[Junie]: feat: make client property in ApplicationTestBuilder mutable

ktor-server/ktor-server-test-host/api/ktor-server-test-host.api

@@ -2,12 +2,14 @@ public final class io/ktor/server/testing/ApplicationTestBuilder : io/ktor/serve
 	public fun <init> ()V
 	public fun createClient (Lkotlin/jvm/functions/Function1;)Lio/ktor/client/HttpClient;
 	public fun getClient ()Lio/ktor/client/HttpClient;
+	public fun setClient (Lio/ktor/client/HttpClient;)V
 	public final fun startApplication (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
 }
 
 public abstract interface class io/ktor/server/testing/ClientProvider {
 	public abstract fun createClient (Lkotlin/jvm/functions/Function1;)Lio/ktor/client/HttpClient;
 	public abstract fun getClient ()Lio/ktor/client/HttpClient;
+	public abstract fun setClient (Lio/ktor/client/HttpClient;)V
 }
 
 public class io/ktor/server/testing/ExpectedTestException : java/lang/Throwable {
@@ -29,6 +31,7 @@ public final class io/ktor/server/testing/On {
 public final class io/ktor/server/testing/TestApplication : io/ktor/server/testing/ClientProvider {
 	public fun createClient (Lkotlin/jvm/functions/Function1;)Lio/ktor/client/HttpClient;
 	public fun getClient ()Lio/ktor/client/HttpClient;
+	public fun setClient (Lio/ktor/client/HttpClient;)V
 	public final fun start (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
 	public final fun stop (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
 }

ktor-server/ktor-server-test-host/api/ktor-server-test-host.klib.api

@@ -8,8 +8,9 @@
 
 // Library unique name: <io.ktor:ktor-server-test-host>
 abstract interface io.ktor.server.testing/ClientProvider { // io.ktor.server.testing/ClientProvider|null[0]
-    abstract val client // io.ktor.server.testing/ClientProvider.client|{}client[0]
+    abstract var client // io.ktor.server.testing/ClientProvider.client|{}client[0]
         abstract fun <get-client>(): io.ktor.client/HttpClient // io.ktor.server.testing/ClientProvider.client.<get-client>|<get-client>(){}[0]
+        abstract fun <set-client>(io.ktor.client/HttpClient) // io.ktor.server.testing/ClientProvider.client.<set-client>|<set-client>(io.ktor.client.HttpClient){}[0]
 
     abstract fun createClient(kotlin/Function1<io.ktor.client/HttpClientConfig<out io.ktor.client.engine/HttpClientEngineConfig>, kotlin/Unit>): io.ktor.client/HttpClient // io.ktor.server.testing/ClientProvider.createClient|createClient(kotlin.Function1<io.ktor.client.HttpClientConfig<out|io.ktor.client.engine.HttpClientEngineConfig>,kotlin.Unit>){}[0]
 }
@@ -47,8 +48,9 @@ final class io.ktor.server.testing.client/TestHttpClientEngine : io.ktor.client.
 final class io.ktor.server.testing/ApplicationTestBuilder : io.ktor.server.testing/ClientProvider, io.ktor.server.testing/TestApplicationBuilder { // io.ktor.server.testing/ApplicationTestBuilder|null[0]
     constructor <init>() // io.ktor.server.testing/ApplicationTestBuilder.<init>|<init>(){}[0]
 
-    final val client // io.ktor.server.testing/ApplicationTestBuilder.client|{}client[0]
+    final var client // io.ktor.server.testing/ApplicationTestBuilder.client|{}client[0]
         final fun <get-client>(): io.ktor.client/HttpClient // io.ktor.server.testing/ApplicationTestBuilder.client.<get-client>|<get-client>(){}[0]
+        final fun <set-client>(io.ktor.client/HttpClient) // io.ktor.server.testing/ApplicationTestBuilder.client.<set-client>|<set-client>(io.ktor.client.HttpClient){}[0]
 
