Scala | Annotation

Annotations in Scala are used to attach metadata to definitions, similar to Java. This metadata can be read at compile time and run-time, and can influence the behavior of the compiler.

Scala supports both its own annotations and Java annotations.

Defining an Annotation:

Scala annotations are defined by creating a class that extends the scala.annotation.Annotation trait. However, for practicality, it's more common to use Java-style annotations (using abstract class).

For example, here's a simple annotation that marks a method as deprecated:

import scala.annotation.StaticAnnotation

class deprecatedMethod(message: String) extends StaticAnnotation

Using an Annotation:

You can use the above-defined annotation on any method:

@deprecatedMethod("Use newMethod instead.")
def oldMethod() = println("This is the old method.")

Some Built-in Scala Annotations:

@deprecated: Used to mark methods that should no longer be used.

@deprecated("Use newer version of this method", "2.2.0")
def oldFunction() = "I'm old!"

@tailrec: Ensures that a function is tail-recursive. The compiler will throw an error if the method cannot be optimized into a loop.

import scala.annotation.tailrec

@tailrec
def factorialAcc(acc: Int, n: Int): Int = {
  if (n <= 1) acc
  else factorialAcc(n * acc, n - 1)
}

@transient: Marks a field to be skipped during serialization.
@volatile: Marks a field as volatile, meaning that writes to this variable are immediately made visible to other threads.
@unchecked: Used to suppress unchecked warnings.

Annotations and Compilation:

Some annotations influence the Scala compiler's behavior. For instance, the @inline annotation suggests to the compiler that it should attempt to inline the annotated method.

Java Annotations:

Because Scala interoperates with Java, it's possible to use Java annotations in Scala code. This is especially useful when you're building applications that use frameworks and libraries reliant on Java annotations (like Spring, Hibernate, etc.).

For instance, the Java @Override annotation can be used in Scala:

class Child extends Parent {
  @Override
  def someMethod(): String = "Child implementation"
}

Conclusion:

Annotations provide a way to add metadata to your Scala definitions. They can influence how the compiler operates, give hints about optimization, or be used to communicate with external libraries and frameworks.

Creating and Using Annotations in Scala:
You can create custom annotations by defining an annotation trait and using it on declarations.
```
class MyAnnotation extends scala.annotation.StaticAnnotation

@MyAnnotation
class MyClass
```
Built-in Annotations in Scala:
Scala provides several built-in annotations for various purposes, such as @deprecated, @transient, and @unchecked.
```
@deprecated("Use the newMethod instead", "2.0")
def oldMethod(): Unit = {
  // Deprecated implementation
}
```

Custom Annotations in Scala:

You can create custom annotations with parameters to convey additional information.

class Author(name: String, year: Int) extends scala.annotation.StaticAnnotation

@Author(name = "John Doe", year = 2022)
class MyBook

Annotations for Code Generation in Scala:

Annotations are often used for code generation by writing annotation processors that inspect annotated elements and generate code based on them.

class GenerateToString extends scala.annotation.StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro ToStringMacro.impl
}

object ToStringMacro {
  def impl(c: blackbox.Context)(annottees: c.Expr[Any]*): c.Expr[Any] = {
    // Code generation logic
  }
}

@GenerateToString
case class MyClass(name: String, age: Int)

Retrieving Annotations at Runtime in Scala:

Scala provides runtime reflection (scala.reflect.runtime.universe) to inspect annotations at runtime.

import scala.reflect.runtime.universe._

@Author(name = "Jane Doe", year = 2022)
class MyBook

val annotations: List[Annotation] = typeOf[MyBook].typeSymbol.annotations
annotations.foreach(println)

Annotation Processors in Scala:

Annotation processors are components that analyze annotated elements during compilation and generate code or perform other tasks.

class MyAnnotationProcessor extends scala.tools.nsc.plugins.Plugin {
  val global: Global = _
  import global._

  override def processOptions(options: List[String], error: String => Unit): Unit = {
    // Process options
  }

  override val optionsHelp: Option[String] = Some("Custom options for the annotation processor")

  override val description: String = "My Annotation Processor"

  override def newPhase(prev: Phase): Phase = new StdPhase(prev) {
    override def apply(unit: CompilationUnit): Unit = {
      // Process annotated elements
    }
  }
}

You can then enable the annotation processor in the compiler settings.

Annotation-Driven Development in Scala:
Annotation-driven development involves using annotations to guide the behavior of tools, frameworks, or other components during development.
```
@RequestMapping(path = "/api")
class MyController {
  @GetMapping(path = "/hello")
  def sayHello(): String = "Hello, World!"
}
```
Here, annotations like @RequestMapping and @GetMapping guide a web framework to map requests to specific methods.