This is a custom name service for the JVM so you can add your own entries during testing of a web service/app.

CustomNameService.scala

package misc

import java.net.{InetAddress, UnknownHostException}
import java.util.concurrent.ConcurrentHashMap

import sun.net.spi.nameservice.{NameService, NameServiceDescriptor}

class CustomNameService extends NameService {

  override def lookupAllHostAddr(s: String): Array[InetAddress] =
    Option(CustomNameService.overrides.get(s))
      .map(a => Array(a))
      .getOrElse(throw new UnknownHostException)

  def getHostByAddr(bytes: Array[Byte]) = throw new UnknownHostException
}

object CustomNameService {
  val overrides = new ConcurrentHashMap[String, InetAddress]()
}

class CustomNameServiceDescriptor extends NameServiceDescriptor {
  val getType = "dns"
  val getProviderName = "custom"
  def createNameService(): NameService = {
    println("Creating new CustomNameService")
    new CustomNameService
  }
}

sun.net.spi.nameservice.NameServiceDescriptor

misc.CustomNameServiceDescriptor

Test.scala

object Test extends App {
  CustomNameService.overrides.put("whatever", InetAddress.getByAddress("whatever", InetAddress.getLoopbackAddress.getAddress))
  println(InetAddress.getByName("whatever"))
  println(InetAddress.getByName("www.google.com"))
}

This is activated with:

-Dsun.net.spi.nameservice.provider.1=dns,custom
-Dsun.net.spi.nameservice.provider.2=default

The file sun.net.spi.nameservice.NameServiceDescriptor must be located in META-INF/services

FYI: it will stop to work in JDK9