Application.scala

package controllers

import play.api._
import play.api.mvc._
import org.opencv.core.Mat
import org.opencv.core.CvType

import play.api.Play.current

object Application extends Controller {

  System.load(Play.getFile("./lib/libopencv_java247.so").getPath)

  def getField(obj: Object, clazz: Class[_], fieldName: String) = {
    val field = clazz.getDeclaredField(fieldName)
    field.setAccessible(true)
    field.get(obj)
  }

  def p(cl: ClassLoader): Unit = {

    println(s" --- " + "%-50s".format(cl.getClass) + " [" + cl.toString.take(50) + "]")

    val nativeLibraries = getField(cl, classOf[ClassLoader], "nativeLibraries").asInstanceOf[java.util.Vector[Object]]

    for (i <- 0 until nativeLibraries.size) {
      val lib = nativeLibraries.get(i)
      println(getField(lib, lib.getClass, "name"))
    }

    val parent = cl.getParent
    if (parent != null) {
      p(parent)
    }
  }

  def index = Action {
    p(this.getClass.getClassLoader)

    val m  = Mat.eye(3, 3, CvType.CV_8UC1)
    Ok("m = " + m.dump())
  }
}

btrace output (dev mode)

findNative: [null, Java_sun_nio_ch_ServerSocketChannelImpl_accept0, ]
 --> 36488467888
findNative: [null, Java_sun_nio_ch_FileDispatcherImpl_read0, ]
 --> 36488462208
findNative: [play.PlayRun$$anonfun$9$$anon$1@233e9780, Java_org_opencv_core_Mat_n_1eye, ]
 --> 0
findNative: [play.PlayRun$$anonfun$9$$anon$1@233e9780, Java_org_opencv_core_Mat_n_1eye__III, ]
 --> 0
findNative: [play.PlayRun$$anonfun$9$$anon$1@233e9780, Java_org_opencv_core_Mat_n_1eye, ]
 --> 0
findNative: [play.PlayRun$$anonfun$9$$anon$1@233e9780, Java_org_opencv_core_Mat_n_1eye__III, ]
 --> 0
findNative: [null, Java_sun_nio_ch_FileDispatcherImpl_write0, ]
 --> 36488461952
findNative: [null, Java_sun_nio_ch_FileDispatcherImpl_close0, ]
 --> 36488462384

Probe.java

import com.sun.btrace.AnyType;
import com.sun.btrace.annotations.*;

import static com.sun.btrace.BTraceUtils.*;

@BTrace public class Probe {

@OnMethod(clazz="java.lang.ClassLoader", method="findNative", location = @Location(Kind.RETURN))
  public static void printArgs(AnyType[] args, @Return long retVal) {
    print("findNative: ");
    printArray(args);
    print(" --> ");
    print(retVal);
    println();
  }
}

sbt output (dev mode)

[info] play - Application started (Dev)
 --- class play.PlayRun$$anonfun$10$$anon$2             [ReloadableClassLoader(v1){file:/usr/home/alex/play]
/usr/home/alex/playground/opencv-play-run/lib/libopencv_java247.so
 --- class play.sbtplugin.run.AssetsClassLoader         [play.sbtplugin.run.AssetsClassLoader@26d1bc87]
 --- class play.PlayRun$$anonfun$9$$anon$1              [PlayDependencyClassLoader{file:/usr/home/alex/play]
 --- class play.core.classloader.DelegatingClassLoader  [DelegatingClassLoader, using parent: Common ClassL]
 --- class play.PlayCommands$$anonfun$19$$anon$1        [Common ClassLoader: file:/usr/home/alex/.ivy2/cach]
[error] application - 

! @6jodkcj6a - Internal server error, for (GET) [/] ->

play.api.Application$$anon$1: Execution exception[[RuntimeException: java.lang.UnsatisfiedLinkError: org.opencv.core.Mat.n_eye(III)J]]
	at play.api.Application$class.handleError(Application.scala:296) ~[play_2.11-2.3.4.jar:2.3.4]
	at play.api.DefaultApplication.handleError(Application.scala:402) [play_2.11-2.3.4.jar:2.3.4]
	at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$14$$anonfun$apply$1.applyOrElse(PlayDefaultUpstreamHandler.scala:205) [play_2.11-2.3.4.jar:2.3.4]
	at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$14$$anonfun$apply$1.applyOrElse(PlayDefaultUpstreamHandler.scala:202) [play_2.11-2.3.4.jar:2.3.4]
	at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:36) [scala-library-2.11.1.jar:na]
Caused by: java.lang.RuntimeException: java.lang.UnsatisfiedLinkError: org.opencv.core.Mat.n_eye(III)J
	at play.api.mvc.ActionBuilder$$anon$1.apply(Action.scala:523) ~[play_2.11-2.3.4.jar:2.3.4]
	at play.api.mvc.Action$$anonfun$apply$1$$anonfun$apply$4$$anonfun$apply$5.apply(Action.scala:130) ~[play_2.11-2.3.4.jar:2.3.4]
	at play.api.mvc.Action$$anonfun$apply$1$$anonfun$apply$4$$anonfun$apply$5.apply(Action.scala:130) ~[play_2.11-2.3.4.jar:2.3.4]
	at play.utils.Threads$.withContextClassLoader(Threads.scala:21) ~[play_2.11-2.3.4.jar:2.3.4]
	at play.api.mvc.Action$$anonfun$apply$1$$anonfun$apply$4.apply(Action.scala:129) ~[play_2.11-2.3.4.jar:2.3.4]
Caused by: java.lang.UnsatisfiedLinkError: org.opencv.core.Mat.n_eye(III)J
	at org.opencv.core.Mat.n_eye(Native Method) ~[opencv-247.jar:na]
	at org.opencv.core.Mat.eye(Mat.java:1467) ~[opencv-247.jar:na]
	at controllers.Application$$anonfun$index$1.apply(Application.scala:40) ~[classes/:na]
	at controllers.Application$$anonfun$index$1.apply(Application.scala:37) ~[classes/:na]
	at play.api.mvc.ActionBuilder$$anonfun$apply$17.apply(Action.scala:464) ~[play_2.11-2.3.4.jar:2.3.4]