lazy val
vs. val
MacWire generates new
instance creation code of given classes, using values in the enclosing type for constructor
parameters, with the help of Scala Macros.
For a general introduction to DI in Scala, take a look at the Guide to DI in Scala, which also features MacWire.
MacWire helps to implement the Dependency Injection (DI) pattern, by removing the need to write the class-wiring code by hand. Instead, it is enough to declare which classes should be wired, and how the instances should be accessed (see Scopes).
Classes to be wired should be organized in "modules", which can be Scala trait
s, class
es or object
s.
Multiple modules can be combined using inheritance or composition; values from the inherited/nested modules are also
used for wiring.
MacWire can be in many cases a replacement for DI containers, offering greater control on when and how classes are instantiated, typesafety and using only language (Scala) mechanisms.
Example usage:
class DatabaseAccess()
class SecurityFilter()
class UserFinder(databaseAccess: DatabaseAccess, securityFilter: SecurityFilter)
class UserStatusReader(userFinder: UserFinder)
trait UserModule {
import com.softwaremill.macwire._
lazy val theDatabaseAccess = wire[DatabaseAccess]
lazy val theSecurityFilter = wire[SecurityFilter]
lazy val theUserFinder = wire[UserFinder]
lazy val theUserStatusReader = wire[UserStatusReader]
}
will generate:
trait UserModule {
lazy val theDatabaseAccess = new DatabaseAccess()
lazy val theSecurityFilter = new SecurityFilter()
lazy val theUserFinder = new UserFinder(theDatabaseAccess, theSecurityFilter)
lazy val theUserStatusReader = new UserStatusReader(theUserFinder)
}
For testing, just extend the base module and override any dependencies with mocks/stubs etc, e.g.:
trait UserModuleForTests extends UserModule {
override lazy val theDatabaseAccess = mockDatabaseAccess
override lazy val theSecurityFilter = mockSecurityFilter
}
The core library has no dependencies.
For more motivation behind the project see also these blogs:
For each constructor parameter of the given class, MacWire tries to find a value conforming to the parameter's type in the enclosing method and trait/class/object:
Here value means either a val
or a no-parameter def
, as long as the return type matches.
A compile-time error occurs if:
The generated code is then once again type-checked by the Scala compiler.
A factory is simply a method. The constructor of the wired class can contain parameters both from the factory (method) parameters, and from the enclosing/super type(s).
For example:
class DatabaseAccess()
class TaxDeductionLibrary(databaseAccess: DatabaseAccess)
class TaxCalculator(taxBase: Double, taxDeductionLibrary: TaxDeductionLibrary)
trait TaxModule {
import com.softwaremill.macwire._
lazy val theDatabaseAccess = wire[DatabaseAccess]
lazy val theTaxDeductionLibrary = wire[TaxDeductionLibrary]
def taxCalculator(taxBase: Double) = wire[TaxCalculator]
// or: lazy val taxCalculator = (taxBase: Double) => wire[TaxCalculator]
}
will generate:
trait TaxModule {
lazy val theDatabaseAccess = new DatabaseAccess()
lazy val theTaxDeductionLibrary = new TaxDeductionLibrary(theDatabaseAccess)
def taxCalculator(taxBase: Double) =
new TaxCalculator(taxBase, theTaxDeductionLibrary)
}
You can also wire an object using a factory method, instead of a constructor. For that, use wireWith
instead of
wire
. For example:
class A()
class C(a: A, specialValue: Int)
object C {
def create(a: A) = new C(a, 42)
}
trait MyModule {
lazy val a = wire[A]
lazy val c = wireWith(C.create _)
}
lazy val
vs. val
It is safer to use lazy val
s, as when using val
, if a value is forward-referenced, it's value during initialization
will be null
. With lazy val
the correct order of initialization is resolved by Scala.
When using wire
and a value for a parameter can't be found, an error is reported. wireRec
takes a different
approach - it tries to recursively create an instance, using normal wiring rules. This allows to explicitly wire
only those objects, which are referenced from the code, skipping helper or internal ones.
