How do dependency injection approaches via constructor and via setter differ in software development?

Dependency Injection through constructor and setter are two common approaches, each with its own features:

1. Through constructor:

   • Dependencies are passed during object creation.
   • Ensures immutability of dependencies after creation.
   • Guarantees that the object is always in a correct state with necessary dependencies.
   • Suitable for mandatory dependencies.

2. Through setter (installation method):

   • Dependencies are set after object creation.
   • Allows changing dependencies during the object's lifecycle.
   • May lead to a state where the object is created but dependencies are not yet set (potentially incorrect operation).
   • Convenient for optional dependencies or when dependencies need to be changed dynamically.

In development context:

• Constructor approach promotes creating more reliable and predictable code.
• Setters provide flexibility but require additional checks for object state correctness.

Example:

// Dependency injection via constructor
public class UserService {
    private final UserRepository repo;
    public UserService(UserRepository repo) {
        this.repo = repo;
    }
}

// Dependency injection via setter
public class UserService {
    private UserRepository repo;
    public void setUserRepository(UserRepository repo) {
        this.repo = repo;
    }
}