diff --git a/Interop/Runtime/src/callbacks/c/callbacks.c b/Interop/Runtime/src/callbacks/c/callbacks.c
index 71e546ab411..d187d71a426 100644
--- a/Interop/Runtime/src/callbacks/c/callbacks.c
+++ b/Interop/Runtime/src/callbacks/c/callbacks.c
@@ -1,3 +1,4 @@
+#include <stdint.h>
 #include <stdlib.h>
 #include <assert.h>
 #include <jni.h>
@@ -163,10 +164,12 @@ static void checkException(JNIEnv *env) {
 static void ffi_fun(ffi_cif *cif, void *ret, void **args, void *user_data) {
     JNIEnv* env = getCurrentEnv();
 
-    static jmethodID ffiFunImpl0 = NULL;
+    static jmethodID acceptFun = NULL;
     static jclass cls = NULL;
-    if (ffiFunImpl0 == NULL) {
-        jclass clsLocal = (*env)->FindClass(env, "kotlinx/cinterop/JvmCallbacksKt");
+    if (acceptFun == NULL) {
+        // Note: in some cases [FindClass] below may use a classloader different from the one loaded interop classes,
+        // so stick to JVM-provided class:
+        jclass clsLocal = (*env)->FindClass(env, "java/util/function/LongConsumer");
         checkException(env);
         assert(clsLocal != NULL);
 
@@ -174,12 +177,13 @@ static void ffi_fun(ffi_cif *cif, void *ret, void **args, void *user_data) {
         checkException(env);
         assert(cls != NULL);
 
-        ffiFunImpl0 = (*env)->GetStaticMethodID(env, cls, "ffiFunImpl0", "(JJJLjava/lang/Object;)V");
+        acceptFun = (*env)->GetMethodID(env, cls, "accept", "(J)V");
         checkException(env);
-        assert(ffiFunImpl0 != NULL);
+        assert(acceptFun != NULL);
     }
 
-    (*env)->CallStaticVoidMethod(env, cls, ffiFunImpl0, (jlong) cif, (jlong) ret, (jlong) args, (jobject) user_data);
+    jlong retAndArgs[2] = { (jlong)ret, (jlong)args }; // Unpacked in [ffiClosureImpl].
+    (*env)->CallVoidMethod(env, (jobject) user_data, acceptFun, (jlong)(intptr_t)&retAndArgs[0]);
     checkException(env);
 }
 
diff --git a/Interop/Runtime/src/jvm/kotlin/kotlinx/cinterop/JvmCallbacks.kt b/Interop/Runtime/src/jvm/kotlin/kotlinx/cinterop/JvmCallbacks.kt
index b322ce9d4d4..58b84fd0c48 100644
--- a/Interop/Runtime/src/jvm/kotlin/kotlinx/cinterop/JvmCallbacks.kt
+++ b/Interop/Runtime/src/jvm/kotlin/kotlinx/cinterop/JvmCallbacks.kt
@@ -17,6 +17,7 @@
 package kotlinx.cinterop
 
 import java.util.concurrent.ConcurrentHashMap
+import java.util.function.LongConsumer
 import kotlin.reflect.KClass
 import kotlin.reflect.KFunction
 import kotlin.reflect.KType
@@ -275,8 +276,18 @@ private inline fun ffiClosureImpl(
         returnType: CType<Any?>,
         crossinline invoke: (args: CArrayPointer<COpaquePointerVar>) -> Any?
 ): FfiClosureImpl {
-    return { ret, args ->
+    // Called through [ffi_fun] when a native function created with [ffiCreateClosure] is invoked.
+    return LongConsumer {  retAndArgsRaw ->
+        val retAndArgs = retAndArgsRaw.toCPointer<CPointerVar<*>>()!!
+
+        // Pointer to memory to be filled with return value of the invoked native function:
+        val ret = retAndArgs[0]!!
+
+        // Pointer to array of pointers to arguments passed to the invoked native function:
+        val args = retAndArgs[1]!!.reinterpret<COpaquePointerVar>()
+
         val result = invoke(args)
+
         returnType.write(ret.rawValue, result)
     }
 }
@@ -357,7 +368,7 @@ private class CEnumType(private val rawValueCType: CType<Any>) : CType<CEnum>(ra
     }
 }
 
-private typealias FfiClosureImpl = (ret: COpaquePointer, args: CArrayPointer<COpaquePointerVar>)->Unit
+private typealias FfiClosureImpl = LongConsumer
 private typealias UserData = FfiClosureImpl
 
 private fun loadCallbacksLibrary() {
@@ -429,29 +440,10 @@ private fun ffiCreateCif(returnType: ffi_type, paramTypes: List<ffi_type>): ffi_
     return interpretPointed(res)
 }
 
-@Suppress("UNUSED_PARAMETER", "UNUSED")
-private fun ffiFunImpl0(ffiCif: Long, ret: Long, args: Long, userData: Any) {
-    @Suppress("UNCHECKED_CAST")
-    ffiFunImpl(interpretCPointer(ret)!!,
-            interpretCPointer(args)!!,
-            userData as UserData)
-}
-
-/**
- * This function is called from native code when a native function created with [ffiCreateClosure] is invoked.
- *
- * @param ret pointer to memory to be filled with return value of the invoked native function
- * @param args pointer to array of pointers to arguments passed to the invoked native function
- */
-private fun ffiFunImpl(ret: COpaquePointer, args: CArrayPointer<COpaquePointerVar>, userData: UserData) {
-
-    userData.invoke(ret, args)
-}
-
 private external fun ffiCreateClosure0(ffiCif: Long, userData: Any): Long
 
 /**
- * Uses libffi to allocate a native function which will call [ffiFunImpl] when invoked.
+ * Uses libffi to allocate a native function which will call [impl] when invoked.
  *
  * @param ffiCif describes the type of the function to create
  */