What is the difference between dependency injection via constructor and via setter?

Difference in timing of implementation, dependency obligation, and capabilities when creating an object:

Feature	Constructor Injection	Setter Injection
Implementation Time	At object creation.	After object creation.
Obligation	Dependency is mandatory for object creation.	Dependency is optional.
Object State	Object is created fully initialized.	Object can be created in a partially initialized state.
Immutability	Allows creating an object with immutable dependencies.	Dependencies can be changed after object creation.
Number of Dependencies	Better suited for a small number of dependencies.	More convenient with a large number of optional dependencies.
Cyclic Dependencies	Signals problematic cyclic dependencies.	May hide cyclic dependencies.

Constructor injection is preferable for mandatory dependencies, ensuring object integrity from the moment of creation and allowing immutability.

// Example of constructor injection
public class ServiceA {
    private final DependencyB dependencyB;

    public ServiceA(DependencyB dependencyB) {
        this.dependencyB = dependencyB;
    }

    // ... methods using dependencyB
}

Setter injection is suitable for optional dependencies or when flexibility in changing dependencies after object creation is required.

// Example of setter injection
public class ServiceC {
    private DependencyD dependencyD;

    public void setDependencyD(DependencyD dependencyD) {
        this.dependencyD = dependencyD;
    }

    // ... methods using dependencyD (with null check if dependency is optional)
}