The previous example becomes:
class DatabaseAccess()
class SecurityFilter()
class UserFinder(databaseAccess: DatabaseAccess, securityFilter: SecurityFilter)
class UserStatusReader(userFinder: UserFinder)
trait UserModule {
import com.softwaremill.macwire._
lazy val theUserStatusReader = wireRec[UserStatusReader]
}
and will generate:
trait UserModule {
lazy val theUserStatusReader = new UserStatusReader(
new UserFinder(new DatabaseAccess(), new SecurityFilter()))
}
This feature is inspired by @yakivy's work on jam.
Warning: autowire
is an experimental feature, if you have any feedback regarding its usage, let us know! Future releases might break source/binary compatibility. It is available for Scala 2 only for now.
Dependency: "com.softwaremill.macwire" %% "macrosautocats" % "2.5.9"
In case you need to build an instance from some particular instances and factory methods you can leverage autowire
. This feature is intended to integrate with effect-management libraries (currently we support cats-effect).
autowire
takes as an argument a list of arguments which may contain:
new A()
)C.create _
)cats.effect.Resource
or cats.effect.IO
(e.g. C.createIO _
)cats.effect.Resource
(e.g. cats.effect.Resource[IO].pure(new A())
)cats.effect.IO
(e.g. cats.effect.IO.pure(new A())
)Using the dependencies from the given arguments it creates an instance of the given type. Any missing instances are created using their primary constructor, provided that the dependencies are met. If this is not possible, a compile-time error is reported. In other words, a wireRec
is performed, bypassing the instances search phase.
The result of the wiring is always wrapped in cats.effect.Resource
. For example:
import cats.effect._
class DatabaseAccess()
class SecurityFilter private (databaseAccess: DatabaseAccess)
object SecurityFilter {
def apply(databaseAccess: DatabaseAccess): SecurityFilter = new SecurityFilter(databaseAccess)
}
class UserFinder(databaseAccess: DatabaseAccess, securityFilter: SecurityFilter)
class UserStatusReader(databaseAccess: DatabaseAccess, userFinder: UserFinder)
object UserModule {
import com.softwaremill.macwire.autocats._
val theDatabaseAccess: Resource[IO, DatabaseAccess] = Resource.pure(new DatabaseAccess())
val theUserStatusReader: Resource[IO, UserStatusReader] = autowire[UserStatusReader](theDatabaseAccess)
}
will generate:
[...]
object UserModule {
import com.softwaremill.macwire.autocats._
val theDatabaseAccess: Resource[IO, DatabaseAccess] = Resource.pure(new DatabaseAccess())
val theUserStatusReader: Resource[IO, UserStatusReader] = UserModule.this.theDatabaseAccess.flatMap(
da => Resource.pure[IO, UserStatusReader](new UserStatusReader(da, new UserFinder(da, SecurityFilter.apply(da))))
)
}
Modules (traits or classes containing parts of the object graph) can be combined using inheritance or composition.
The inheritance case is straightforward, as wire
simply looks for values in parent traits/classes. With composition,
you need to tell MacWire that it should look inside the nested modules.
To do that, you can use imports:
class FacebookAccess(userFind: UserFinder)
class UserModule {
lazy val userFinder = ... // as before
}
class SocialModule(userModule: UserModule) {
import userModule._
lazy val facebookAccess = wire[FacebookAccess]
}
Or, if you are using that pattern a lot, you can annotate your modules using @Module
, and they will be used when
searching for values automatically:
class FacebookAccess(userFind: UserFinder)
@Module
class UserModule { ... } // as before
class SocialModule(userModule: UserModule) {
lazy val facebookAccess = wire[FacebookAccess]
}
Warning: the @Module
annotation is an experimental feature, if you have any feedback regarding its usage, let
us know!
There are two "built-in" scopes, depending on how the dependency is defined:
lazy val
/ val
def
MacWire also supports user-defined scopes, which can be used to implement request or session scopes in web applications.
The proxy
subproject defines a Scope
trait, which has two methods:
apply
, to create a scoped valueget
, to get or create the current value from the scopeTo define a dependency as scoped, we need a scope instance, e.g.:
trait WebModule {
lazy val loggedInUser = session(new LoggedInUser)
def session: Scope
}
With abstract scopes as above, it is possible to use no-op scopes for testing (NoOpScope
).
