Why Should Field Injection Be Avoided in Spring Dependency Injection?

Field Injection and Its Detriments

Field injection, whereby beans are injected via @Autowired annotations on fields, is often discouraged. To delineate this, consider the following:

@Component
public class MyComponent {
    @Autowired
    private Cart cart;
}

Alternatively, constructor injection employs the following approach:

@Component
public class MyComponent {
    private final Cart cart;

    @Autowired
    public MyComponent(Cart cart){
       this.cart = cart;
    }
}

Injection Techniques

There are three primary dependency injection methods:

• Constructor injection: Injected through constructors.
• Method injection: Injected through setters or other methods.
• Field injection: Injected directly into fields using reflection.

Field injection, as seen in the first example, corresponds to the third option.

Injection Guidelines

Spring advocates the following injection guidelines:

• Mandatory dependencies: Utilize constructor injection.
• Optional or changeable dependencies: Employ setter injection.
• Avoid field injection: Use it only in exceptional scenarios.

Drawbacks of Field Injection

Field injection is discouraged for several reasons:

• Immutable objects: Constructor injection facilitates the creation of immutable objects, which field injection does not offer.
• Tight coupling: Field injection tightly couples classes with the DI container, hindering their independent usage.
• Instantiation challenges: Without reflection and the DI container, classes cannot be instantiated, hindering unit testing capabilities.
• Hidden dependencies: Field injection conceals dependencies, making it difficult to discern the class's reliance on external components.
• Multiple dependencies: Field injection enables the unfettered accumulation of dependencies.
• Responsibility violation: Classes that handle multiple responsibilities often exhibit excessive dependencies, potentially violating the Single Responsibility Principle.

Conclusion

Constructor and setter injection should be prioritized based on requirements. Field injection should generally be avoided due to its shortcomings, with convenience as its sole advantage.

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