diff --git a/akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/EffectfulActorContext.scala b/akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/EffectfulActorContext.scala
index c261bcf3b5..09969396a5 100644
--- a/akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/EffectfulActorContext.scala
+++ b/akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/EffectfulActorContext.scala
@@ -5,7 +5,6 @@
 package akka.actor.testkit.typed.internal
 
 import java.util.concurrent.ConcurrentLinkedQueue
-import java.util.function
 
 import akka.actor.{ ActorPath, Cancellable }
 import akka.actor.typed.{ ActorRef, Behavior, Props }
@@ -15,7 +14,6 @@ import akka.actor.testkit.typed.Effect._
 
 import scala.concurrent.duration.FiniteDuration
 import scala.reflect.ClassTag
-import scala.compat.java8.FunctionConverters._
 
 /**
  * INTERNAL API
@@ -47,9 +45,9 @@ import scala.compat.java8.FunctionConverters._
     effectQueue.offer(MessageAdapter(implicitly[ClassTag[U]].runtimeClass.asInstanceOf[Class[U]], f))
     ref
   }
-  override def messageAdapter[U](messageClass: Class[U], f: function.Function[U, T]): ActorRef[U] = {
+  override def messageAdapter[U](messageClass: Class[U], f: akka.japi.function.Function[U, T]): ActorRef[U] = {
     val ref = super.messageAdapter(messageClass, f)
-    effectQueue.offer(MessageAdapter[U, T](messageClass, f.asScala))
+    effectQueue.offer(MessageAdapter[U, T](messageClass, f.apply))
     ref
   }
   override def spawn[U](behavior: Behavior[U], name: String, props: Props = Props.empty): ActorRef[U] = {
diff --git a/akka-actor-testkit-typed/src/test/java/akka/actor/testkit/typed/javadsl/BehaviorTestKitTest.java b/akka-actor-testkit-typed/src/test/java/akka/actor/testkit/typed/javadsl/BehaviorTestKitTest.java
index 578d968895..6ad0cf1ee5 100644
--- a/akka-actor-testkit-typed/src/test/java/akka/actor/testkit/typed/javadsl/BehaviorTestKitTest.java
+++ b/akka-actor-testkit-typed/src/test/java/akka/actor/testkit/typed/javadsl/BehaviorTestKitTest.java
@@ -70,10 +70,10 @@ public class BehaviorTestKitTest extends JUnitSuite {
 
   public static class CreateMessageAdapter implements Command {
     private final Class<Object> clazz;
-    private final Function<Object, Command> f;
+    private final akka.japi.function.Function<Object, Command> f;
 
