Akka http with json4s

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Akka Http uses the spray-json library for JSON support. But a few days back while working on a project which was using the json4s library for marshalling/unmarshalling, I got stuck during parsing of JSON request and extracting that parsed values into my target Scala data model.

Common transformations are summarized in the following picture.

Description :

I have a sample Scala web service created using Akka HTTP for performing CRUD operations which is not using any database for the time being as this blog focuses on json4s and Akka HTTP.
The user input contains objects which I want to extract cleanly. To keep things simpler, I have a Student case class which contains two fields roll number and name of the student. CRUD operations are performed on this Student data model.

Dependencies :

I am using SBT, so here is the relevant code from build.sbt

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	version := "0.1"

	scalaVersion := "2.12.6"

	libraryDependencies ++= Seq("com.typesafe.akka" %% "akka-http" % "10.0.11",
	"org.json4s" %% "json4s-native" % "3.2.11",
	"org.scalatest" %% "scalatest" % "3.0.1" % Test,
	"org.mockito" % "mockito-core" % "2.11.0" % Test,
	"com.typesafe.akka" %% "akka-http-testkit" % "10.1.1")

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build.sbt

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Scala data model as discussed above :

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	import scala.collection.mutable.ListBuffer

	case class Student(rollNum: Int, name: String)

	trait Students {

	val students = ListBuffer(Student(1, "Ayush"), Student(2, "deepankar"))

	def fetchStudents: ListBuffer[Student] = students

	def addStudent(student: Student): ListBuffer[Student] = {
	student +: students
	}

	def updateStudent(student: Student): ListBuffer[Student] = {
	val oldRecord = students.filter(_.rollNum == student.rollNum)
	students —= oldRecord
	student +: students
	}

	def deleteStudent(rollNum: Int): ListBuffer[Student] = {
	val deleteRecord = students.filter(_.rollNum == rollNum)
	students —= deleteRecord
	}

	}

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Students.scala

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To get Akka HTTP and json4s to play nicely together, I am going to implement a custom trait that defines some helper methods for marshalling/unmarshalling to and from JSON along with necessary implicit values.

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	import org.json4s.{DefaultFormats, Formats, JNothing, JValue}
	import org.json4s.native.JsonMethods.{parse => jParser}
	import org.json4s.native.Serialization
	import org.json4s.native.Serialization.{write => jWrite}

	trait JsonHelper extends {

	val EMPTY_STRING = ""


	implicit val serialization: Serialization.type = Serialization

	implicit val formats: Formats = DefaultFormats

	/*
	takes any type of value and serializes it to string
	eg: val student = Student(1,"deepankar")
	write(student) will serialize it to {"rollNum":1,"name":"deepankar"}
	*/
	def write[T <: AnyRef](value: T): String = jWrite(value)

	/*
	Any valid json can be parsed into internal AST(Abstract Syntax Tree) format.
	eg : If this is our json string coming from a post request
	val jsonString = {"rollNum":1,"name":"deepankar"}
	parse(jsonString) will parse it to something like
	JObject(List((rollNum,JInt(1)), (name,JString(deepankar))))
	*/
	protected def parse(value: String): JValue = jParser(value)

	/*
	extract method is used after parsing the json string, as parse method returns JValue so we
	can map this JValue to our target scala data model
	eg : val jsonString = {"rollNum":1,"name":"deepankar"}
	val parseString = parse(jsonString)
	extract(parseString) will return Student(?1,"deepankar")
	*/

	implicit protected def extractOrEmptyString(json: JValue): String = {
	json match {
	case JNothing => EMPTY_STRING
	case data => data.extract[String]
	}
	}

	}

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JsonHelper.scala

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Here’s our sample web server file that handles all CRUD related requests (GET, POST, PUT, DELETE) and responds with JSON. This file extends JsonHelper defined above and also shows the output generated by these helper methods to make your understanding even better.

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	import akka.actor.ActorSystem
	import akka.http.scaladsl.Http
	import akka.http.scaladsl.model._
	import akka.http.scaladsl.server.Directives._
	import akka.stream.ActorMaterializer

	import scala.concurrent.ExecutionContextExecutor

	//the usual config code required for an Akka Http server, mixing in our JsonHelper trait too
	object Routes extends App with JsonHelper {

	implicit val system: ActorSystem = ActorSystem("my-system")
	implicit val materializer: ActorMaterializer = ActorMaterializer()
	// needed for the future flatMap/onComplete in the end
	implicit val executionContext: ExecutionContextExecutor = system.dispatcher

	val studentsClass = new Students

	val route =
	pathPrefix("demo") {
	get {
	path("fetchAllStudents") {
	complete {
	val addedStudents = studentsClass.fetchStudents
	val jsonResponse = write(addedStudents)

	/* json for list of Students
	[{"rollNum":1,"name":"Ayush"},{"rollNum":2,"name":"deepankar"}]
	*/
	HttpResponse(entity = HttpEntity(ContentTypes.`application/json`, jsonResponse))
	}
	}
	} ~
	post {
	path("addStudent") {
	entity(as[String]) { studentJson => {
	complete {
	val studentRecord = parse(studentJson).extract[studentsClass.Student]

	/*
	Student(3,randhir)
	*/
	val updatedList = studentsClass.addStudent(studentRecord)
	val jsonResponse = write(updatedList)
	HttpResponse(entity = HttpEntity(ContentTypes.`application/json`, jsonResponse))
	}
	}
	}
	}
	} ~
	put {

	//for updating student name student roll number has to be same as entered previously
	path("updateStudent") {
	entity(as[String]) { studentJson => {
	complete {
	val studentRecord = parse(studentJson).extract[studentsClass.Student]
	val updatedList = studentsClass.updateStudent(studentRecord)
	val jsonResponse = write(updatedList)
	HttpResponse(entity = HttpEntity(ContentTypes.`application/json`, jsonResponse))
	}
	}
	}
	}
	} ~
	delete {
	path("deleteStudent" / IntNumber) { rollNum =>
	complete {
	val deleteRecord = studentsClass.deleteStudent(rollNum)
	val jsonResponse = write(deleteRecord)
	HttpResponse(entity = HttpEntity(ContentTypes.`application/json`, jsonResponse))
	}
	}
	}
	}

	val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)
	println(s"Server online at http://localhost:8080")
	}