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MacWire

Ideas, suggestions, problems, questions Maven Central

Zero-cost, compile-time, type-safe dependency injection library.

The core of MacWire is a set of macros, that is code which runs at compile-time. The macros generate the new instance creation code automatically, allowing you to avoid a ton of boilerplate code, while maintaining type-safety.

The core library has no dependencies, and incurs no run-time overhead. The main mechanism of defining dependencies of a class are plain constructors or apply methods in companion objects. MacWire doesn't impact the code of your classes in any way, there are no annotations that you have to use or conventions to follow.

There's a couple of wiring variants that you can choose from:

  • autowire create an instance of the given type, using the provided dependencies. Any missing dependencies are created using constructors/apply methods.
  • wire create an instance of the given type, using dependencies from the context, within which it is called. Dependencies are looked up in the enclosing trait/class/object and parents (via inheritance).
  • wireRec is a variant of wire, which creates missing dependencies using constructors.

In other words, autowire is context-free, while the wire family of macros is context-dependent.

MacWire is available for Scala 2.12, 2.13 and 3 on the JVM, JS and Native platforms. Not all functionalities are available for each Scala version.

To use, add the following dependency:

// sbt
"com.softwaremill.macwire" %% "macros" % "2.6.4" % "provided"

// scala-cli
//> using dep com.softwaremill.macwire::macros:2.6.4

Table of Contents

autowire

Scala 3 direct-style

Autowire generates the code needed to instantiate the given type. To create the instance, the public primary constructor is used, or if one is absent - the apply method from the companion object. Any dependencies are created recursively. For example, given the following classes:

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

invoking autowire as below:

import com.softwaremill.macwire.*
val userStatusReader = autowire[UserStatusReader]()

generates following code:

val userStatusReader =
  val wiredDatabaseAccess   = new DatabaseAccess()
  val wiredSecurityFilter   = new SecurityFilter()
  val wiredUserFinder       = new UserFinder(wiredDatabaseAccess, wiredSecurityFilter)
  val wiredUserStatusReader = new UserStatusReader(wiredUserFinder)
  wiredUserStatusReader

autowire accepts an arbitrary number of parameters, which specify how to lookup or create dependencies, instead of the default construct/apply mechanism. These are described in detail below. Each such parameter might be:

  • an instance to use
  • a function to create an instance
  • a class to instantiate to provide a dependency for the types it implements (provided as: classOf[SomeType])
  • a autowireMembersOf(instance) call, to use the members of the given instance as dependencies

autowire is context-free: its result does not depend on the environment, within which it is called (except for implicit parameters, which are looked up using the usual mechanism). It only depends on the type that is specified for wiring, and any parameters that are passed.

Providing instantiated dependencies

In some cases it's necessary to instantiate a dependency by hand, e.g. to initialise it with some configuration, or to manage its lifecycle. In such cases, dependencies can be provided as parameters to the autowire invocation. They will be used whenever a value of the given type is needed.

As an example, consider a DataSource, which needs to be configured with a JDBC connection string, and has a managed life-cycle:

import java.io.Closeable

class DataSource(jdbcConn: String) extends Closeable { def close() = () }
class DatabaseAccess(ds: DataSource)
class SecurityFilter()
class UserFinder(databaseAccess: DatabaseAccess, securityFilter: SecurityFilter)
class UserStatusReader(userFinder: UserFinder)

We can provide a DataSource instance to be used by autowire:

import com.softwaremill.macwire.*
import scala.util.Using

Using.resource(DataSource("jdbc:h2:~/test")): ds =>
  autowire[UserStatusReader](ds)

Using factories

In addition to instances, which should be used by autowire, it's also possible to provide factory methods. They will be used to create instances of types which is the result of the provided function, using the dependencies which are the function's parameters. Any dependencies are recursively created.

For example, we can provide a custom way to create a UserFinder:

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

autowire[UserStatusReader](UserFinder(_, _, adminOnly = true))

Specifying implementations to use

You can specify which classes to use to create instances. This is useful when the dependencies are expressed for example using traits, not the concrete types. Such a dependency can be expressed by providing a classOf[] parameter to autowire:

trait DatabaseAccess
class DatabaseAccessImpl() extends DatabaseAccess

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

autowire[UserStatusReader](classOf[DatabaseAccessImpl])

Without the classOf[], MacWire wouldn't know how to create an instance implementing DatabaseAccess.

