When to Use Constructor-based and Setter-based DI in Spring?

In this blog post, we will discuss the critical topic of when to use constructor-based and setter-based dependency injection (DI) in Spring-based applications. Understanding the appropriate use cases for each type of DI is essential for building robust and maintainable applications.

Constructor-based Dependency Injection

Constructor-based DI is achieved by the container invoking a constructor with several arguments, each representing a dependency. This approach ensures that all required dependencies are provided when the object is created, making the object fully initialized and ready to use.

Here is an example of constructor-based DI:

public class Car {
    private Engine engine;

    @Autowired
    public Car(Engine engine) {
        this.engine = engine;
    }
}

In the diagram below, the highlighted part shows the constructor-based dependency injection:

Setter-based Dependency Injection

Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean. This method is often used for optional dependencies or when you need to re-inject dependencies.

Here is an example of setter-based DI:

public class Car {
    private Engine engine;

    @Autowired
    public void setEngine(Engine engine) {
        this.engine = engine;
    }
}

In the diagram below, the highlighted part shows the setter-based dependency injection:

When to Use Constructor-based and Setter-based DI?

According to the Spring Framework documentation, you can mix constructor-based and setter-based DI. However, here are some guidelines:

1. Use Constructor-based DI for Mandatory Dependencies: Ensure that your bean is ready to use when it is first called.
2. Use Setter-based DI for Optional Dependencies: This allows for re-injection of dependencies later if needed.
3. Avoid Constructor-based DI for Beans with Many Dependencies: If a constructor has many arguments, it indicates that the bean might be doing too much and should be refactored.
4. Avoid Circular Dependencies: Constructor-based DI can lead to circular dependencies. Use setter-based DI in such cases.

Best Practices

• Mandatory Dependencies: Use constructor-based DI.
• Optional Dependencies: Use setter-based DI.
• Refactoring Large Constructors: Break down beans with many constructor arguments into smaller, more manageable units.
• Handling Circular Dependencies: Prefer setter-based DI to avoid circular dependencies.
• Mixing DI Types: Depending on the nature of dependencies, it is common to mix constructor-based and setter-based DI for the same bean.

Conclusion

Choosing between constructor-based and setter-based dependency injection in Spring depends on your specific use case and requirements. Constructor-based DI is best for mandatory dependencies, ensuring the bean is fully initialized upon creation. Setter-based DI is suitable for optional dependencies and scenarios where you need to re-inject dependencies. By following the guidelines and best practices outlined above, you can build more maintainable and robust Spring applications.

Related Spring Dependency Injection Articles

1. Guide to Dependency Injection in Spring

   • This guide covers what dependency injection is in the Spring framework and the different types of DI supported by Spring with examples.

2. Spring Dependency Injection via Setter Example

   • Learn how to use setter-based dependency injection in Spring Applications.

3. Spring Dependency Injection via Constructor Example

   • Learn how to use constructor-based dependency injection in Spring Applications.

4. Spring - @DependsOn Annotation Example

   • Discusses how to use the @DependsOn annotation in Spring Applications with an example. The @DependsOn annotation can force the Spring IoC container to initialize one or more beans before the bean is annotated with @DependsOn.

By understanding and appropriately applying these DI methods, you can ensure your Spring-based applications are well-architected and maintainable.