Differences Between Constructor Injection and Setter Injection

What is the difference between constructor injection and setter injection?

Constructor injection and setter injection are two common methods of dependency injection in object-oriented programming, particularly in frameworks that support inversion of control (IoC). Both methods are used to provide an object with its dependencies, but they do so in different ways. Here are the key differences between the two:

Constructor Injection

1. Definition: Dependencies are provided to a class through its constructor. When an instance of the class is created, the required dependencies are passed as parameters.

2. Immutability: Constructor injection allows for the creation of immutable objects. Once the object is constructed, its dependencies cannot be changed, which can lead to more predictable and reliable code.

3. Mandatory Dependencies: It is clear from the constructor signature which dependencies are required for the class to function. This makes it easier to understand the class's requirements and ensures that all necessary dependencies are provided at the time of instantiation.

4. Tight Coupling: While constructor injection can lead to tighter coupling between the class and its dependencies, it also makes the dependencies explicit, which can be beneficial for understanding and maintaining the code.

5. Example:

   public class Service {
       private final Repository repository;
   
       public Service(Repository repository) {
           this.repository = repository;
       }
   }
   

Setter Injection

1. Definition: Dependencies are provided to a class through setter methods after the object has been constructed. This allows for the dependencies to be set or changed at any time.

2. Mutability: Setter injection allows for mutable objects, meaning that dependencies can be changed after the object has been created. This can lead to more flexible designs but may also introduce risks if the object is used before all dependencies are set.

3. Optional Dependencies: Setter injection can be used for optional dependencies, allowing the class to function even if some dependencies are not provided. This can make the class more flexible but may also lead to situations where the class is in an invalid state if required dependencies are not set.

4. Loose Coupling: Setter injection can lead to looser coupling between the class and its dependencies, as dependencies can be changed or replaced at runtime.

5. Example:

   public class Service {
       private Repository repository;
   
       public void setRepository(Repository repository) {
           this.repository = repository;
       }
   }
   

Summary

• Constructor Injection: Dependencies are required at the time of object creation, leading to immutable objects and clear dependency requirements.
• Setter Injection: Dependencies can be set or changed after object creation, allowing for optional dependencies and mutable objects.

The choice between constructor and setter injection often depends on the specific use case, design preferences, and the need for flexibility versus immutability.