diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/ActorRefResolver.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/ActorRefResolver.scala
index 4d91187b51..fffcb13f83 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/ActorRefResolver.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/ActorRefResolver.scala
@@ -5,21 +5,25 @@
 package akka.actor.typed
 
 import akka.actor.ExtendedActorSystem
+import akka.annotation.DoNotInherit
+import akka.annotation.InternalApi
 
 object ActorRefResolver extends ExtensionId[ActorRefResolver] {
   def get(system: ActorSystem[_]): ActorRefResolver = apply(system)
 
   override def createExtension(system: ActorSystem[_]): ActorRefResolver =
-    new ActorRefResolver(system)
+    new ActorRefResolverImpl(system)
 }
 
 /**
  * Serialization and deserialization of `ActorRef`.
+ *
+ * This class is not intended for user extension other than for test purposes (e.g.
+ * stub implementation). More methods may be added in the future and that may break
+ * such implementations.
  */
-class ActorRefResolver(system: ActorSystem[_]) extends Extension {
-  import akka.actor.typed.scaladsl.adapter._
-
-  private val untypedSystem = system.toUntyped.asInstanceOf[ExtendedActorSystem]
+@DoNotInherit
+abstract class ActorRefResolver extends Extension {
 
   /**
    * Generate full String representation including the uid for the actor cell
@@ -28,13 +32,27 @@ class ActorRefResolver(system: ActorSystem[_]) extends Extension {
    * information. This representation should be used as serialized
    * representation.
    */
-  def toSerializationFormat[T](ref: ActorRef[T]): String =
-    ref.path.toSerializationFormatWithAddress(untypedSystem.provider.getDefaultAddress)
+  def toSerializationFormat[T](ref: ActorRef[T]): String
 
   /**
    * Deserialize an `ActorRef` in the [[#toSerializationFormat]].
    */
-  def resolveActorRef[T](serializedActorRef: String): ActorRef[T] =
+  def resolveActorRef[T](serializedActorRef: String): ActorRef[T]
+
+}
+
+/**
+ * INTERNAL API
+ */
+@InternalApi private[akka] class ActorRefResolverImpl(system: ActorSystem[_]) extends ActorRefResolver {
+  import akka.actor.typed.scaladsl.adapter._
+
+  private val untypedSystem = system.toUntyped.asInstanceOf[ExtendedActorSystem]
+
+  override def toSerializationFormat[T](ref: ActorRef[T]): String =
+    ref.path.toSerializationFormatWithAddress(untypedSystem.provider.getDefaultAddress)
+
+  override def resolveActorRef[T](serializedActorRef: String): ActorRef[T] =
     untypedSystem.provider.resolveActorRef(serializedActorRef)
 }
 
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/internal/ExtensionsImpl.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/internal/ExtensionsImpl.scala
index 092f3d34c1..d45d1d8fd8 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/internal/ExtensionsImpl.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/internal/ExtensionsImpl.scala
@@ -69,7 +69,7 @@ trait ExtensionsImpl extends Extensions { self: ActorSystem[_] ⇒
   final override def hasExtension(ext: ExtensionId[_ <: Extension]): Boolean = findExtension(ext) != null
 
   final override def extension[T <: Extension](ext: ExtensionId[T]): T = findExtension(ext) match {
-    case null ⇒ throw new IllegalArgumentException("Trying to get non-registered extension [" + ext + "]")
+    case null ⇒ throw new IllegalArgumentException(s"Trying to get non-registered extension [$ext]")
     case some ⇒ some.asInstanceOf[T]
   }
 
@@ -88,7 +88,7 @@ trait ExtensionsImpl extends Extensions { self: ActorSystem[_] ⇒
           case (_, extSetup: ExtensionSetup[_]) if extSetup.extId == ext ⇒ extSetup.createExtension(self)
         }.getOrElse(ext.createExtension(self))
         instance match {
-          case null ⇒ throw new IllegalStateException("Extension instance created as 'null' for extension [" + ext + "]")
+          case null ⇒ throw new IllegalStateException(s"Extension instance created as 'null' for extension [$ext]")
           case instance: T @unchecked ⇒
             // Replace our in process signal with the initialized extension
             extensions.replace(ext, inProcessOfRegistration, instance)
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/receptionist/Receptionist.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/receptionist/Receptionist.scala
index c4cba544e7..fb431be32c 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/receptionist/Receptionist.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/receptionist/Receptionist.scala
@@ -12,8 +12,22 @@ import scala.concurrent.duration._
 import scala.reflect.ClassTag
 
