renghen/jam

Incredibly simple DI Scala library.

Jam

Jam is an incredibly simple DI Scala library.

Essential differences from macwire:

• injects a whole object tree, not only constructor arguments
• can be easily migrated on Scala 3, since doesn't use context scans
• incredibly simple: 1 exposed method, ~50 lines of code

Quick start

Add jam dependency:

"com.github.yakivy" %% "jam-core" % "0.0.1"

Usage example:

class DatabaseAccess()
class SecurityFilter(databaseAccess: DatabaseAccess)
class UserFinder(databaseAccess: DatabaseAccess, securityFilter: SecurityFilter)
class UserStatusReader(userFinder: UserFinder)

trait UserModule {
    val singletonDatabaseAccess = jam.brew[DatabaseAccess]
    val userStatusReader        = jam.brew[UserStatusReader]
}

will generate:

trait UserModule {
    val singletonDatabaseAccess = new DatabaseAccess()
    val userStatusReader = new UserStatusReader(
        new UserFinder(
            singletonDatabaseAccess,
            new SecurityFilter(singletonDatabaseAccess)
        )
    )
}

Implementation details

• injection candidates is being searched in this instance, so to provide an instance for future injection you need to make it a member of this. Examples:

trait A {
    val a = new A
    ...brewing //val a will be used
}

val container = new {
    val a = new A
    ...brewing //val a will be used
}

trait A {
    def b(): String = {
        val a = new A
        ...brewing //new A instance will be created
    }
}

trait A {
    val a1 = new A
    {
        val a2 = new A
        ...brewing //val a1 will be used
    }
}

• library injects only non implicit constructor arguments, implicits will be resolved by scalac
• jam is intended to be minimal, features like scopes, object lifecycles or factory methods should be implemented manually, for example:

//to release resources use cats.effect.Resource
dbClientResource.use { dbClient =>
    val container = new {
        //to create an object with custom constructor define val or def member
        val config = Config.fromFile("config.conf")
        //to create singleton define val member
        val userStorage = new UserStorage(dbClient)
        //to create custom factory method define def member
        def userService = new UserService(
            userStorage,
            config.withFallback(new Config())
        )

        ...brewing //provided members will be used here
    }
    ...logic that utilizes container
} // client will be closed