     @SuppressWarnings("unchecked")
-    public CreateMessageAdapter(Class clazz, Function<Object, Command> f) {
+    public CreateMessageAdapter(Class clazz, akka.japi.function.Function<Object, Command> f) {
       this.clazz = clazz;
       this.f = f;
     }
diff --git a/akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/ActorCompile.java b/akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/ActorCompile.java
index e045bb5829..dc8fb0b6a5 100644
--- a/akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/ActorCompile.java
+++ b/akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/ActorCompile.java
@@ -170,4 +170,50 @@ public class ActorCompile {
                     .onFailure(IllegalStateException.class, strategy6))
             .onFailure(RuntimeException.class, strategy1);
   }
+
+  // actor context
+  {
+    final ActorRef<Object> otherActor = null;
+    Behavior<String> behavior =
+        Behaviors.setup(
+            context -> {
+              context.ask(
+                  String.class,
+                  otherActor,
+                  Duration.ofSeconds(10),
+                  (ActorRef<String> respRef) -> new Object(),
+                  (String res, Throwable failure) -> {
+                    // checked exception should be ok
+                    if (failure != null) throw new Exception(failure);
+                    else return "success";
+                  });
+
+              ActorRef<Integer> adapter =
+                  context.messageAdapter(
+                      Integer.class,
+                      (number) -> {
+                        // checked exception should be ok
+                        if (number < 10) throw new Exception("too small number");
+                        else return number.toString();
+                      });
+
+              return Behaviors.empty();
+            });
+  }
+
+  // stash buffer
+  {
+    Behavior<String> behavior =
+        Behaviors.withStash(
+            5,
+            stash -> {
+              stash.forEach(
+                  msg -> {
+                    // checked is ok
+                    throw new Exception("checked");
+                  });
+
+              return Behaviors.empty();
+            });
+  }
 }
diff --git a/akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/ReceiveBuilderTest.java b/akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/ReceiveBuilderTest.java
index 21dc6385a4..50c67bc5a5 100644
--- a/akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/ReceiveBuilderTest.java
+++ b/akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/ReceiveBuilderTest.java
@@ -8,6 +8,7 @@ import akka.actor.testkit.typed.javadsl.LogCapturing;
 import akka.actor.testkit.typed.javadsl.TestKitJunitResource;
 import akka.actor.testkit.typed.javadsl.TestProbe;
 import akka.actor.typed.ActorRef;
+import akka.actor.typed.PostStop;
 import org.junit.ClassRule;
 import org.junit.Rule;
 import org.junit.Test;
@@ -139,4 +140,42 @@ public class ReceiveBuilderTest extends JUnitSuite {
     ref.tell("message");
     probe.expectMessage("message");
   }
+
+  public void compileOnlyHandlerAllowedToThrowCheckedException() {
+    Behavior<Object> behavior =
+        ReceiveBuilder.create()
+            .onMessageEquals(
+                "exactly",
+                () -> {
+                  throw new Exception("checked");
+                })
+            .onMessage(
+                String.class,
+                msg -> {
+                  throw new Exception("checked");
+                })
+            .onMessage(
+                Integer.class,
+                msg -> true,
+                msg -> {
+                  throw new Exception("checked");
+                })
+            .onSignalEquals(
+                PostStop.instance(),
+                () -> {
+                  throw new Exception("checked");
+                })
+            .onSignal(
+                PostStop.class,
+                (signal) -> {
+                  throw new Exception("checked");
+                })
+            .onSignal(
+                PostStop.class,
+                signal -> true,
+                signal -> {
+                  throw new Exception("checked");
+                })
+            .build();
+  }
 }
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 f90187027f..5c55db8be2 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
@@ -81,9 +81,7 @@ abstract class ActorRefResolver extends Extension {
 
 object ActorRefResolverSetup {
   def apply[T <: Extension](createExtension: ActorSystem[_] => ActorRefResolver): ActorRefResolverSetup =
-    new ActorRefResolverSetup(new java.util.function.Function[ActorSystem[_], ActorRefResolver] {
-      override def apply(sys: ActorSystem[_]): ActorRefResolver = createExtension(sys)
-    }) // TODO can be simplified when compiled only with Scala >= 2.12
+    new ActorRefResolverSetup(sys => createExtension(sys))
 
 }
 
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/Extensions.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/Extensions.scala
index 34fb4644a8..2ebdc91252 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/Extensions.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/Extensions.scala
@@ -174,6 +174,4 @@ abstract class ExtensionSetup[T <: Extension](
  * extension with stub/mock implementations.
  */
 abstract class AbstractExtensionSetup[T <: Extension](extId: ExtensionId[T], createExtension: ActorSystem[_] => T)
-    extends ExtensionSetup[T](extId, new java.util.function.Function[ActorSystem[_], T] {
-      override def apply(sys: ActorSystem[_]): T = createExtension.apply(sys)
-    }) // TODO can be simplified when compiled only with Scala >= 2.12
+    extends ExtensionSetup[T](extId, createExtension.apply)
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/internal/ActorContextImpl.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/internal/ActorContextImpl.scala
index a33455a314..5ac9bbdf54 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/internal/ActorContextImpl.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/internal/ActorContextImpl.scala
@@ -6,21 +6,18 @@ package akka.actor.typed
 package internal
 
 import java.time.Duration
-import java.util.function.{ Function => JFunction }
 import java.util.ArrayList
 import java.util.Optional
 import java.util.concurrent.CompletionStage
-import java.util.function.BiConsumer
-import java.util.function.BiFunction
 
 import scala.concurrent.{ ExecutionContextExecutor, Future }
 import scala.reflect.ClassTag
 import scala.util.Try
-
 import akka.annotation.InternalApi
 import akka.util.OptionVal
 import akka.util.Timeout
 import akka.util.JavaDurationConverters._
+import com.github.ghik.silencer.silent
 import org.slf4j.Logger
 import org.slf4j.LoggerFactory
 