Using dependencies contained in objects

Finally, it's possible to use the members of a given instance as dependencies. Simply pass a autowireMembersOf(someInstance) as a parameter to autowire.

Errors

autowire reports an error when:

  • a dependency can't be created (e.g. there are no public constructors / apply methods)
  • there are circular dependencies
  • the provided dependencies contain a duplicate
  • a provided dependency is not used
  • a primitive or String type is used as a dependency (instead, use e.g. an opaque type)

Each error contains the wiring path. For example, below there's no public constructor for DatabaseAccess, which results in a compile-time error:

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

autowire[UserStatusReader]()

// compile-time error:
// cannot find a provided dependency, constructor or apply method for: DatabaseAccess;
// wiring path: UserStatusReader -> UserFinder -> DatabaseAccess

Limitiations of autowire

autowire doesn't handle propagating generic type parameters, when creating missing dependencies using constructors or apply methods. E.g. if you have a class B[X](a: A[X]), and a B[Int] needs to be created, the specific value for the type parameter X won't be propagated when resolving the dependencies.

As a work-around, you need to explicitly provide the generic dependencies with concrete type parameters. A generic dependency might appear multiple times, with different type parameters. To wire the classes from the example, you'd need: autowire[...](B[Int](_), someInstance: A[Int]) or similar.

wire

Scala 2 Scala 3 direct-style

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 traits, classes or objects. 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:

How wiring works

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:

  • first it tries to find a unique value declared as a value in the current block, argument of enclosing methods and anonymous functions.
  • then it tries to find a unique value declared or imported in the enclosing type
  • then it tries to find a unique value in parent types (traits/classes)
  • if the parameter is marked as implicit, it is ignored by MacWire and handled by the normal implicit resolution mechanism

Here value means either a val or a no-parameter def, as long as the return type matches.

A compile-time error occurs if:

  • there are multiple values of a given type declared in the enclosing block/method/function's arguments list, enclosing type or its parents.
  • parameter is marked as implicit and implicit lookup fails to find a value
  • there is no value of a given type

The generated code is then once again type-checked by the Scala compiler.

Factories

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)
}

Factory methods

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 vals, 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.

Recursive wiring

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.

Composing modules

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]
}

Avoiding imports

Dependency:

"com.softwaremill.macwire" %% "util" % "2.6.4"

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]
}

Scopes

Dependency:

"com.softwaremill.macwire" %% "proxy" % "2.6.4"

There are two "built-in" scopes, depending on how the dependency is defined:

  • singleton: lazy val / val
  • dependent - separate instance for each dependency usage: 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 value
  • get, to get or create the current value from the scope

To 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:

  • request scope, we need a new empty storage for every request. The associateWithEmptyStorage is useful here
  • session scope, the storage (a Map) should be stored in the HttpSession. The associate(Map) method is useful here

For 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.

Accessing wired instances dynamically

Dependency:

"com.softwaremill.macwire" %% "util" % "2.6.4"

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 vals and parameter-less defs (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")

Limitations

When:

  • referencing wired values within the trait/class/object
  • using multiple modules in the same compilation unit
  • using multiple modules with scopes

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.

Akka integration

Dependency:

"com.softwaremill.macwire" %% "macrosakka" % "2.6.4" % "provided"

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 ActorRefs? The simplest way is to pass them as arguments to the constructor. But how to distinguish two actorRefs 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")

Multi Wiring (wireSet)

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]
}

Autowire for cats-effect

Scala 2 cats-effect

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" % "2.6.4"

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:

  • values (e.g. new A())
  • factory methods (e.g. C.create _)
  • factory methods that return 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))))
  )
}

Interceptors

Dependency:

"com.softwaremill.macwire" %% "proxy" % "2.6.4"

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 changes
  • ProxyingInterceptor: proxies the instance, and returns the proxy. A provided function is called with information on the invocation

Interceptors 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.

Qualifiers

Note

While the below works both in Scala 2 & Scala 3, in Scala 3, you can use the built-in opaque types instead.

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.

Development

Debugging

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, start using sbt -Dmacwire.debug.

Future development - vote!

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?

Platform and version-specifics

Scala.js

Macwire also works with Scala.js. For an example, see here: Macwire+Scala.js example.

Scala3 support

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 .

Other

Commercial Support

We offer commercial support for MacWire and related technologies, as well as development services. Contact us to learn more about our offer!

Copyright

Copyright (C) 2013-2024 SoftwareMill https://softwaremill.com.