     final fun createClient(kotlin/Function1<io.ktor.client/HttpClientConfig<out io.ktor.client.engine/HttpClientEngineConfig>, kotlin/Unit>): io.ktor.client/HttpClient // io.ktor.server.testing/ApplicationTestBuilder.createClient|createClient(kotlin.Function1<io.ktor.client.HttpClientConfig<out|io.ktor.client.engine.HttpClientEngineConfig>,kotlin.Unit>){}[0]
     final suspend fun startApplication() // io.ktor.server.testing/ApplicationTestBuilder.startApplication|startApplication(){}[0]
@@ -59,8 +61,9 @@ final class io.ktor.server.testing/ExternalServicesBuilder { // io.ktor.server.t
 }
 
 final class io.ktor.server.testing/TestApplication : io.ktor.server.testing/ClientProvider { // io.ktor.server.testing/TestApplication|null[0]
-    final val client // io.ktor.server.testing/TestApplication.client|{}client[0]
+    final var client // io.ktor.server.testing/TestApplication.client|{}client[0]
         final fun <get-client>(): io.ktor.client/HttpClient // io.ktor.server.testing/TestApplication.client.<get-client>|<get-client>(){}[0]
+        final fun <set-client>(io.ktor.client/HttpClient) // io.ktor.server.testing/TestApplication.client.<set-client>|<set-client>(io.ktor.client.HttpClient){}[0]
 
     final fun createClient(kotlin/Function1<io.ktor.client/HttpClientConfig<out io.ktor.client.engine/HttpClientEngineConfig>, kotlin/Unit>): io.ktor.client/HttpClient // io.ktor.server.testing/TestApplication.createClient|createClient(kotlin.Function1<io.ktor.client.HttpClientConfig<out|io.ktor.client.engine.HttpClientEngineConfig>,kotlin.Unit>){}[0]
     final suspend fun start() // io.ktor.server.testing/TestApplication.start|start(){}[0]

ktor-server/ktor-server-test-host/common/src/io/ktor/server/testing/TestApplication.kt

@@ -30,13 +30,14 @@ import kotlin.coroutines.EmptyCoroutineContext
 @KtorDsl
 public interface ClientProvider {
     /**
-     * Returns a client with the default configuration.
+     * A client with the default configuration.
+     * This property is mutable, so you can set your own configured client instance.
      *
      * [Report a problem](https://ktor.io/feedback/?fqname=io.ktor.server.testing.ClientProvider.client)
      *
      * @see [testApplication]
      */
-    public val client: HttpClient
+    public var client: HttpClient
 
     /**
      * Creates a client with a custom configuration.
@@ -360,7 +361,17 @@ public open class TestApplicationBuilder {
 @KtorDsl
 public class ApplicationTestBuilder : TestApplicationBuilder(), ClientProvider {
 
-    override val client: HttpClient by lazy { createClient { } }
+    private var _client: HttpClient? = null
+    override var client: HttpClient
+        get() {
+            if (_client == null) {
+                _client = createClient { }
+            }
+            return _client!!
+        }
+        set(value) {
+            _client = value
+        }
 
     internal val application: TestApplication by lazy {
         TestApplication(

ktor-server/ktor-server-test-host/jvm/test/ClientConfigurationTest.kt

@@ -0,0 +1,49 @@
+/*
+ * 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.tests.server.testing
+
+import io.ktor.client.*
+import io.ktor.client.request.*
+import io.ktor.client.statement.*
+import io.ktor.http.*
+import io.ktor.server.application.*
+import io.ktor.server.response.*
+import io.ktor.server.routing.*
+import io.ktor.server.testing.*
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotSame
+
+class ClientConfigurationTest {
+
+    @Test
+    fun testClientConfiguration() = testApplication {
+        // Set up a simple route
+        application {
+            routing {
+                get("/hello") {
+                    call.respondText("Hello, World!")
+                }
+            }
+        }
+
+        // Store the original client
+        val originalClient = client
+
+        // Create a new client
+        val configuredClient = createClient { }
+
+        // Set the new client as the default
+        client = configuredClient
+
+        // Verify that the client has been changed
+        assertNotSame(originalClient, client)
+
+        // Test the configured client
+        val response = client.get("/hello")
+        assertEquals(HttpStatusCode.OK, response.status)
+        assertEquals("Hello, World!", response.bodyAsText())
+    }
+}