@@ -139,17 +136,15 @@ import org.slf4j.LoggerFactory
   }
 
   // Java API impl
-  def ask[Req, Res](
+  @silent("never used") // resClass is just a pretend param
+  override def ask[Req, Res](
       resClass: Class[Res],
       target: RecipientRef[Req],
       responseTimeout: Duration,
-      createRequest: JFunction[ActorRef[Res], Req],
-      applyToResponse: BiFunction[Res, Throwable, T]): Unit = {
+      createRequest: akka.japi.function.Function[ActorRef[Res], Req],
+      applyToResponse: akka.japi.function.Function2[Res, Throwable, T]): Unit = {
     import akka.actor.typed.javadsl.AskPattern
-    val message = new akka.japi.function.Function[ActorRef[Res], Req] {
-      def apply(ref: ActorRef[Res]): Req = createRequest(ref)
-    }
-    pipeToSelf(AskPattern.ask(target, message, responseTimeout, system.scheduler), applyToResponse)
+    pipeToSelf(AskPattern.ask(target, (ref) => createRequest(ref), responseTimeout, system.scheduler), applyToResponse)
   }
 
   // Scala API impl
@@ -158,14 +153,14 @@ import org.slf4j.LoggerFactory
   }
 
   // Java API impl
-  def pipeToSelf[Value](future: CompletionStage[Value], applyToResult: BiFunction[Value, Throwable, T]): Unit = {
-    future.whenComplete(new BiConsumer[Value, Throwable] {
-      def accept(value: Value, ex: Throwable): Unit = {
-        if (value != null) self.unsafeUpcast ! AdaptMessage(value, applyToResult.apply(_: Value, null))
-        if (ex != null)
-          self.unsafeUpcast ! AdaptMessage(ex, applyToResult.apply(null.asInstanceOf[Value], _: Throwable))
-      }
-    })
+  def pipeToSelf[Value](
+      future: CompletionStage[Value],
+      applyToResult: akka.japi.function.Function2[Value, Throwable, T]): Unit = {
+    future.whenComplete { (value, ex) =>
+      if (value != null) self.unsafeUpcast ! AdaptMessage(value, applyToResult.apply(_: Value, null))
+      if (ex != null)
+        self.unsafeUpcast ! AdaptMessage(ex, applyToResult.apply(null.asInstanceOf[Value], _: Throwable))
+    }
   }
 
   private[akka] override def spawnMessageAdapter[U](f: U => T, name: String): ActorRef[U] =
@@ -185,7 +180,7 @@ import org.slf4j.LoggerFactory
     internalMessageAdapter(messageClass, f)
   }
 
-  override def messageAdapter[U](messageClass: Class[U], f: JFunction[U, T]): ActorRef[U] =
+  override def messageAdapter[U](messageClass: Class[U], f: akka.japi.function.Function[U, T]): ActorRef[U] =
     internalMessageAdapter(messageClass, f.apply)
 
   private def internalMessageAdapter[U](messageClass: Class[U], f: U => T): ActorRef[U] = {
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/internal/StashBufferImpl.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/internal/StashBufferImpl.scala
index d6edb93333..d70e8c5f6b 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/internal/StashBufferImpl.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/internal/StashBufferImpl.scala
@@ -4,7 +4,6 @@
 
 package akka.actor.typed.internal
 
-import java.util.function.Consumer
 import java.util.function.{ Function => JFunction }
 
 import akka.actor.DeadLetter
@@ -18,6 +17,7 @@ import akka.actor.typed.javadsl
 import akka.actor.typed.scaladsl
 import akka.actor.typed.scaladsl.ActorContext
 import akka.annotation.{ InternalApi, InternalStableApi }
+import akka.japi.function.Procedure
 import akka.util.{ unused, ConstantFun }
 
