A separate function to load single service inmplementation

ktor-client/ktor-client-core/jvm/src/io/ktor/client/HttpClientJvm.kt

@@ -36,9 +36,7 @@ public interface HttpClientEngineContainer {
 }
 
 @OptIn(InternalAPI::class)
-private val engines = loadService<HttpClientEngineContainer>()
-
-private val FACTORY = engines.firstOrNull()?.factory ?: error(
+private val FACTORY = loadServiceOrNull<HttpClientEngineContainer>()?.factory ?: error(
     "Failed to find HTTP client engine implementation: consider adding client engine dependency. " +
         "See https://ktor.io/docs/http-client-engines.html"
 )

ktor-client/ktor-client-plugins/ktor-client-json/jvm/src/io/ktor/client/plugins/json/DefaultJvm.kt

@@ -7,10 +7,8 @@ package io.ktor.client.plugins.json
 import io.ktor.util.reflect.*
 import io.ktor.utils.io.*
 
-@OptIn(InternalAPI::class)
 @Suppress("DEPRECATION_ERROR")
 public actual fun defaultSerializer(): JsonSerializer {
-    val serializers = loadService<JsonSerializer>()
     return serializers.maxByOrNull { it::javaClass.name } ?: error(
         """
         Failed to find serializer. Consider adding one of the following dependencies: 
@@ -20,3 +18,7 @@ public actual fun defaultSerializer(): JsonSerializer {
         """.trimIndent()
     )
 }
+
+@OptIn(InternalAPI::class)
+@Suppress("DEPRECATION_ERROR")
+private val serializers = loadServices<JsonSerializer>()

ktor-client/ktor-client-tests/jvm/src/io/ktor/client/tests/utils/ClientLoaderJvm.kt

@@ -19,7 +19,7 @@ import kotlin.time.Duration.Companion.seconds
 actual abstract class ClientLoader actual constructor(val timeoutSeconds: Int) {
 
     @OptIn(InternalAPI::class)
-    private val engines: Iterable<HttpClientEngineContainer> by lazy { loadService<HttpClientEngineContainer>() }
+    private val engines: List<HttpClientEngineContainer> by lazy { loadServices<HttpClientEngineContainer>() }
 
     /**
      * Perform test against all clients from dependencies.

ktor-server/ktor-server-core/jvm/src/io/ktor/server/config/ConfigLoadersJvm.kt

@@ -16,4 +16,4 @@ internal actual val CONFIG_PATH: List<String>
     )
 
 @OptIn(InternalAPI::class)
-public actual val configLoaders: List<ConfigLoader> = loadService<ConfigLoader>().toList()
+public actual val configLoaders: List<ConfigLoader> = loadServices<ConfigLoader>()

ktor-shared/ktor-serialization/ktor-serialization-kotlinx/jvm/src/io/ktor/serialization/kotlinx/ExtensionsJvm.kt

@@ -9,4 +9,4 @@ import io.ktor.utils.io.*
 
 @OptIn(InternalAPI::class)
 internal actual val providers: Iterable<KotlinxSerializationExtensionProvider> =
-    loadService<KotlinxSerializationExtensionProvider>()
+    loadServices<KotlinxSerializationExtensionProvider>()

ktor-utils/jvm/src/io/ktor/util/reflect/ServiceLoader.kt

@@ -7,17 +7,28 @@ package io.ktor.util.reflect
 import io.ktor.utils.io.*
 import java.util.*
 
+// NOTE: ServiceLoader should use specific call convention to be optimized by R8 on Android:
+// `ServiceLoader.load(X.class, X.class.getClassLoader()).iterator()`
+// See: https://r8.googlesource.com/r8/+/refs/heads/main/src/main/java/com/android/tools/r8/ir/optimize/ServiceLoaderRewriter.java
+
+/**
+ * Loads all implementations of the service [T] using [ServiceLoader.load].
+ * @see loadServiceOrNull
+ */
+@InternalAPI
+public inline fun <reified T : Any> loadServices(): List<T> = ServiceLoader.load(
+    T::class.java,
+    T::class.java.classLoader,
+).iterator().asSequence().toList()
+
+
 /**
- * Loads implementations of service [T] using [ServiceLoader.load].
- *
- * NOTE: ServiceLoader should use specific call convention to be optimized by R8 on Android:
- * `ServiceLoader.load(X.class, X.class.getClassLoader()).iterator()`
- * See: https://r8.googlesource.com/r8/+/refs/heads/main/src/main/java/com/android/tools/r8/ir/optimize/ServiceLoaderRewriter.java
+ * Loads single implementation of the service [T] using [ServiceLoader.load]
+ * returns `null` if there are no any implementations.
+ * @see loadServices
  */
 @InternalAPI
-public inline fun <reified T> loadService(): Iterable<T> = Iterable {
-    ServiceLoader.load(
-        T::class.java,
-        T::class.java.classLoader,
-    ).iterator()
-}
+public inline fun <reified T : Any> loadServiceOrNull(): T? = ServiceLoader.load(
+    T::class.java,
+    T::class.java.classLoader,
+).iterator().asSequence().firstOrNull()