Spring Dependency Injection: @Autowired vs Constructor Injection

Last Updated : 04 Jan, 2025

In Spring framework, dependency injection is a key idea that lets you inject dependencies into a class in preference to hard coding them. This provides loose coupling and makes the code extra maintainable and testable. There are distinct methods to perform dependency injection.

Common Approaches to Perform Dependency Injection

Below is the list of common approaches:

Constructor Injection: Dependencies are injected via the constructor of the elegance.
Setter Injection: Dependencies are injected through setter techniques of elegance.
Field Injection: Dependencies are injected directly into fields of the class using annotations like @Autowired.
Method Injection: Dependencies are injected through methods of the class, typically using annotations like @Autowired.
Interface-Based Injection: Dependencies are injected through interfaces, and the actual implementation is resolved at runtime.
Qualifiers: When there is more than one bean of the same kind, you could use qualifiers to specify which bean should be injected.
Primary Annotation: Use the @Primary annotation on a bean to indicate it as the primary candidate for injection while multiple applicants of the identical type exist.
Resource Annotation: The @Resource annotation can be used for dependency injection with the aid of name, type, or a combination of each.
Java Configuration: Instead of relying on XML for configuration, you can use Java-primarily based configuration with the usage of @Configuration and @Bean annotations.
Component Scanning: Spring can automatically find out and sign in beans by way of scanning particular packages for instructions annotated with @Component and associated stereotypes.
Autowire via Type or Qualifier: The @Autowired annotation can be used with the @Qualifier annotation to specify which bean has to be injected when there are a couple of applicants.
Lazy Initialization: Beans may be marked for lazy initialization by the usage of the @Lazy annotation, which means that the bean is created handiest while it's far first asked.
Lookup Method Injection: Using the lookup-method attribute in XML or the @Lookup annotation to allow a method to return the actual dependency instance.
Value Injection: Using the @Value annotation to inject values from property files or other sources into bean properties.

@Autowired vs Constructor Injection

@Autowired Annotation: @Autowired is a Spring annotation used for automatic dependency injection. It can be applied to fields, techniques, and constructors. When Spring encounters @Autowired, it routinely injects the proper bean at that factor.

Consider the beneath instance for @Autowired Annotation:

Java

import org.springframework.beans.factory.annotation.Autowired;

public class GeekService {
    @Autowired
    private GeekRepository geekRepository;

    // Other methods using geekRepository
}

Constructor Injection: Constructor injection is a shape of dependency injection in which the dependencies of a category are furnished through its constructor. In the Spring framework, constructor injection is a commonplace and endorsed manner to inject dependencies into a class.

Java

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

@Service
public class GeekService {
    private GeekRepository geekRepository;

    // Constructor Injection
    @Autowired
    public GeekService(GeekRepository geekRepository) {
        this.geekRepository = geekRepository;
    }

    // rest of the class
}

Key differences between @Autowired and Constructor Injection

Feature	@Autowired	Constructor Injection
Usage	Can be applied to fields, methods, or constructors.	Applied only to the constructor.
Annotation Location	Applied directly to fields or methods.	Applied to the constructor.
Dependencies Initialization	Dependencies can be injected after the object is created (method injection).	Dependencies are injected during object creation.
Preferred Approach	Suitable for scenarios where flexibility in changing dependencies at runtime is needed.	Preferred approach for better code readability, testability, and immutability.
Nullability	Fields may be null if not initialized properly, leading to potential Null Pointer Exceptions.	Nullability is less of a concern, as dependencies are initialized during construction.
Flexibility	Offers flexibility in changing dependencies dynamically at runtime.	Promotes a more rigid structure, discouraging changes to dependencies after construction.
Compile-time Safety	May result in runtime errors if dependencies are not present or misconfigured.	Ensures compile-time safety as dependencies are resolved during compilation.
Pros	Offers flexibility for dynamic setups.	Emphasizes simplicity and clarity.
Cons	May scatter dependencies, potentially reducing readability.	Centralizes dependencies, making code more straightforward.

Note: While @Autowired provides flexibility, Constructor Injection is favored for clean, maintainable code.

Conclusion

In summary, the choice between @Autowired and Constructor Injection in Spring, consider your project's needs. @Autowired offers flexibility but may compromise code clarity and introduce runtime errors. Conversely, Constructor Injection, prioritizing simplicity and readability, is widely favored for clean coding practices, compile-time safety, and ease of testing. Below are the preferred recommendations.

Best Practices: Prefer Constructor Injection for clean coding practices.
Readability: Constructor Injection centralizes dependencies, improving clarity.
Testing: Constructor Injection simplifies testing with a clear approach.