There's an implementation of Scope
targeted at classical synchronous frameworks, ThreadLocalScope
. The apply method
of this scope creates a proxy (using javassist); the get method
stores the value in a thread local. The proxy should be defined as a val
or lazy val
.
In a web application, the scopes have to be associated and disassociated with storages. This can be done for example in a servlet filter. To implement a:
associateWithEmptyStorage
is useful hereMap
) should be stored in the HttpSession
. The associate(Map)
method is useful hereFor example usage see the MacWire+Scalatra example sources.
You can run the example with sbt examples-scalatra/run
and going to http://localhost:8080.
Note that the proxy
subproject does not depend on MacWire core, and can be used stand-alone with manual wiring or any other
frameworks.
To integrate with some frameworks (e.g. Play 2) or to create instances of classes
which names are only known at run-time (e.g. plugins) it is necessary to access the wired instances dynamically.
MacWire contains a utility class in the util
subproject, Wired
, to support such functionality.
An instance of Wired
can be obtained using the wiredInModule
macro, given an instance of a module containing the
wired object graph. Any vals
, lazy val
s and parameter-less def
s (factories) from the module which are references
will be available in the Wired
instance.
The object graph in the module can be hand-wired, wired using wire
, or a result of any computation.
Wired
has two basic functionalities: looking up an instance by its class (or trait it implements), and instantiating
new objects using the available dependencies. You can also extend Wired
with new instances/instance factories.
For example:
// 1. Defining the object graph and the module
trait DatabaseConnector
class MysqlDatabaseConnector extends DatabaseConnector
class MyApp {
def securityFilter = new SecurityFilter()
val databaseConnector = new MysqlDatabaseConnector()
}
// 2. Creating a Wired instance
import com.softwaremill.macwire._
val wired = wiredInModule(new MyApp)
// 3. Dynamic lookup of instances
wired.lookup(classOf[SecurityFilter])
// Returns the mysql database connector, even though its type is MysqlDatabaseConnector, which is
// assignable to DatabaseConnector.
wired.lookup(classOf[DatabaseConnector])
// 4. Instantiation using the available dependencies
{
package com.softwaremill
class AuthenticationPlugin(databaseConnector: DatabaseConnector)
}
// Creates a new instance of the given class using the dependencies available in MyApp
wired.wireClassInstanceByName("com.softwaremill.AuthenticationPlugin")
MacWire contains an implementation of interceptors, which can be applied to class instances in the modules.
Similarly to scopes, the proxy
subproject defines an Interceptor
trait, which has only one method: apply
.
When applied to an instance, it should return an instance of the same class, but with the interceptor applied.
There are two implementations of the Interceptor
trait provided:
NoOpInterceptor
: returns the given instance without changesProxyingInterceptor
: proxies the instance, and returns the proxy. A provided function is called
with information on the invocationInterceptors can be abstract in modules. E.g.:
trait BusinessLogicModule {
lazy val moneyTransferer = transactional(wire[MoneyTransferer])
def transactional: Interceptor
}
During tests, you can then use the NoOpInterceptor
. In production code or integration tests, you can specify a real
interceptor, either by extending the ProxyingInterceptor
trait, or by passing a function to the
ProxyingInterceptor
object:
object MyApplication extends BusinessLogicModule {
lazy val tm = wire[TransactionManager]
lazy val transactional = ProxyingInterceptor { ctx =>
try {
tm.begin()
val result = ctx.proceed()
tm.commit()
result
} catch {
case e: Exception => tm.rollback()
}
}
}
The ctx
is an instance of an InvocationContext
, and contains information on the parameters passed to the method,
the method itself, and the target object. It also allows to proceed with the invocation with the same or changed
parameters.
For more general AOP, e.g. if you want to apply an interceptor to all methods matching a given pointcut expression, you should use AspectJ or an equivalent library. The interceptors that are implemented in MacWire correspond to annotation-based interceptors in Java.
