Kotlin多平台项目自定义Gradle插件同步报错求助

问题概述 #

我在为Kotlin多平台项目的app模块实现自定义Gradle插件时，同步项目反复报错：提示需初始化至少一个Kotlin Target，但我已完成配置。添加extensions.getByType(ApplicationExtension::class.java)后触发NoClassDefFoundError，尝试在插件中初始化target也无效果。

报错堆栈信息 #

> Configure project :app
mmd is Coming
com.android.application
e: Please initialize at least one Kotlin target in 'app (:app)'.
Read more https://kotl.in/set-up-targets

w: The following Kotlin source sets were configured but not added to any Kotlin compilation:
 * commonMain
 * commonTest
You can add a source set to a target's compilation by connecting it with the compilation's default source set using 'dependsOn'.
See https://kotl.in/connecting-source-sets

w: Missing 'androidTarget()' Kotlin target in multiplatform project 'app (:app)'.
The Android Gradle plugin was applied without creating a corresponding 'android()' Kotlin Target:

plugins {
id("com.android.application")
kotlin("multiplatform")
}

kotlin {
androidTarget() // <-- please register this Android target
}

FAILURE: Build failed with an exception.

* What went wrong:
  com/android/build/api/dsl/ApplicationExtension

> com/android/build/api/dsl/ApplicationExtension

* Try:

> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.

* Exception is:
  java.lang.NoClassDefFoundError: com.android.build.api.dsl.ApplicationExtension
  at ApplicationGradlePlugin.apply(ApplicationGradlePlugin.kt:24)
  ...
  Caused by: java.lang.ClassNotFoundException: com.android.build.api.dsl.ApplicationExtension
  at org.gradle.internal.classloader.VisitableURLClassLoader$InstrumentingVisitableURLClassLoader.findClass(VisitableURLClassLoader.java:187)
  ... 215 more

CONFIGURE FAILED in 6s

BuildSrc中的自定义插件代码 #

class ApplicationGradlePlugin : Plugin<Project> {
    override fun apply(target: Project) {
        with(target) {
            val libs = extensions.getByType<VersionCatalogsExtension>().named("libs")

            println("mmd is Coming")
            println(libs.findPlugin("androidApplication").get().get().pluginId)
            pluginManager.apply(libs.findPlugin("kotlinMultiplatform").get().get().pluginId)

            pluginManager.apply(libs.findPlugin("androidApplication").get().get().pluginId)

            extensions.getByType(ApplicationExtension::class.java)

            println("mmd done his job..")
        }
    }
}

app模块的build.gradle.kts配置 #

import org.jetbrains.compose.desktop.application.dsl.TargetFormat

plugins {
    alias(libs.plugins.kotlinMultiplatform)
    alias(libs.plugins.composeMultiplatform)
    alias(libs.plugins.composeCompiler)
    alias(libs.plugins.kotlin.serialization)
    alias(libs.plugins.androidApplication)
    id("com.sample.myproject.application-plugin")// --> this is my custom plugin
}

kotlin {
    android {
        compilations.all {
            kotlinOptions.jvmTarget = "21"
        }
    }

    jvm("desktop")

    sourceSets {
        val desktopMain by getting

        androidMain.dependencies {
            // android dependencies
        }
        commonMain.dependencies {
            // common dependencies
        }
        desktopMain.dependencies {
           // desktop dependencies
        }
    }
}

android {
    namespace = "com.sample.myproject"
    compileSdk = libs.versions.android.compileSdk.get().toInt()

    defaultConfig {
        applicationId = "com.sample.myproject"
    // other configurations
}

已尝试的无效操作 #

在插件中初始化target，无效果：

extensions.getByType(KotlinMultiplatformExtension::class).apply {
     androidTarget()
     jvm("desktop")
}

解决方案 #

问题根源 #

1. 插件依赖缺失：BuildSrc未添加Android Gradle插件的依赖，导致运行时找不到ApplicationExtension类。
2. 插件应用顺序混乱：自定义插件手动重复应用kotlinMultiplatform和androidApplication插件，与app模块plugins块的声明冲突，导致KMP插件先执行时无法找到Android配置，触发target未初始化报错。
3. 扩展获取时机错误：在Android插件未完全应用时就尝试获取ApplicationExtension，导致类加载失败。

修复步骤 #

1. 给BuildSrc添加Android Gradle插件依赖

在buildSrc/build.gradle.kts中添加依赖，确保能找到Android相关类：

plugins {
    `kotlin-dsl`
}

repositories {
    google()
    mavenCentral()
}

dependencies {
    // 使用libs catalog引用（推荐）
    implementation(libs.plugins.android.application.get().get().pluginId)
    implementation(libs.plugins.kotlin.multiplatform.get().get().pluginId)
    
    // 或者直接指定版本（如果未用libs catalog）
    // implementation("com.android.tools.build:gradle:8.4.0")
    // implementation("org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.20")
}

2. 重构自定义插件逻辑

移除重复应用插件的代码，改用插件监听机制，确保在目标插件加载完成后再处理扩展：

class ApplicationGradlePlugin : Plugin<Project> {
    override fun apply(target: Project) {
        with(target) {
            val libs = extensions.getByType<VersionCatalogsExtension>().named("libs")
            
            println("mmd is Coming")

            // 监听Android Application插件加载完成事件
            pluginManager.withPlugin("com.android.application") {
                val androidExt = extensions.getByType(ApplicationExtension::class.java)
                // 在这里添加你的自定义Android配置逻辑，比如统一设置compileSdk等
                println("mmd done his job..")
            }

            // 可选：监听Kotlin Multiplatform插件加载完成事件，做默认配置
            pluginManager.withPlugin("org.jetbrains.kotlin.multiplatform") {
                val kmpExt = extensions.getByType(KotlinMultiplatformExtension::class.java)
                // 比如添加默认target或配置sourceSets
            }
        }
    }
}

3. 调整app模块plugins块顺序

将自定义插件移到所有基础插件之后，确保依赖插件先加载：

plugins {
    alias(libs.plugins.kotlinMultiplatform)
    alias(libs.plugins.composeMultiplatform)
    alias(libs.plugins.composeCompiler)
    alias(libs.plugins.kotlin.serialization)
    alias(libs.plugins.androidApplication)
    id("com.sample.myproject.application-plugin") // 移到最后
}

4. 验证Kotlin Target配置

app模块中的kotlin块已经正确配置了android和desktop target，无需修改，确保插件逻辑不会覆盖这些配置即可。

内容的提问来源于stack exchange，提问作者AbolfazlHaghpanah