This example perfectly illustrates the problem we're discussing. It shows how a seemingly simple dependency injection setup can fail when dealing with package-protected types. The next step is to figure out why this happens and what we can do about it.

Why This Happens: Avaje's Component Scanning and Visibility

To understand why Avaje struggles with package-protected types, we need to delve into how Avaje performs component scanning. Component scanning is the process of automatically discovering classes that should be managed as beans within the dependency injection container. Avaje, like other DI frameworks, uses annotations like @Singleton, @Component, and @Factory to identify these beans. However, the visibility of these classes plays a crucial role in whether they are discovered during the scanning process. By default, Avaje's component scanning mechanism might be configured to only look for classes with public visibility. This is a common optimization, as public classes are typically the ones intended for external use and dependency injection. When a class is package-protected, it falls outside this default scope. The scanning process might simply skip over it, leading to the "No dependency provided" error we saw earlier. Visibility modifiers in Java are designed to control access levels, and this directly impacts how dependency injection frameworks like Avaje operate. This behavior is not necessarily a bug; it's a design choice that prioritizes performance and avoids unintended bean creation. However, it does highlight a limitation when dealing with package-protected types that are intentionally part of the internal dependency graph.

Potential Solutions and Workarounds

So, what can we do to address this issue? Here are a few potential solutions and workarounds:

1. Make the Implementation Public: The simplest solution, if feasible, is to make the AdderImpl class public. This would allow Avaje's component scanning to discover it without any issues. However, this approach might not be desirable if you're intentionally using package-private access to restrict visibility and maintain encapsulation. Changing the visibility can impact the overall design of your application and might expose internal implementation details that you'd prefer to keep hidden.

2. Configure Avaje to Scan Package-Protected Types: Avaje might offer configuration options to customize its component scanning behavior. It's worth exploring the documentation to see if there's a way to explicitly instruct Avaje to include package-protected classes in the scanning process. This could involve setting a specific flag or providing a custom component scanner implementation. The exact configuration mechanism would depend on the version of Avaje you're using, so it's essential to refer to the official documentation for the most up-to-date information.

3. Use a Factory or Provider: Another approach is to use a @Factory or @Provider method to explicitly define how the package-protected type should be created and provided to the dependency injection container. This gives you more control over the instantiation process and allows you to bypass the default component scanning mechanism. For example, you could create a factory class within the same package as AdderImpl and define a @Bean method that returns an instance of AdderImpl. This would make the AdderImpl available for injection without making it publicly accessible.

4. Consider Using Modules: Some dependency injection frameworks use the concept of modules to group related beans and configure dependencies. Avaje might have a similar mechanism that allows you to define a module that explicitly includes the package-protected type. This can be a cleaner way to manage dependencies within a specific part of your application.

5. Request for Enhancement in Avaje: As highlighted in the original discussion, it would be beneficial if Avaje could natively support injection of package-protected types. Consider raising a feature request or contributing to the Avaje project to address this limitation. This would make Avaje more flexible and easier to use in scenarios where package-private access is intentionally used for encapsulation.

Avaje's Potential Improvement: Allowing Package-Protected Types for Injection

The original discussion raises a valid point: Avaje could be improved to handle package-protected types more gracefully. It would be ideal if Avaje allowed package-protected types to be eligible for injection without requiring extensive workarounds. This would align with the common practice of using package-private access for internal implementation details and promote better encapsulation. One way to achieve this is by enhancing Avaje's component scanning mechanism to optionally include package-protected classes within the scope of a component. This could be controlled by a configuration setting or a specific annotation. Another approach is to allow explicit registration of package-protected types as beans within a module or factory. This would provide a more fine-grained control over which package-protected types are exposed for injection. By addressing this limitation, Avaje could become even more powerful and versatile, catering to a wider range of application designs and coding styles. The goal is to make dependency injection as seamless and intuitive as possible, regardless of the visibility modifiers used within the code. This would ultimately lead to more maintainable and robust applications.

Conclusion: Navigating Package-Protected Types with Avaje

In conclusion, dealing with package-protected types and dependency injection frameworks like Avaje can present some challenges. The default component scanning behavior might not pick up these types, leading to injection errors. However, by understanding the underlying reasons and exploring the various solutions and workarounds, you can effectively manage dependencies in your application while maintaining proper encapsulation. Whether it's making the implementation public (if feasible), configuring Avaje's scanning behavior, using factories or providers, or considering modules, there are several ways to address this issue. Furthermore, contributing to the Avaje project by requesting a feature enhancement to natively support package-protected types can benefit the entire community. Remember, the key is to strike a balance between leveraging the power of dependency injection and adhering to sound software design principles. By carefully considering the visibility of your classes and dependencies, you can build robust and maintainable applications with Avaje Inject. And that's a wrap, guys! Keep coding, and don't let those package-protected types get you down!