Sometimes you have multiple objects of the same type that you want to use during wiring. Macwire needs to have some
way of telling the instances apart. As with other things, the answer is: types! Even when not using wire
, it may
be useful to give the instances distinct types, to get compile-time checking.
For that purpose Macwire includes support for tagging via scala-common, which lets you attach tags to instances to qualify them. This is a compile-time only operation, and doesn't affect the runtime. The tags are derived from Miles Sabin's gist.
To bring the tagging into scope, import com.softwaremill.tagging._
.
Using tagging has two sides. In the constructor, when declaring a dependency, you need to declare what tag it needs
to have. You can do this with the _ @@ _
type constructor, or if you prefer another syntax Tagged[_, _]
. The first
type parameter is the type of the dependency, the second is a tag.
The tag can be any type, but usually it is just an empty marker trait.
When defining the available instances, you need to specify which instance has which tag. This can be done with the
taggedWith[_]
method, which returns a tagged instance (A.taggedWith[T]: A @@ T
). Tagged instances can be used
as regular ones, without any constraints.
The wire
macro does not contain any special support for tagging, everything is handled by subtyping. For example:
class Berry()
trait Black
trait Blue
case class Basket(blueberry: Berry @@ Blue, blackberry: Berry @@ Black)
lazy val blueberry = wire[Berry].taggedWith[Blue]
lazy val blackberry = wire[Berry].taggedWith[Black]
lazy val basket = wire[Basket]
Multiple tags can be combined using the andTaggedWith
method. E.g. if we had a berry that is both blue and black:
lazy val blackblueberry = wire[Berry].taggedWith[Black].andTaggedWith[Blue]
The resulting value has type Berry @ (Black with Blue)
and can be used both as a blackberry and as a blueberry.
Using wireSet
you can obtain a set of multiple instances of the same type. This is done without constructing the set explicitly. All instances of the same type which are found by MacWire are used to construct the set.
Consider the below example. Let's suppose that you want to create a RockBand(musicians: Set[Musician])
object. It's easy to do so using the wireSet
functionality:
trait Musician
class RockBand(musicians: Set[Musician])
trait RockBandModule {
lazy val singer = new Musician {}
lazy val guitarist = new Musician {}
lazy val drummer = new Musician {}
lazy val bassist = new Musician {}
lazy val musicians = wireSet[Musician] // all above musicians will be wired together
// musicians has type Set[Musician]
lazy val rockBand = wire[RockBand]
}
When:
due to limitations of the current macros implementation in Scala (for more details see this discussion) to avoid compilation errors it is recommended to add type ascriptions to the dependencies. This is a way of helping the type-checker that is invoked by the macro to figure out the types of the values which can be wired.
For example:
class A()
class B(a: A)
// note the explicit type. Without it wiring would fail with recursive type compile errors
lazy val theA: A = wire[A]
// reference to theA; if for some reason we need explicitly write the constructor call
lazy val theB = new B(theA)
This is an inconvenience, but hopefully will get resolved once post-typer macros are introduced to the language.
Also, wiring will probably not work properly for traits and classes defined inside the containing trait/class, or in super traits/classes.
Note that the type ascription may be a subtype of the wired type. This can be useful if you want to expose e.g. a trait that the wired class extends, instead of the full implementation.
Macwire provides wiring suport for akka through the macrosAkka
module.
Here
you can find example code. The module adds three macros wireAnonymousActor[A]
, wireActor[A]
and wireProps[A]
which help create instances of akka.actor.ActorRef
and akka.actor.Props
.
These macros require an ActoRefFactory
(ActorSystem
or Actor.context
) to be in scope as a dependency.
If actor's primary constructor has dependencies - they are required to be in scope as well.
Example usage:
import akka.actor.{Actor, ActorRef, ActorSystem}
class DatabaseAccess()
class SecurityFilter()
class UserFinderActor(databaseAccess: DatabaseAccess, securityFilter: SecurityFilter) extends Actor {
override def receive: Receive = {
case m => // ...
