KTOR-7298: Load modules dynamically only if development mode is enabled (#4715)

* Make the module list immutable after ServerConfig creation
* Handle cases when lambda has more or less than one constructor

Author: osipxd

ktor-server/ktor-server-core/common/src/io/ktor/server/application/Application.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2019 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2014-2025 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package io.ktor.server.application
@@ -9,8 +9,11 @@ import io.ktor.server.engine.*
 import io.ktor.util.*
 import io.ktor.util.logging.*
 import io.ktor.utils.io.*
-import kotlinx.coroutines.*
-import kotlin.coroutines.*
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.SupervisorJob
+import kotlin.coroutines.CoroutineContext
+import kotlin.coroutines.EmptyCoroutineContext
 
 /**
  * A builder for [ServerConfig].
@@ -61,7 +64,7 @@ public class ServerConfigBuilder(
     }
 
     internal fun build(): ServerConfig =
-        ServerConfig(environment, modules, watchPaths, rootPath, developmentMode, parentCoroutineContext)
+        ServerConfig(environment, modules.toList(), watchPaths, rootPath, developmentMode, parentCoroutineContext)
 }
 
 /**
@@ -72,7 +75,7 @@ public class ServerConfigBuilder(
  */
 public class ServerConfig internal constructor(
     public val environment: ApplicationEnvironment,
-    internal val modules: MutableList<Application.() -> Unit>,
+    internal val modules: List<Application.() -> Unit>,
     internal val watchPaths: List<String>,
     public val rootPath: String,
     public val developmentMode: Boolean = PlatformUtils.IS_DEVELOPMENT_MODE,

ktor-server/ktor-server-core/jvm/src/io/ktor/server/engine/EmbeddedServerJvm.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2024 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2014-2025 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package io.ktor.server.engine
@@ -14,15 +14,24 @@ import io.ktor.util.logging.*
 import io.ktor.util.pipeline.*
 import io.ktor.utils.io.*
 import io.ktor.utils.io.core.*
-import kotlinx.coroutines.*
-import java.io.*
-import java.net.*
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
+import java.io.File
+import java.net.URL
+import java.net.URLDecoder
 import java.nio.file.*
 import java.nio.file.StandardWatchEventKinds.*
-import java.nio.file.attribute.*
-import java.util.concurrent.*
-import java.util.concurrent.locks.*
-import kotlin.concurrent.*
+import java.nio.file.attribute.BasicFileAttributes
+import java.util.concurrent.TimeUnit
+import java.util.concurrent.locks.ReentrantReadWriteLock
+import kotlin.concurrent.getOrSet
+import kotlin.concurrent.read
+import kotlin.concurrent.write
+
+private typealias ApplicationModule = Application.() -> Unit
+private typealias DynamicApplicationModule = Application.(ClassLoader) -> Unit
 
 public actual class EmbeddedServer<
     TEngine : ApplicationEngine,
@@ -34,6 +43,7 @@ actual constructor(
     engineConfigBlock: TConfiguration.() -> Unit
 ) {
 
+    @Suppress("DEPRECATION")
     public actual val monitor: Events = rootConfig.environment.monitor
 
     public actual val environment: ApplicationEnvironment = rootConfig.environment
@@ -50,11 +60,13 @@ actual constructor(
     private val configuredWatchPath = environment.config.propertyOrNull("ktor.deployment.watch")?.getList().orEmpty()
     private val watchPatterns: List<String> = configuredWatchPath + rootConfig.watchPaths
 
-    private val configModulesNames: List<String> = run {
-        environment.config.propertyOrNull("ktor.application.modules")?.getList() ?: emptyList()
-    }
+    private val configModulesNames: List<String> =
+        environment.config.propertyOrNull("ktor.application.modules")?.getList().orEmpty()
 
-    private val modulesNames: List<String> = configModulesNames
+    private val modules by lazy {
+        configModulesNames.map(::dynamicModule) +
+            rootConfig.modules.map { module -> module.toDynamicModuleOrNull() ?: module.wrapWithDynamicModule() }
+    }
 
     private var applicationInstance: Application? = Application(
         environment,
@@ -356,23 +368,53 @@ actual constructor(
         safeRaiseEvent(ApplicationStarting, newInstance)
 
         avoidingDoubleStartup {
-            modulesNames.forEach { name ->
-                launchModuleByName(name, currentClassLoader, newInstance)
-            }
+            modules.forEach { module -> module(newInstance, currentClassLoader) }
+        }
 
