Introduction
Guide to DI in Scala (external link)
How wiring works
Factories
Limitations
lazy val vs. val
Scopes
Accessing wired instances dynamically
Interceptors
Wiring with implicit values
Qualifiers
Installation, using with SBT
Debugging
Scala.js
Future development - vote!

MacWire

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 that should be wired should be organized in "modules", which can be Scala traits, classes or objects. Multiple modules can be combined using inheritance; values from the inherited 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.MacwireMacros._

    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
}

Instead of importing the wire method in each module you can also extend com.softwaremill.macwire.Macwire trait.

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 which is a subtype of the parameter's type in the enclosing method and trait/class/object:

first it tries to find a unique value declared as the argument of enclosing methods and anonymous functions.
then it tries to find a unique value declared in the enclosing type
then it tries to find a unique value in parent types (traits/classes)
if the parameter is marked as implicit, additionally the usual implicit lookup mechanism is used

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 method/function's arguments list, enclosing type or its parents.
parameter is marked as implicit and both implicit lookup and searching in enclosing/parent types 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).

Unlike wiring in other places, if multiple values of the desired type are found, they are additionally filtered by-name. In general this could give unpredictable results, but should be safe in the scope of a method.

For example:

class DatabaseAccess()
class TaxDeductionLibrary(databaseAccess: DatabaseAccess)
class TaxCalculator(taxBase: Double, taxFreeAmount: Double, taxDeductionLibrary: TaxDeductionLibrary)

trait TaxModule {
    import com.softwaremill.macwire.MacwireMacros._

    lazy val theDatabaseAccess      = wire[DatabaseAccess]
    lazy val theTaxDeductionLibrary = wire[TaxDeductionLibrary]
    def taxCalculator(taxBase: Double, taxFreeAmount: Double) = wire[TaxCalculator]
    // or: lazy val taxCalculator = (taxBase: Double, taxFreeAmount: Double) => wire[TaxCalculator]
}

will generate:

trait TaxModule {
    lazy val theDatabaseAccess      = new DatabaseAccess()
    lazy val theTaxDeductionLibrary = new TaxDeductionLibrary(theDatabaseAccess)
    def taxCalculator(taxBase: Double, taxFreeAmount: Double) =
       new TaxCalculator(taxBase, taxFreeAmount, theTaxDeductionLibrary)
}

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.

`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.

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 runtime 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 runtime 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, 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 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 MacwireMacros._
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 runtime 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.

Wiring with implicit values

It is also possible to wire an object taking into account implicit values and values in scope using wireImplicit. This makes it possible to wire with objects that are defined inside a method body, as default implicit values and others which aren't part of the usual scope where MacWire looks.

wireImplicit for each constructor parameter does an implicit lookup and looks in the current scope. If multiple values are found, an error is reported.

For example:

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

object SecurityFilter {
    implicit val default = wire[SecurityFilter]
}

object Implicits {
    implicit lazy val theDatabaseAccess = wire[DatabaseAccess]
}

trait UserModule {
    def run() {
        import com.softwaremill.macwire.MacwireMacros._
        import Implicits._

        lazy val theUserFinder = wireImplicit[UserFinder]
    }
}

Note that the signature of UserFinder is unaffected by the way the wiring is done, that is its parameter list doesn't have to be marked as implicit.

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, 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](via https://gist.github.com/milessabin/89c9b47a91017973a35f).

To bring the tagging into scope, import com.softwaremill.macwire._, or the more specific com.softwaremill.macwire.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]

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" % "0.8.0"

libraryDependencies += "com.softwaremill.macwire" %% "runtime" % "0.8.0"

To use the snapshot version:

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

libraryDependencies += "com.softwaremill.macwire" %% "macros" % "0.8.1-SNAPSHOT"

libraryDependencies += "com.softwaremill.macwire" %% "runtime" % "0.8.1-SNAPSHOT"

MacWire works with Scala 2.11. The last release for Scala 2.10 is 0.7.3.

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?

mkubala/macwire

Table of Contents

MacWire

How wiring works

Factories

Limitations

`lazy val` vs. `val`

Scopes

Accessing wired instances dynamically

Interceptors

Wiring with implicit values

Qualifiers

Installation, using with SBT

Debugging

Scala.js

Future development - vote!

mkubala/macwire

Table of Contents

MacWire

How wiring works

Factories

Limitations

lazy val vs. val

Scopes

Accessing wired instances dynamically

Interceptors

Wiring with implicit values

Qualifiers

Installation, using with SBT

Debugging

Scala.js

Future development - vote!

`lazy val` vs. `val`