 import akka.actor.typed.ExtensionSetup
+import akka.annotation.InternalApi
 
-class Receptionist(system: ActorSystem[_]) extends Extension {
+/**
+ * This class is not intended for user extension other than for test purposes (e.g.
+ * stub implementation). More methods may be added in the future and that may break
+ * such implementations.
+ */
+@DoNotInherit
+abstract class Receptionist extends Extension {
+  def ref: ActorRef[Receptionist.Command]
+}
+
+/**
+ * INTERNAL API
+ */
+@InternalApi private[akka] class ReceptionistImpl(system: ActorSystem[_]) extends Receptionist {
 
   private def hasCluster: Boolean = {
     // FIXME: replace with better indicator that cluster is enabled
@@ -21,7 +35,7 @@ class Receptionist(system: ActorSystem[_]) extends Extension {
     provider == "akka.cluster.ClusterActorRefProvider"
   }
 
-  val ref: ActorRef[Receptionist.Command] = {
+  override val ref: ActorRef[Receptionist.Command] = {
     val provider: ReceptionistBehaviorProvider =
       if (hasCluster) {
         system.dynamicAccess
@@ -99,7 +113,7 @@ abstract class ServiceKey[T] extends AbstractServiceKey { key ⇒
  * The receptionist is easiest accessed through the system: [[ActorSystem.receptionist]]
  */
 object Receptionist extends ExtensionId[Receptionist] {
-  def createExtension(system: ActorSystem[_]): Receptionist = new Receptionist(system)
+  def createExtension(system: ActorSystem[_]): Receptionist = new ReceptionistImpl(system)
   def get(system: ActorSystem[_]): Receptionist = apply(system)
 
   /**
diff --git a/akka-actor/src/main/scala/akka/actor/ActorSystem.scala b/akka-actor/src/main/scala/akka/actor/ActorSystem.scala
index 30cef964fa..20b65221d7 100644
--- a/akka-actor/src/main/scala/akka/actor/ActorSystem.scala
+++ b/akka-actor/src/main/scala/akka/actor/ActorSystem.scala
@@ -911,7 +911,7 @@ private[akka] class ActorSystemImpl(
         extensions.putIfAbsent(ext, inProcessOfRegistration) match { // Signal that registration is in process
           case null ⇒ try { // Signal was successfully sent
             ext.createExtension(this) match { // Create and initialize the extension
-              case null ⇒ throw new IllegalStateException("Extension instance created as 'null' for extension [" + ext + "]")
+              case null ⇒ throw new IllegalStateException(s"Extension instance created as 'null' for extension [$ext]")
               case instance ⇒
                 extensions.replace(ext, inProcessOfRegistration, instance) //Replace our in process signal with the initialized extension
                 instance //Profit!
@@ -930,7 +930,7 @@ private[akka] class ActorSystemImpl(
   }
 
   def extension[T <: Extension](ext: ExtensionId[T]): T = findExtension(ext) match {
-    case null ⇒ throw new IllegalArgumentException("Trying to get non-registered extension [" + ext + "]")
+    case null ⇒ throw new IllegalArgumentException(s"Trying to get non-registered extension [$ext]")
     case some ⇒ some.asInstanceOf[T]
   }
 
