Spring Boot with Scala

A while back I had tried out a small Spring Boot based sample with Scala as the language and found that the combination works out quite nicely - no big surprises there actually as Scala programs ultimately run in the JVM. I have now updated the sample with the latest version of Spring Boot and some of the supporting libraries.

To very quickly revisit the sample, it is a very simple web application with a UI to manage a "Hotel" domain object managed via JPA, represented in scala the following way:

import javax.persistence.Id
import javax.persistence.GeneratedValue
import java.lang.Long
import javax.persistence.Entity
import scala.beans.BeanProperty
import org.hibernate.validator.constraints.NotEmpty

@Entity
class Hotel {

  @Id
  @GeneratedValue
  @BeanProperty
  var id: Long = _

  @BeanProperty
  @NotEmpty
  var name: String = _

  @BeanProperty
  @NotEmpty
  var address: String = _

  @BeanProperty
  @NotEmpty
  var zip: String = _
}

JPA annotations carry over quite well, one wrinkle may be the additional @BeanProperty annotation though, this is required for JPA implementations as this makes the scala compiler generate the normal Java Beans type getters and setters instead of the scala default getters and setters which don't follow the Java Bean conventions.

Spring Data makes it ridiculously simple to manage this domain type, all it requires is a marker interface and it generates a runtime implementation:

import org.springframework.data.repository.CrudRepository
import mvctest.domain.Hotel
import java.lang.Long

trait HotelRepository extends CrudRepository[Hotel, Long]

Now I have a toolkit available for managing the Hotel domain:

//save or update a hotel
hotelRepository.save(hotel)

//find one hotel
hotelRepository.findOne(id)

//find all hotels
val hotels = hotelRepository.findAll()

//delete a hotel
hotelRepository.delete(id)

And finally a controller to manage the UI flow with this repository:

@Controller
@RequestMapping(Array("/hotels"))
class HotelController @Autowired()(private val hotelRepository: HotelRepository) {

  @RequestMapping(method = Array(RequestMethod.GET))
  def list(model: Model) = {
    val hotels = hotelRepository.findAll()
    model.addAttribute("hotels", hotels)
    "hotels/list"
  }

  @RequestMapping(Array("/edit/{id}"))
  def edit(@PathVariable("id") id: Long, model: Model) = {
    model.addAttribute("hotel", hotelRepository.findOne(id))
    "hotels/edit"
  }

  @RequestMapping(method = Array(RequestMethod.GET), params = Array("form"))
  def createForm(model: Model) = {
    model.addAttribute("hotel", new Hotel())
    "hotels/create"
  }

  @RequestMapping(method = Array(RequestMethod.POST))
  def create(@Valid hotel: Hotel, bindingResult: BindingResult) = {
    if (bindingResult.hasErrors()) {
      "hotels/create"
    } else {
      hotelRepository.save(hotel)
      "redirect:/hotels"
    }
  }

  @RequestMapping(value = Array("/update"), method = Array(RequestMethod.POST))
  def update(@Valid hotel: Hotel, bindingResult: BindingResult) = {
    if (bindingResult.hasErrors()) {
      "hotels/edit"
    } else {
      hotelRepository.save(hotel)
      "redirect:/hotels"
    }
  }

  @RequestMapping(value = Array("/delete/{id}"))
  def delete(@PathVariable("id") id: Long) = {
    hotelRepository.delete(id)
    "redirect:/hotels"
  }
}

There are some wrinkles here too but should mostly make sense, the way the repository is autowired is a little non-intuitive and the way an explicit Array type has to be provided for request mapping paths and methods may be confusing.

Beyond these small concerns the code just works, do play with it, I would love any feedback on ways to improve this sample. Here is the git location of this sample - https://github.com/bijukunjummen/spring-boot-scala-web

Marble Diagrams - Rxjava operators

I love the use of Marble Diagrams for representing the different ReactiveX operations. It really clarifies the behavior of some complex operations. RxJava uses these diagrams in its Javadocs and provides the following legend to explain Marble diagrams:

Keeping the marble diagrams in mind, here is a sample test for flatMap operation, written using the Rx-Scala library:

val colors = Observable.just("Red", "Green", "Blue")

val f: String => Observable[String]  = (x: String) => Observable.interval(x.length() seconds).map(_ => x).take(2)

val obs: Observable[String] = colors.flatMap(f)

assert(obs.toBlocking.toList == List("Red", "Blue", "Green", "Red", "Blue", "Green"))

and the marble diagram for the operation:

Given this, the flow in the test should become very clear - we start with an Observable which emits three values "Red", "Green", "Blue", the function transforms an element to another Observable, calling the flatMap now makes this mapping and flattens the result to an Observable.

Another a little more complex variation of the flatMap operation has the following signature in scala:

def flatMap[R](onNext: (T) ⇒ Observable[R], onError: (Throwable) ⇒ Observable[R], onCompleted: () ⇒ Observable[R]): Observable[R]

or the following in Java:

public final <R> Observable<R> flatMap(Func1<? super T,? extends Observable<? extends R>> onNext,
                        Func1<? super java.lang.Throwable,? extends Observable<? extends R>> onError,
                        Func0<? extends Observable<? extends R>> onCompleted)

again best explained using its Marble diagram:

Here is a test for this variation of flatMap:

val colors = Observable.just("Red", "Green", "Blue")

val f: String => Observable[String]  = (x: String) => Observable.just(x, x)

val d = () => Observable.just("done")

val e: Throwable => Observable[String] = e => Observable.just(e.getMessage)

val obs: Observable[String] = colors.flatMap(f, e, d)

assert(obs.toBlocking.toList == List("Red", "Red", "Green", "Green", "Blue", "Blue", "done"))

In conclusion, I really appreciate the effort that goes behind creating these marble diagrams by the authors of ReactiveX and feel that they clarify the purpose of the operations neatly.