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Lightweight and Nonintrusive Scala Dependency Injection Library
Scala Java
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MacWire

Build Status Join the chat at https://gitter.im/adamw/macwire Maven Central Dependencies

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:

You can also try MacWire through Typesafe Activator.

A similar project for Java is Dagger.

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.

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] 
}

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!

Scopes

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

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")

Interceptors

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

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.

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.

Installation, using with SBT

The jars are deployed to Sonatype's OSS repository. To use MacWire in your project, add a dependency:

libraryDependencies += "com.softwaremill.macwire" %% "macros" % "2.2.3" % "provided"

libraryDependencies += "com.softwaremill.macwire" %% "util" % "2.2.3"

libraryDependencies += "com.softwaremill.macwire" %% "proxy" % "2.2.3"

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.

To use the snapshot version:

resolvers += "Sonatype OSS Snapshots" at "https://oss.sonatype.org/content/repositories/snapshots"

libraryDependencies += "com.softwaremill.macwire" %% "macros" % "2.2.4-SNAPSHOT" % "provided"

libraryDependencies += "com.softwaremill.macwire" %% "util" % "2.2.4-SNAPSHOT"

Currently 2.x supports only Scala 2.11.

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"

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, just add a System.setProperty("macwire.debug", "") line to your build file.

Scala.js

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

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?

Activators

There are two Typesafe Activators which can help you to get started with Scala, Dependency Injection and Macwire:

Migrating from 1.x

  • changed how code is split across modules. You'll need to depend on util to get tagging & Wired, and proxy to get interceptors and scopes
  • tagging moved to a separate package. If you use tagging, you'll need to import com.softwaremill.tagging._
  • removed wireImplicit
  • implicit parameters aren't handled by wire at all (they used to be subject to the same lookup procedure as normal parameters + implicit lookup)

Play 2.4.x

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:

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