}
}
import com.softwaremill.macwire._
import com.softwaremill.macwire.akkasupport._
val theDatabaseAccess = wire[DatabaseAccess] //1st dependency for UserFinderActor
//it compiles to: val theDatabaseAccess = new DatabaseAccess
val theSecurityFilter = wire[SecurityFilter] //2nd dependency for UserFinderActor
//it compiles to: val theSecurityFilter = new SecurityFilter
val system = ActorSystem("actor-system") //there must be instance of ActoRefFactory in scope
val theUserFinder = wireActor[UserFinderActor]("userFinder")
//this compiles to:
//lazy val theUserFinder = system.actorOf(Props(classOf[UserFinderActor], theDatabaseAccess, theSecurityFilter), "userFinder")
In order to make it working all dependencies created Actor
's (UserFinderActor
's) primary constructor and
instance of the akka.actor.ActorRefFactory
must be in scope. In above example this is all true. Dependencies
of the UserFinderActor
are DatabaseAccess
and SecurityFilter
and they are in scope.
The ActorRefFactory
is in scope as well because ActorSystem
which is subtype of it is there.
Creating actor within another actor is even simpler than in first example because we don't need to have ActorSystem
in scope.
The ActorRefFactory
is here because Actor.context
is subtype of it. Let's see this in action:
class UserStatusReaderActor(theDatabaseAccess: DatabaseAccess) extends Actor {
val theSecurityFilter = wire[SecurityFilter]
val userFinder = wireActor[UserFinderActor]("userFinder")
//this compiles to:
//val userFinder = context.actorOf(Props(classOf[UserFinderActor], theDatabaseAccess, theSecurityFilter), "userFinder")
override def receive = ...
}
The difference is that previously macro expanded into system.actorOf(...)
and when inside another actor it expanded into context.actorOf(...)
.
It's possible to create anonymous actors. wireAnonymousActor
is for it:
val userFinder = wireAnonymousActor[UserFinderActor]
//this compiles to:
//val userFinder = context.actorOf(Props(classOf[UserFinderActor], theDatabaseAccess, theSecurityFilter))
How about creating akka.actor.Props
? It's there and can be achieved by calling wireProps[A]
.
Wiring only Props
can be handy when it's required to setup the Props
before passing them to the actorOf(...)
method.
Let's say we want to create some actor with router. It can be done as below:
val userFinderProps = wireProps[UserFinderActor] //create Props
//This compiles to: Props(classOf[UserFinderActor], theDatabaseAccess, theSecurityFilter)
.withRouter(RoundRobinPool(4)) //change it according requirements
val userFinderActor = system.actorOf(userFinderProps, "userFinder") //create the actor
How about creating actors which depends on ActorRef
s? The simplest way is to
pass them as arguments to the constructor. But how to distinguish two actorRef
s representing two different actors?
They have the same type though.
class DatabaseAccessActor extends Actor { ... }
class SecurityFilterActor extends Actor { ... }
val db: ActorRef = wireActor[DatabaseAccessActor]("db")
val filter: ActorRef = wireActor[SecurityFilterActor]("filter")
class UserFinderActor(databaseAccess: ActorRef, securityFilter: ActorRef) extends Actor {...}
//val userFinder = wireActor[UserFinderActor] wont work here
We can't just call wireActor[UserFinderActor]
because it's not obvious which instance of ActorRef
is for databaseAccess
and which are for securityFilter
. They are both of the same type - ActorRef
.
The solution for it is to use earlier described qualifiers. In above example solution for wiring may look like this:
sealed trait DatabaseAccess //marker type
class DatabaseAccessActor extends Actor { ... }
sealed trait SecurityFilter //marker type
class SecurityFilterActor extends Actor { ... }
val db: ActorRef @@ DatabaseAccess = wireActor[DatabaseAccessActor]("db").taggedWith[DatabaseAccess]
val filter: ActorRef @@ SecurityFilter = wireActor[SecurityFilterActor]("filter").taggedWith[SecurityFilter]
class UserFinderActor(databaseAccess: ActorRef @@ DatabaseAccess, securityFilter: ActorRef @@ SecurityFilter) extends Actor {...}
val userFinder = wireActor[UserFinderActor]
It is also possible to wire an actor using a factory function.