 /**
@@ -107,7 +107,7 @@ import akka.util.{ unused, ConstantFun }
     }
   }
 
-  override def forEach(f: Consumer[T]): Unit = foreach(f.accept)
+  override def forEach(f: Procedure[T]): Unit = foreach(f.apply)
 
   override def unstashAll(behavior: Behavior[T]): Behavior[T] = {
     val behav = unstash(behavior, size, ConstantFun.scalaIdentityFunction[T])
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/ActorContext.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/ActorContext.scala
index 70628d41ff..f367687dbc 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/ActorContext.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/ActorContext.scala
@@ -5,7 +5,6 @@
 package akka.actor.typed.javadsl
 
 import java.time.Duration
-import java.util.function.{ BiFunction, Function => JFunction }
 
 import akka.annotation.DoNotInherit
 import akka.actor.ClassicActorContextProvider
@@ -267,7 +266,7 @@ trait ActorContext[T] extends TypedActorContext[T] with ClassicActorContextProvi
    * *Warning*: This method is not thread-safe and must not be accessed from threads other
    * than the ordinary actor message processing thread, such as [[java.util.concurrent.CompletionStage]] callbacks.
    */
-  def messageAdapter[U](messageClass: Class[U], f: JFunction[U, T]): ActorRef[U]
+  def messageAdapter[U](messageClass: Class[U], f: akka.japi.function.Function[U, T]): ActorRef[U]
 
   /**
    * Perform a single request-response message interaction with another actor, and transform the messages back to
@@ -299,8 +298,8 @@ trait ActorContext[T] extends TypedActorContext[T] with ClassicActorContextProvi
       resClass: Class[Res],
       target: RecipientRef[Req],
       responseTimeout: Duration,
-      createRequest: java.util.function.Function[ActorRef[Res], Req],
-      applyToResponse: BiFunction[Res, Throwable, T]): Unit
+      createRequest: akka.japi.function.Function[ActorRef[Res], Req],
+      applyToResponse: akka.japi.function.Function2[Res, Throwable, T]): Unit
 
   /**
    * Sends the result of the given `CompletionStage` to this Actor (“`self`”), after adapted it with
@@ -309,6 +308,8 @@ trait ActorContext[T] extends TypedActorContext[T] with ClassicActorContextProvi
    * This method is thread-safe and can be called from other threads than the ordinary
    * actor message processing thread, such as [[java.util.concurrent.CompletionStage]] callbacks.
    */
-  def pipeToSelf[Value](future: CompletionStage[Value], applyToResult: BiFunction[Value, Throwable, T]): Unit
+  def pipeToSelf[Value](
+      future: CompletionStage[Value],
+      applyToResult: akka.japi.function.Function2[Value, Throwable, T]): Unit
 
 }
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/AskPattern.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/AskPattern.scala
index 6bf2e76d93..0e5e3ef90a 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/AskPattern.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/AskPattern.scala
@@ -37,8 +37,8 @@ object AskPattern {
    */
   def ask[Req, Res](
       actor: RecipientRef[Req],
-      message: JFunction[ActorRef[Res], Req],
+      messageFactory: JFunction[ActorRef[Res], Req],
       timeout: Duration,
       scheduler: Scheduler): CompletionStage[Res] =
-    (actor.ask(message.apply)(timeout.asScala, scheduler)).toJava
+    (actor.ask(messageFactory.apply)(timeout.asScala, scheduler)).toJava
 }
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/BehaviorBuilder.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/BehaviorBuilder.scala
index f11150e207..20cc188e80 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/BehaviorBuilder.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/BehaviorBuilder.scala
@@ -4,11 +4,10 @@
 
 package akka.actor.typed.javadsl
 
-import java.util.function.Supplier
-
 import scala.annotation.tailrec
 
-import akka.japi.function.{ Function => JFunction }
+import akka.japi.function.Function
+import akka.japi.function.Creator
 import akka.japi.function.{ Predicate => JPredicate }
 import akka.annotation.InternalApi
 import akka.actor.typed.Behavior
@@ -24,6 +23,9 @@ import akka.util.OptionVal
  * When handling a message or signal, this [[Behavior]] will consider all handlers in the order they were added,
  * looking for the first handler for which both the type and the (optional) predicate match.
  *
+ * Akka `akka.japi.function` lambda types are used throughout to allow handlers to throw checked exceptions
+ * (which will fail the actor).
+ *
  * @tparam T the common superclass of all supported messages.
  */
 final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signalHandlers: List[Case[T, Signal]]) {
@@ -41,7 +43,7 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
    * @tparam M type of message to match
    * @return a new behavior builder with the specified handling appended
    */
-  def onMessage[M <: T](`type`: Class[M], handler: JFunction[M, Behavior[T]]): BehaviorBuilder[T] =
+  def onMessage[M <: T](`type`: Class[M], handler: Function[M, Behavior[T]]): BehaviorBuilder[T] =
     withMessage(OptionVal.Some(`type`), OptionVal.None, handler)
 
