Overview

Injector is lightweight and zero-overhead dependency injection library for JVM developers. It was carefully designed to make no-use of reflection by having all required meta-information computed at compile time. At runtime it performs only the necessary tasks required to instantiate classes and have its dependencies injected.

Why Injector is different?

"Often in discussions about dependency injection, people mix the pattern with how it is supported by containers. There is plenty to criticize when it comes to many of these containers, but the core pattern is easy to appreciate: instead of letting objects assemble their own dependencies, classes state what dependencies they require from clients in order to be constructed and used." [1]

Injector would be better described as small library with a single responsibility: inject dependencies on your objects. Despite of its small footprint (<7kb), it optimizes everything it requires to perform its tasks at compilation time, providing a zero-overhead runtime for your projects.

Dependency injection in action

Kotlin

import injector.*;

data class User(
    val id: UUID = UUID.randomUUID()
    val name: String,
    val isPayingUser: Boolean
)

@Singleton class UserPersistence {
    private val users = mutableMapOf<UUID, User>()

    fun save( user: User ) {
        users.put( user.id, user );
    }

    fun loadAll(): Map<UUID, User> = users
}

@Singleton class UserService( val persistence: UserPersistence ) {

    fun save( user: User ){
        if (user.isPayingUser)
            persistence.save( user )
    }
}

fun main() {
    val service = Injector.create().instanceOf( UserService.class )
    val user = User("joe@dinner.com", true)
    service.save( user )
}

Java

import injector.*;
import java.util.*;

class User {
    final UUID id = UUID.randomUUID();
    final String name;
    final Boolean isPayingUser;

    User( String name, Boolean isPayingUser ){
        this.name = name;
        this.isPayingUser = isPayingUser;
    }
}

@Singleton
class UserPersistence {
    private final Map<UUID,User> users = new HashMap<>();

    void save( User user ) {
        users.put( user.id, user );
    }
}

@Singleton
class UserService {

    final UserPersistence persistence;

    UserService( UserPersistence persistence ){
        this.persistence = persistence;
    }

    void save( User user ){
        if (user.isPayingUser)
            persistence.save( user );
    }
}

class Main {
    public static void main(String[] args) {
        UserService service = Injector.create().instanceOf(UserService.class);
        User user = new User("joe@dinner.com", true);
        service.save( user );
    }
}

License

Injector is release under Apache License 2 terms.