For that, the module provides three additional macros wireAnonymousActorWith
, wireActorWith
and wirePropsWith
.
Their usage is similar to wireWith
(see Factory methods).
For example:
class UserFinderActor(databaseAccess: DatabaseAccess, securityFilter: SecurityFilter) extends Actor { ... }
object UserFinderActor {
def get(databaseAccess: DatabaseAccess) = new UserFinderActor(databaseAccess, new SimpleSecurityFilter())
}
val theUserFinder = wireActorWith(UserFinderActor.get _)("userFinder")
The jars are deployed to Sonatype's OSS repository. To use MacWire in your project, add a dependency:
libraryDependencies += "com.softwaremill.macwire" %% "macros" % "2.5.9" % "provided"
libraryDependencies += "com.softwaremill.macwire" %% "macrosakka" % "2.5.9" % "provided"
libraryDependencies += "com.softwaremill.macwire" %% "util" % "2.5.9"
libraryDependencies += "com.softwaremill.macwire" %% "proxy" % "2.5.9"
MacWire is available for Scala 2.12, 2.13, 3 on the JVM and JS.
The macros
subproject contains only code which is used at compile-time, hence the provided
scope.
The util
subproject contains tagging, Wired
and the @Module
annotation; if you don't use these features, you don't
need to include this dependency.
The proxy
subproject contains interceptors and scopes, and has a dependency on javassist
.
Older 1.x release for Scala 2.10 and 2.11:
libraryDependencies += "com.softwaremill.macwire" %% "macros" % "1.0.7"
libraryDependencies += "com.softwaremill.macwire" %% "runtime" % "1.0.7"
To print debugging information on what MacWire does when looking for values, and what code is generated, set the
macwire.debug
system property. E.g. with SBT, just add a System.setProperty("macwire.debug", "")
line to your
build file.
Macwire also works with Scala.js. For an example, see here: Macwire+Scala.js example.
Take a look at the available issues. If you'd like to see one developed please vote on it. Or maybe you'll attempt to create a pull request?
util
to get tagging & Wired
, and proxy
to get interceptors and scopescom.softwaremill.tagging._
wireImplicit
wire
at all (they used to be subject to the same lookup procedure as normal
parameters + implicit lookup)In Play 2.4.x, you can no longer use getControllerInstance in GlobalSettings for injection. Play has a new pattern for injecting controllers. You must extend ApplicationLoader, from there you can mix in your modules.
import controllers.{Application, Assets}
import play.api.ApplicationLoader.Context
import play.api._
import play.api.routing.Router
import router.Routes
import com.softwaremill.macwire._
class AppApplicationLoader extends ApplicationLoader {
def load(context: Context) = {
// make sure logging is configured
Logger.configure(context.environment)
(new BuiltInComponentsFromContext(context) with AppComponents).application
}
}
trait AppComponents extends BuiltInComponents with AppModule {
lazy val assets: Assets = wire[Assets]
lazy val prefix: String = "/"
lazy val router: Router = wire[Routes]
}
trait AppModule {
// Define your dependencies and controllers
lazy val applicationController = wire[Application]
}
In application.conf, add the reference to the ApplicationLoader.
play.application.loader = "AppApplicationLoader"
For more information and to see the sample project, go to examples/play24
Reference Play docs for more information:
For Play 2.5.x, you must do the same as for Play 2.4.x, except the Logger
configuration.
import play.api.LoggerConfigurator
class AppApplicationLoader extends ApplicationLoader {
def load(context: Context) = {
LoggerConfigurator(context.environment.classLoader).foreach {
_.configure(context.environment)
}
// ... do the same as for Play 2.4.x
}
}
The Scala 3 version is written to be compatible with Scala 2 where possible. Currently there are a few missing features:
For full list of incompatibilities take a look at tests/src/test/resources/test-cases
and util-tests/src/test/resources/test-cases
.
We offer commercial support for MacWire and related technologies, as well as development services. Contact us to learn more about our offer!
Copyright (C) 2013-2021 SoftwareMill https://softwaremill.com.