Targeting annotations to Javabean accessors in Scala

On a recent project I had written a simple scheduling service. I wanted to expose some properties and methods to JMX so I could view the configuration at runtime and so I could perform some basic operations, like starting the service.

Spring provides a convenient way of exposing your classes to JMX using annotations, eliminating a ton of boilerplate. My initial stab at decorating the service for JMX looked like:

@ManagedResource
@Component("schedulerService")
class SchedulerServiceImpl extends SchedulerService {

  @Value("${scheduler.enabled:false}")
  @ManagedAttribute @BeanProperty
  val enabled: Boolean = false

  @ManagedAttribute @BeanProperty
  var started: Boolean = _

  @ManagedOperation
  def start(): String = {
    //...
  }
  //...
}

@ManagedAttribute has to be put on Javabean getters and setters so I added @BeanProperty to the properties I wanted to expose. Actually I only wanted to expose the getter, but initially I wasn’t worried about that part.

I fired up jconsole and saw that the start() method was exposed but the started and enabled properties were not. The usual tool for debugging Scala-Java interop, javap, doesn’t display information on annotations, so I used jclasslib bytecode viewer. There I saw that the annotation had actually been set on the private field that Scala generated, not the Javabean setter.

After some asking around, Iulian Dragos told me about the meta-annotations in the scala.annotation.target package. These let you target annotations at specific accessors.

This allows you to create Javabean accessor-targeted annotations like this:

  
@(ManagedAttribute @beanGetter) @BeanProperty
val enabled: Boolean = false

That is cool but pretty verbose. Fortunately you can clean it up with a type alias:

object JmxConfig {
  type ManagedGetter = ManagedAttribute @beanGetter
  type ManagedSetter = ManagedAttribute @beanSetter
}

So the end result can look like:

import JmxConfig._

@ManagedResource
@Component("schedulerService")
class SchedulerServiceImpl extends SchedulerService {

  @Value("${scheduler.enabled:false}")
  @ManagedGetter @BeanProperty
  val enabled: Boolean = false

  @ManagedGetter @BeanProperty
  var started: Boolean = _

  //....
}