   /**
@@ -53,7 +55,7 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
    * @tparam M type of message to match
    * @return a new behavior builder with the specified handling appended
    */
-  def onMessage[M <: T](`type`: Class[M], test: JPredicate[M], handler: JFunction[M, Behavior[T]]): BehaviorBuilder[T] =
+  def onMessage[M <: T](`type`: Class[M], test: JPredicate[M], handler: Function[M, Behavior[T]]): BehaviorBuilder[T] =
     withMessage(OptionVal.Some(`type`), OptionVal.Some((t: T) => test.test(t.asInstanceOf[M])), handler)
 
   /**
@@ -66,7 +68,7 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
    * @param handler action to apply when the type matches
    * @return a new behavior builder with the specified handling appended
    */
-  def onMessageUnchecked[M <: T](`type`: Class[_ <: T], handler: JFunction[M, Behavior[T]]): BehaviorBuilder[T] =
+  def onMessageUnchecked[M <: T](`type`: Class[_ <: T], handler: Function[M, Behavior[T]]): BehaviorBuilder[T] =
     withMessage[M](OptionVal.Some(`type`.asInstanceOf[Class[M]]), OptionVal.None, handler)
 
   /**
@@ -76,13 +78,11 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
    * @param handler action to apply when the message matches
    * @return a new behavior builder with the specified handling appended
    */
-  def onMessageEquals(msg: T, handler: Supplier[Behavior[T]]): BehaviorBuilder[T] =
+  def onMessageEquals(msg: T, handler: Creator[Behavior[T]]): BehaviorBuilder[T] =
     withMessage[T](
       OptionVal.Some(msg.getClass.asInstanceOf[Class[T]]),
       OptionVal.Some(_.equals(msg)),
-      new JFunction[T, Behavior[T]] {
-        override def apply(msg: T): Behavior[T] = handler.get()
-      })
+      (_: T) => handler.create())
 
   /**
    * Add a new case to the message handling matching any message. Subsequent `onMessage` clauses will
@@ -91,7 +91,7 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
    * @param handler action to apply for any message
    * @return a new behavior builder with the specified handling appended
    */
-  def onAnyMessage(handler: JFunction[T, Behavior[T]]): BehaviorBuilder[T] =
+  def onAnyMessage(handler: Function[T, Behavior[T]]): BehaviorBuilder[T] =
     withMessage(OptionVal.None, OptionVal.None, handler)
 
   /**
@@ -102,8 +102,8 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
    * @tparam M type of signal to match
    * @return a new behavior builder with the specified handling appended
    */
-  def onSignal[M <: Signal](`type`: Class[M], handler: JFunction[M, Behavior[T]]): BehaviorBuilder[T] =
-    withSignal(`type`, OptionVal.None, handler.asInstanceOf[JFunction[Signal, Behavior[T]]])
+  def onSignal[M <: Signal](`type`: Class[M], handler: Function[M, Behavior[T]]): BehaviorBuilder[T] =
+    withSignal(`type`, OptionVal.None, handler.asInstanceOf[Function[Signal, Behavior[T]]])
 
   /**
    * Add a new predicated case to the signal handling.
@@ -117,11 +117,11 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
   def onSignal[M <: Signal](
       `type`: Class[M],
       test: JPredicate[M],
-      handler: JFunction[M, Behavior[T]]): BehaviorBuilder[T] =
+      handler: Function[M, Behavior[T]]): BehaviorBuilder[T] =
     withSignal(
       `type`,
       OptionVal.Some((t: Signal) => test.test(t.asInstanceOf[M])),
-      handler.asInstanceOf[JFunction[Signal, Behavior[T]]])
+      handler.asInstanceOf[Function[Signal, Behavior[T]]])
 