-            rootConfig.modules.forEach { module ->
-                val name = module.methodName()
+        safeRaiseEvent(ApplicationStarted, newInstance)
+        return newInstance
+    }
 
-                try {
-                    launchModuleByName(name, currentClassLoader, newInstance)
-                } catch (_: ReloadingException) {
-                    module(newInstance)
-                }
+    private fun dynamicModule(name: String): DynamicApplicationModule {
+        return { classLoader ->
+            val application = this
+            launchModuleByName(name, classLoader, application)
+        }
+    }
+
+    private fun ApplicationModule.toDynamicModuleOrNull(): DynamicApplicationModule? {
+        // Programmatic modules are loaded dynamically only when development mode is active
+        if (!rootConfig.developmentMode) return null
+
+        val module = this
+        // Method name getting might fail if method signature has been changed after compilation
+        // (for example by R8 or ProGuard)
+        val name = runCatching { module.methodName() }
+            .onFailure { cause ->
+                environment.log.debug(
+                    "Module can't be loaded dynamically, auto-reloading won't work for this module",
+                    cause,
+                )
+            }
+            .getOrElse { return null }
+
+        return { classLoader ->
+            val application = this
+            try {
+                launchModuleByName(name, classLoader, application)
+            } catch (cause: ReloadingException) {
+                environment.log.debug(
+                    "Failed to load module '$name' by classpath reference, falling back to currently loaded value",
+                    cause,
+                )
+                module.invoke(application)
             }
         }
+    }
 
-        safeRaiseEvent(ApplicationStarted, newInstance)
-        return newInstance
+    private fun ApplicationModule.wrapWithDynamicModule(): DynamicApplicationModule {
+        val module = this
+        return { module() }
     }
 
     private fun launchModuleByName(name: String, currentClassLoader: ClassLoader, newInstance: Application) {

ktor-server/ktor-server-core/jvm/src/io/ktor/server/engine/ServerHostUtils.kt

@@ -1,19 +1,19 @@
 /*
-* Copyright 2014-2021 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
-*/
+ * Copyright 2014-2025 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
 
 package io.ktor.server.engine
 
-import kotlin.reflect.*
-import kotlin.reflect.jvm.*
+import kotlin.reflect.KFunction
+import kotlin.reflect.jvm.javaMethod
 
 /**
  * Obtain function FQName.
  */
 internal fun Function<*>.methodName(): String {
     val method = (this as? KFunction<*>)?.javaMethod ?: return "${javaClass.name}.invoke"
 
-    val clazz = method.declaringClass
+    val className = method.declaringClass.name
     val name = method.name
-    return "${clazz.name}.$name"
+    return "$className.$name"
 }

ktor-server/ktor-server-core/jvm/src/io/ktor/server/engine/internal/CallableUtils.kt

@@ -1,14 +1,18 @@
 /*
- * Copyright 2014-2021 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2014-2025 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package io.ktor.server.engine.internal
 
 import io.ktor.server.application.*
-import java.lang.reflect.*
-import kotlin.reflect.*
-import kotlin.reflect.full.*
-import kotlin.reflect.jvm.*
+import java.lang.reflect.InvocationTargetException
+import java.lang.reflect.Modifier
+import kotlin.reflect.KClass
+import kotlin.reflect.KFunction
+import kotlin.reflect.KParameter
+import kotlin.reflect.full.functions
+import kotlin.reflect.jvm.javaType
+import kotlin.reflect.jvm.kotlinFunction
 
 internal fun executeModuleFunction(
     classLoader: ClassLoader,
@@ -40,10 +44,13 @@ internal fun executeModuleFunction(
 
     try {
         if (Function1::class.java.isAssignableFrom(clazz)) {
-            val constructor = clazz.declaredConstructors.single()
-            if (constructor.parameterCount != 0) {
-                throw ReloadingException("Module function with captured variables cannot be instantiated '$fqName'")
+            // Normally lambda has a single constructor, but this could change after R8/ProGuard optimizations
+            val constructors = clazz.declaredConstructors
+            if (constructors.isEmpty()) {
+                throw ReloadingException("Module function cannot be instantiated '$fqName'")
             }
+            val constructor = constructors.find { it.parameterCount == 0 }
+                ?: throw ReloadingException("Module function with captured variables cannot be instantiated '$fqName'")
 
             constructor.isAccessible = true
             @Suppress("UNCHECKED_CAST")