Spring - using constructor injection with `@Autowired`
Published: 2023-10-30 05:55:28

Using constructor injection with `@Autowired` is generally considered a best practice in Spring because it promotes better code readability, makes dependencies explicit, and ensures that dependencies are available when an object is created. Here's what your service code might look like when using constructor injection:

```java
@Service
public class ProductService {
    private final ProductRepository productRepository;

    @Autowired
    public ProductService(ProductRepository productRepository) {
        this.productRepository = productRepository;
    }

    // ... other methods that use productRepository ...
}
```

In this code:

- The `ProductRepository` dependency is injected via the constructor, and it's marked as `final`, indicating that it cannot be changed after initialization. This helps guarantee that the `ProductRepository` is available for use throughout the service's lifecycle.

- Constructor injection makes it clear what dependencies the `ProductService` requires. Anyone reading the code can immediately see that it depends on a `ProductRepository`.

- The `@Autowired` annotation on the constructor is not strictly necessary if you're using Spring 4.3+ and later versions. Spring can automatically detect a single constructor and inject dependencies into it without the need for the `@Autowired` annotation. However, adding `@Autowired` for clarity is still a good practice.

Here are some advantages of using constructor injection:

1. **Readability**: The dependencies of the class are clearly visible in the constructor parameters, making the code more self-documenting.

2. **Immutability**: Marking the dependencies as `final` ensures that they cannot be changed after initialization, making the code more robust.

3. **Testability**: Constructor injection makes it easy to provide mock or stub dependencies when writing unit tests for the class.

4. **Consistency**: It's consistent with the Dependency Injection principle, where dependencies are provided from the outside rather than created within the class.

Overall, using constructor injection with `@Autowired` is a best practice because it leads to cleaner, more maintainable code and promotes good design principles like dependency inversion and dependency injection.

## What potential risks if i don't use contructor injection on above service?
If you don't use constructor injection for your service in the Spring framework and instead rely on field or method injection, it can introduce several potential risks and issues:

1. **Null Pointers and Dependencies Not Guaranteed:**
   - Without constructor injection, there is no guarantee that the required dependencies will be provided during object creation.
   - You may encounter `NullPointerExceptions` if you forget to initialize or inject dependencies in the right order.

2. **Mutable State:**
   - Field injection, where dependencies are injected directly into fields, can lead to mutable states. This can make the class less thread-safe and harder to reason about.

3. **Testing Challenges:**
   - Constructor injection makes it easier to write unit tests for your classes because you can inject mock or test dependencies when creating objects.
   - With field or method injection, you often need to rely on reflection or Spring-specific test annotations to inject dependencies for testing, making your tests more complex.

4. **Initialization Order:**
   - Field and method injection rely on the order of initialization, which can be problematic in complex dependency hierarchies or when multiple beans are involved.
   - With constructor injection, the dependencies are guaranteed to be available when the object is constructed.

5. **Immutability:**
   - Constructor injection promotes immutability because once the dependencies are set during construction, they cannot be changed.
   - With field injection, you might accidentally or intentionally change the injected dependencies after the object's creation, leading to unpredictable behavior.

6. **Complex Configuration:**
   - In cases with many dependencies, field or method injection can lead to a long list of annotated fields or methods, making the configuration complex and harder to manage.

7. **Hidden Dependencies:**
   - Constructor injection makes dependencies explicit in the constructor's signature, making it clear which dependencies are required for the class to function properly.
   - Field or method injection can hide dependencies and make it less obvious what the class relies on.

8. **Code Smells:**
   - In some cases, excessive use of field or method injection can be considered a code smell because it can indicate poor design and a lack of control over your components.

In summary, while it's possible to use field or method injection in Spring, constructor injection is generally considered a best practice because it ensures that dependencies are provided during object creation, promotes immutability, simplifies testing, and leads to more robust and maintainable code. It helps reduce the risks associated with relying on the Spring framework to inject dependencies at the appropriate time.

## JUnit tests - address some of the potential risks

Testing all the potential risks associated with not using constructor injection in Spring components can be a complex task, and the specific test cases will depend on your application's architecture and requirements. However, I can provide you with a simplified example that demonstrates how to write JUnit tests to identify some of the risks. This example will cover potential risks related to null pointers, mutable state, and hidden dependencies.

Suppose you have the following Spring component without constructor injection:

```java
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class MyService {
    @Autowired
    private MyRepository myRepository;

    private String data;

    public void processData() {
        data = myRepository.getData();
    }

    public String getResult() {
        return data;
    }
}
```

Here's a JUnit test class that aims to identify some of the potential risks:

```java
import org.junit.Before;
import org.junit.Test;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.MockitoAnnotations;
import org.springframework.test.util.ReflectionTestUtils;

public class MyServiceTest {

    private MyService myService;

    @Mock
    private MyRepository myRepository;

    @Before
    public void setUp() {
        MockitoAnnotations.initMocks(this);
        myService = new MyService();
        ReflectionTestUtils.setField(myService, "myRepository", myRepository);
    }

    @Test
    public void testNullPointerRisk() {
        // Ensure that MyRepository is not null
        Mockito.when(myRepository.getData()).thenReturn("Test Data");
        myService.processData();
        // Check if getResult() does not return null
        assert myService.getResult() != null;
    }

    @Test
    public void testMutableStateRisk() {
        // Ensure that data is not modified externally
        Mockito.when(myRepository.getData()).thenReturn("Test Data");
        myService.processData();
        String result = myService.getResult();
        result = "Modified Data";
        // The original data in MyService should not change
        assert !myService.getResult().equals(result);
    }

    @Test
    public void testHiddenDependenciesRisk() {
        // Ensure that MyRepository is correctly injected
        assert myService != null;
        assert myService.getResult() != null;
    }
}
```

In this example:

- We use the Mockito framework to mock the `MyRepository` dependency.
- The `testNullPointerRisk` test checks that there are no null pointer exceptions when calling `getResult()`.
- The `testMutableStateRisk` test checks that the `data` field in `MyService` is not affected by external changes.
- The `testHiddenDependenciesRisk` test checks that `MyService` has a non-null instance of `MyRepository`.

Please note that this is a simplified example, and in a real-world application, you would likely have more complex scenarios to test. The specific tests you write will depend on your application's requirements and the risks you want to address.