   /**
    * Add a new case to the signal handling matching equal signals.
@@ -130,17 +130,13 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
    * @param handler action to apply when the message matches
    * @return a new behavior builder with the specified handling appended
    */
-  def onSignalEquals(signal: Signal, handler: Supplier[Behavior[T]]): BehaviorBuilder[T] =
-    withSignal(signal.getClass, OptionVal.Some(_.equals(signal)), new JFunction[Signal, Behavior[T]] {
-      override def apply(signal: Signal): Behavior[T] = {
-        handler.get()
-      }
-    })
+  def onSignalEquals(signal: Signal, handler: Creator[Behavior[T]]): BehaviorBuilder[T] =
+    withSignal(signal.getClass, OptionVal.Some(_.equals(signal)), (_: Signal) => handler.create())
 
   private def withMessage[M <: T](
       clazz: OptionVal[Class[M]],
       test: OptionVal[M => Boolean],
-      handler: JFunction[M, Behavior[T]]): BehaviorBuilder[T] = {
+      handler: Function[M, Behavior[T]]): BehaviorBuilder[T] = {
     val newCase = Case(clazz, test, handler)
     new BehaviorBuilder[T](newCase.asInstanceOf[Case[T, T]] +: messageHandlers, signalHandlers)
   }
@@ -148,7 +144,7 @@ final class BehaviorBuilder[T] private (messageHandlers: List[Case[T, T]], signa
   private def withSignal[M <: Signal](
       `type`: Class[M],
       test: OptionVal[Signal => Boolean],
-      handler: JFunction[Signal, Behavior[T]]): BehaviorBuilder[T] = {
+      handler: Function[Signal, Behavior[T]]): BehaviorBuilder[T] = {
     new BehaviorBuilder[T](
       messageHandlers,
       Case(OptionVal.Some(`type`), test, handler).asInstanceOf[Case[T, Signal]] +: signalHandlers)
@@ -165,7 +161,7 @@ object BehaviorBuilder {
   private[javadsl] final case class Case[BT, MT](
       `type`: OptionVal[Class[_ <: MT]],
       test: OptionVal[MT => Boolean],
-      handler: JFunction[MT, Behavior[BT]])
+      handler: Function[MT, Behavior[BT]])
 
   /**
    * @return new empty immutable behavior builder.
diff --git a/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/StashBuffer.scala b/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/StashBuffer.scala
index 768b6813e6..671d1775a1 100644
--- a/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/StashBuffer.scala
+++ b/akka-actor-typed/src/main/scala/akka/actor/typed/javadsl/StashBuffer.scala
@@ -4,12 +4,12 @@
 
 package akka.actor.typed.javadsl
 
-import java.util.function.Consumer
 import java.util.function.{ Function => JFunction }
 
 import akka.actor.typed.Behavior
 import akka.actor.typed.scaladsl
 import akka.annotation.DoNotInherit
+import akka.japi.function.Procedure
 
 /**
  * A non thread safe mutable message buffer that can be used to buffer messages inside actors
@@ -72,7 +72,7 @@ import akka.annotation.DoNotInherit
    *
    * @param f the function to apply to each element
    */
-  def forEach(f: Consumer[T]): Unit
+  def forEach(f: Procedure[T]): Unit
 
   /**
    * Process all stashed messages with the `behavior` and the returned
diff --git a/akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/javadsl/ReplicatorMessageAdapter.scala b/akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/javadsl/ReplicatorMessageAdapter.scala
index b265d1f5ee..44003f64e4 100644
--- a/akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/javadsl/ReplicatorMessageAdapter.scala
+++ b/akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/javadsl/ReplicatorMessageAdapter.scala
@@ -64,7 +64,7 @@ class ReplicatorMessageAdapter[A, B <: ReplicatedData](
    * the replicator are transformed to the message protocol of the requesting actor with
    * the given `responseAdapter` function.
    */
-  def subscribe(key: Key[B], responseAdapter: JFunction[Replicator.SubscribeResponse[B], A]): Unit = {
+  def subscribe(key: Key[B], responseAdapter: akka.japi.function.Function[Replicator.SubscribeResponse[B], A]): Unit = {
     // unsubscribe in case it's called more than once per key
     unsubscribe(key)
     changedMessageAdapters.get(key).foreach { subscriber =>