Updated April 4, 2023
Introduction to Scala Dependency Injection
Dependency Injection is the software design pattern used to simplify our logic. Dependency Injection separates our implemented methods, and interfaces provide us benefits from debugging our code easily. Also, with the help of dependency injection, we can replace our object type according to our needs. Like in Java, we have spring IOC for this, but which we can manage dependency injection in a better way. But in the case of scala, we have some options available to work with dependency injection. We can use spring in scala also because it is work on JVM.
Syntax
In scala, we have two types of dependency injection which are as follows;
1) Runtime dependency Injection: For this dependency injection, we can use @Inject annotation to create our object; see below;
e.g. >>
import javax.inject._
class MyDemo @Inject() {
// your logic
}2) Compile-time Dependency Injection: We will use Application Loader to declare it for compile-time dependency injection. Please follow the syntax below for more understanding.
e.g. >>
class DemoCTApplicationLoader extends ApplicationLoader {
override def load(context: Context): Application
= new ApplicationComponents(context).application
}How Dependency Injection work in Scala
Dependency injection provides us with loose coupling among the services and their implementation classes. It also separates our business logic into the code. By using this, we do not need to create all dependent services inside the same service; instead, we can define our dependent services independently and inject them separately whenever they are needed. This will also give us more understanding of code, make it easy to debug, make it readable and reusable, and remove the tight coupling between classes and interfaces.
We can inject the dependency using two things;
1) via the constructor: through parameter
2) via setter or getter: through parameter
Scala provides us two types of dependency injection;
1) Compile-time dependency Injection: To work with compile-time dependency injection, we have to use the application loader for this. We do not need to create a separate module or add any other configuration to our program. All the dependencies will get managed by the application loader itself. If you want to make your program and configuration easier than for this, we have Deadbolt components that can be available by the DeadboltComponent trait. The list of the component provided by the Deadbolt are mentioned as follows;
- ecContextProvider: This component provides us context, which helps in the concurrent operation of the program.
- patternCache: This component helps in the cache of the regular expression. This component also provides us with the default implementation, which is recommended to use. To use this component, we have to define this in our application loader.
- actionBuilders: These components are used to create and build actions.
- scalaAnalyzer: This component is used to constraint logic.
- handlers: This is the implementation of the handlers Cache.
- configuration: This component is used to provide the application configuration.
- templateFailureListenerProvider: This component is used for error listening. It got revoked when two render the templates.
- deadboltActions: This component is used to create actions.
- viewSupport: This component is used for the implementation of constraints.
For Dependency Injection, scala uses the play framework. This play framework supports both the runtime and compiles time dependency Injection.
2) Runtime Dependency Injection: All the things that happened at runtimes like wiring, dependency grape creation, and validation is also at runtime. Play framework support this dependency injection. Suppose you have injected some dependency for a service, but you will not be able to see the error while writing the code or at compile time until we run the application. This is happening only in the case if it is not able to find the dependency for an injected class.
We can have a look at the example to understand it better;
import javax.inject._
class Deomo @Inject() (sd: ServiceOneDemo) {
// code logic here ...
}To inject any runtime dependency, we use @inject followed by the service or a class name that we want to inject and use into our code for future reference. But at compile-time, we won’t see an error if it is unavailable to find the service. That can only be tested when we run it. In java spring, we also use this annotation, but now we majorly use @Autowire for injection.
But we should always keep in mind that the inject keyword always followed by the () brackets and always followed by the class name. This is because, in scala, they always recommend using constructor injection because it is always more testable, clear, and concise.
Also, we can bind any class by using its constructor. For this, we can use @Inject annotation on the class constructor; this is called just in time-binding. So we do not need to make any explicit configuration for this.
Example of Scala Dependency Injection
In this example, we are using compile-time dependency injection to load the objects.
Code:
import play.api._
import play.api.ApplicationLoader.Context
import play.api.routing.Router
import play.filters.HttpFiltersComponents
class MyDemoApplicationCompileLoader extends ApplicationLoader {
def load(contextObject: Context) = {
new MyDemoComponents(contextObject).application
}
}
class MyDemoComponents(conObject: Context) extends BuiltInComponentsFromContext(conObject) with HttpFiltersComponents {
lazy val router1 = Router.empty
println("application loaded successfully. !!!")
}Output:
Advantages of using Dependency Injection in Scala?
Scala Dependency Injection provides us with various advantages, which are mentioned below.
- Dependency Injection allows us to lose coupling among the module that we create. It helps us remove the dependency between the classes and the interface.
- It differentiates the business logic, making our code clearer, readable, and untestable. Also, it makes it easier to debug.
- With the use of dependency injection, we can also switch our object type at run time.
- We can also provide the default injection of objects by introducing @inject annotation on top of the constructor of the class.
Conclusion
Dependency Injection separates our code and removes unwanted dependency; it does not support tight coupling among the classes and interfaces.We have compile-time and runtime dependency available in scala. For compile-time, we need to use application Loader, and for runtime, we have a play framework with the support of various annotations available.
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