diff --git a/akka-cluster-sharding-typed/src/main/scala/akka/cluster/sharding/typed/javadsl/ClusterSharding.scala b/akka-cluster-sharding-typed/src/main/scala/akka/cluster/sharding/typed/javadsl/ClusterSharding.scala
index db2530a460..7b0a458867 100644
--- a/akka-cluster-sharding-typed/src/main/scala/akka/cluster/sharding/typed/javadsl/ClusterSharding.scala
+++ b/akka-cluster-sharding-typed/src/main/scala/akka/cluster/sharding/typed/javadsl/ClusterSharding.scala
@@ -136,6 +136,9 @@ object ClusterSharding {
  * between reception of `Passivate` and termination of the entity. Such buffered messages
  * are thereafter delivered to a new incarnation of the entity.
  *
+ * This class is not intended for user extension other than for test purposes (e.g.
+ * stub implementation). More methods may be added in the future and that may break
+ * such implementations.
  */
 @DoNotInherit
 abstract class ClusterSharding {
diff --git a/akka-cluster-sharding-typed/src/main/scala/akka/cluster/sharding/typed/scaladsl/ClusterSharding.scala b/akka-cluster-sharding-typed/src/main/scala/akka/cluster/sharding/typed/scaladsl/ClusterSharding.scala
index bb0d7252ae..f91c84f983 100644
--- a/akka-cluster-sharding-typed/src/main/scala/akka/cluster/sharding/typed/scaladsl/ClusterSharding.scala
+++ b/akka-cluster-sharding-typed/src/main/scala/akka/cluster/sharding/typed/scaladsl/ClusterSharding.scala
@@ -138,6 +138,9 @@ object ClusterSharding extends ExtensionId[ClusterSharding] {
  * between reception of `Passivate` and termination of the entity. Such buffered messages
  * are thereafter delivered to a new incarnation of the entity.
  *
+ * This class is not intended for user extension other than for test purposes (e.g.
+ * stub implementation). More methods may be added in the future and that may break
+ * such implementations.
  */
 @DoNotInherit
 trait ClusterSharding extends Extension { javadslSelf: javadsl.ClusterSharding ⇒
diff --git a/akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/javadsl/DistributedData.scala b/akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/javadsl/DistributedData.scala
index 6ed83bb7c7..ac9247aeb7 100644
--- a/akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/javadsl/DistributedData.scala
+++ b/akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/javadsl/DistributedData.scala
@@ -9,12 +9,14 @@ import akka.actor.typed.Extension
 import akka.actor.typed.ExtensionId
 import akka.actor.typed.ActorRef
 import akka.actor.typed.ExtensionSetup
+import akka.annotation.DoNotInherit
+import akka.annotation.InternalApi
 
 object DistributedData extends ExtensionId[DistributedData] {
   def get(system: ActorSystem[_]): DistributedData = apply(system)
 
   override def createExtension(system: ActorSystem[_]): DistributedData =
-    new DistributedData(system)
+    new DistributedDataImpl(system)
 }
 
 /**
@@ -25,13 +27,25 @@ object DistributedData extends ExtensionId[DistributedData] {
  * This is using the same underlying `Replicator` instance as
  * [[akka.cluster.ddata.DistributedData]] and that means that typed
  * and untyped actors can share the same data.
+ *
+ * This class is not intended for user extension other than for test purposes (e.g.
+ * stub implementation). More methods may be added in the future and that may break
+ * such implementations.
  */
-class DistributedData(system: ActorSystem[_]) extends Extension {
-
+@DoNotInherit
+abstract class DistributedData extends Extension {
   /**
    * `ActorRef` of the [[Replicator]] .
    */
-  val replicator: ActorRef[Replicator.Command] =
+  def replicator: ActorRef[Replicator.Command]
+}
+
+/**
+ * INTERNAL API
+ */
+@InternalApi private[akka] class DistributedDataImpl(system: ActorSystem[_]) extends DistributedData {
+
+  override val replicator: ActorRef[Replicator.Command] =
     akka.cluster.ddata.typed.scaladsl.DistributedData(system).replicator.narrow[Replicator.Command]
 
 }
diff --git a/akka-cluster-typed/src/main/scala/akka/cluster/typed/Cluster.scala b/akka-cluster-typed/src/main/scala/akka/cluster/typed/Cluster.scala
index 5e638e1fbe..2cf27f726b 100644
--- a/akka-cluster-typed/src/main/scala/akka/cluster/typed/Cluster.scala
+++ b/akka-cluster-typed/src/main/scala/akka/cluster/typed/Cluster.scala
@@ -159,7 +159,9 @@ object Cluster extends ExtensionId[Cluster] {
 }
 
 /**
- * Not intended for user extension.
+ * This class is not intended for user extension other than for test purposes (e.g.
+ * stub implementation). More methods may be added in the future and that may break
+ * such implementations.
  */
 @DoNotInherit
 abstract class Cluster extends Extension {
diff --git a/akka-cluster-typed/src/main/scala/akka/cluster/typed/ClusterSingleton.scala b/akka-cluster-typed/src/main/scala/akka/cluster/typed/ClusterSingleton.scala
index 2c4bc2c3d7..7d5a0e5286 100644
--- a/akka-cluster-typed/src/main/scala/akka/cluster/typed/ClusterSingleton.scala
+++ b/akka-cluster-typed/src/main/scala/akka/cluster/typed/ClusterSingleton.scala
@@ -125,7 +125,9 @@ private[akka] object ClusterSingletonImpl {
 }
 
 /**
- * Not intended for user extension.
+ * This class is not intended for user extension other than for test purposes (e.g.
+ * stub implementation). More methods may be added in the future and that may break
+ * such implementations.
  */
 @DoNotInherit
 abstract class ClusterSingleton extends Extension {