Using Spring Boot on Scala

Assumption

This example uses a project generated/seeded via Activator using the “minimal-scala” template. However, any project using SBT will probably work just as well.

Add Spring Boot to dependencies

Add Spring Boot dependencies to build.sbt:

libraryDependencies += "org.springframework.boot" % "spring-boot-starter" % "1.3.2.RELEASE"
libraryDependencies += "org.springframework.boot" % "spring-boot-starter-test" % "1.3.2.RELEASE"

Create a SpringBootApplication and run it

package com.example

@SpringBootApplication
class HelloApp {

}

object Hello {
 def main(args: Array[String]): Unit = {
   SpringApplication run classOf[HelloApp]
 }
}

This class serves as a package “anchor” for component scanning. The singleton exports an equivalence of a “public static void main(String args[])” entry point.

At this point, “sbt run” will bring up the app, but it will quit soon afterward.

Time to decide if this will be a Web app or a Command line app.

Branch to Web or Command line (console)

Web app

To create a Web app, add the spring-boot-starter-web dependency. Add this to build.sbt:

libraryDependencies += "org.springframework.boot" % "spring-boot-starter-web" % "1.3.2.RELEASE"

That’s it! Now running “sbt run” will bring up the Web app listening on port 8080.

Command line app

To create a command line app (or to add a command line runner to a Web app such as above), modify the main app class:

@SpringBootApplication
class HelloApp extends CommandLineRunner {
  override def run(args: String*): Unit = {
    println("Hello from Command line runner!")
  }
}

Extending CommandLineRunner in Scala is the same as implementing CommandLineRunner in Java. So just implement the run method.

Try Autowiring something

Assuming the app class is com.example.HelloApp. The component scanning starts at com.example. So create a service class under that package (or a subpackage thereof). I will create one under com.example.services:

package com.example.services

import org.springframework.stereotype.Service

@Service
class HelloService {
  def hello(): String = {
    "Hello from HelloService!"
  }
}

Autowire into a bean using the service (HelloApp in this example):

@SpringBootApplication
class HelloApp extends CommandLineRunner {

  @Autowired
  var helloSvc: HelloService = null

  override def run(args: String*): Unit = {
    println(helloSvc.hello())
  }
}

Configuration properties

Configuration properties can go into src/main/resources/application.properties, same as in Java world:

titlemsg=Hello App

Accessing properties is also pretty much the same (e.g. shown here using @Value):

@SpringBootApplication
class HelloApp extends CommandLineRunner {
  ...

  @Value("${titlemsg}")
  var titleMsg: String = null

  override def run(args: String*): Unit = {
    println(titleMsg)
  }
}

There are several implementations of the @Value annotation available, be sure to use the one from Spring:

org.springframework.beans.factory.annotation.Value

Logging

And logging. Adding src/main/resources/logback.xml will allow tweaking of logging behavior:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <include resource="org/springframework/boot/logging/logback/base.xml"/>
    <logger name="com.example" level="DEBUG"/>
</configuration>

I use Log4j, so here’s how to add a logger into the class to log with:

class HelloApp extends CommandLineRunner {

  val logger = Logger getLogger classOf[HelloApp]

  override def run(args: String*): Unit = {
    logger info "Hello using logger"
  }
}