What is dependency injection, and what forms does it take?

Question

Accepted Answer

Summary

Dependency injection means an object receives its collaborators from outside instead of creating them. The forms are constructor injection (the default), setter or property injection (optional or changeable dependencies) and method injection (one call needs it).

Longer Answer

DI makes dependencies explicit and swappable: tests pass fakes (an in-memory mailer, a fixed clock), and production wires real implementations in one composition root. Frameworks like Spring, Laravel's container, NestJS and FastAPI's Depends automate the wiring.

Inject things that do I/O, hold state or need faking: databases, HTTP clients, clocks, random numbers. Ten constructor parameters signal an SRP problem, not a DI problem.

Code Example

class FixedClock:
    def today(self):
        return "2030-01-01"


class Signup:
    def __init__(self, clock):  # constructor injection
        self.clock = clock

    def welcome(self, email):
        return f"{email} joined on {self.clock.today()}"


print(Signup(FixedClock()).welcome("[email protected]"))

interface Clock { today(): string; }

class FixedClock implements Clock {
  today() { return "2030-01-01"; }
}

class Signup {
  constructor(private clock: Clock) {} // constructor injection
  welcome(email: string) { return `${email} joined on ${this.clock.today()}`; }
}

console.log(new Signup(new FixedClock()).welcome("[email protected]"));

public class Main {
    public static void main(String[] args) {
        System.out.println(new Signup(new FixedClock()).welcome("[email protected]"));
    }
}

interface Clock { String today(); }

class FixedClock implements Clock {
    public String today() { return "2030-01-01"; }
}

class Signup {
    private final Clock clock;
    Signup(Clock clock) { this.clock = clock; } // constructor injection
    String welcome(String email) { return email + " joined on " + clock.today(); }
}

<?php
interface Clock { public function today(): string; }

class FixedClock implements Clock {
    public function today(): string { return "2030-01-01"; }
}

class Signup {
    public function __construct(private Clock $clock) {} // constructor injection
    public function welcome(string $email): string { return "$email joined on {$this->clock->today()}"; }
}

echo (new Signup(new FixedClock()))->welcome("[email protected]"), "\n";

Key Points

Pass dependencies in.
Constructor injection is the default.
Composition root wires real objects.
Too many dependencies = too many responsibilities.

Deep dive: Design Patterns, Explained Simply: Dependency Injection Without a Framework

What is dependency injection, and what forms does it take?

Summary

Longer Answer

Code Example

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Explanation

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