Kotlin-KCP的应用-第一篇

前言

KCP的应用计划分两篇,本文是第一篇

本文主要记录从发现问题到使用KCP解决问题的折腾过程,下一篇记录KCP的应用

背景

Kotlin 号称百分百兼容 Java ,所以在 Kotlin 中一些修饰符,比如 internal ,在编译后放在纯 Java 的项目中使用(没有Kotlin环境),Java 仍然可以访问被 internal 修饰的类、方法、字段等

在使用 Kotlin 开发过程中需要对外提供 SDK 包,在 SDK 中有一些 API 不想被外部调用,并且已经添加了 internal 修饰,但是受限于上诉问题且第三方使用 SDK 的环境不可控(不能要求第三方必须使用Kotlin)

带着问题Google一番,查到以下几个解决方案:

  1. 使用 JvmName 注解设置一个不符合 Java 命名规则的标识符[1]
  2. 使用 ˋˋKotlin 中把一个不合法的标识符强行合法化[1]
  3. 使用 JvmSynthetic 注解[2]

以上方案可以满足大部分需求,但是以上方案都不满足隐藏构造方法,可能会想什么情景下需要隐藏构造方法,例如:

class Builder(internal val a: Int, internal val b: Int) {
    
    /**
     * non-public constructor for java
     */
    internal constructor() : this(-1, -1)
}

为此我还提了个Issue[3],期望官方把 JvmSynthetic 的作用域扩展到构造方法,不过官方好像没有打算实现:joy:

为解决隐藏构造方法,可以把构造方法私有化,对外暴露静态工厂方法:

class Builder private constructor (internal val a: Int, internal val b: Int) {
    
    /**
     * non-public constructor for java
     */
    private constructor() : this(-1, -1)
    
    companion object {

        @JvmStatic
        fun newBuilder(a: Int, b: Int) = Builder(a, b)
    }
}

解决方案说完了,大家散了吧,散了吧~

开玩笑,开玩笑:stuck_out_tongue:,必然要折腾一番

折腾

探索JvmSynthetic实现原理

先看下 JvmSynthetic 注解的注释文档

/**
 * Sets `ACC_SYNTHETIC` flag on the annotated target in the Java bytecode.
 *
 * Synthetic targets become inaccessible for Java sources at compile time while still being accessible for Kotlin sources.
 * Marking target as synthetic is a binary compatible change, already compiled Java code will be able to access such target.
 *
 * This annotation is intended for *rare cases* when API designer needs to hide Kotlin-specific target from Java API
 * while keeping it a part of Kotlin API so the resulting API is idiomatic for both languages.
 */

好家伙,实现原理都说了:在 Java 字节码中的注解目标上设置 ACC_SYNTHETIC 标识

此处涉及 Java 字节码知识点,ACC_SYNTHETIC 标识可以简单理解是 Java 隐藏的,非公开的一种修饰符,可以修饰类、方法、字段等[4]

得看看 Kotlin 是如何设置 ACC_SYNTHETIC 标识的,打开 Github Kotlin 仓库,在仓库内搜索 JvmSynthetic 关键字 Search · JvmSynthetic (github.com)

在搜索结果中分析发现 JVM_SYNTHETIC_ANNOTATION_FQ_NAME 关联性较大,继续在仓库内搜索 JVM_SYNTHETIC_ANNOTATION_FQ_NAME 关键字 Search · JVM_SYNTHETIC_ANNOTATION_FQ_NAME (github.com)

在搜索结果中发现几个类名与代码生成相关,这里以 ClassCodegen.kt 为例,附上相关代码

// 获取Class的SynthAccessFlag
private fun IrClass.getSynthAccessFlag(languageVersionSettings: LanguageVersionSettings): Int {
    // 如果有`JvmSynthetic`注解,返回`ACC_SYNTHETIC`标识
    if (hasAnnotation(JVM_SYNTHETIC_ANNOTATION_FQ_NAME))
        return Opcodes.ACC_SYNTHETIC
    if (origin == IrDeclarationOrigin.GENERATED_SAM_IMPLEMENTATION &&
        languageVersionSettings.supportsFeature(LanguageFeature.SamWrapperClassesAreSynthetic)
    )
        return Opcodes.ACC_SYNTHETIC
    return 0
}

// 计算字段的AccessFlag
private fun IrField.computeFieldFlags(context: JvmBackendContext, languageVersionSettings: LanguageVersionSettings): Int =
    origin.flags or visibility.flags or
            (if (isDeprecatedCallable(context) ||
                correspondingPropertySymbol?.owner?.isDeprecatedCallable(context) == true
            ) Opcodes.ACC_DEPRECATED else 0) or
            (if (isFinal) Opcodes.ACC_FINAL else 0) or
            (if (isStatic) Opcodes.ACC_STATIC else 0) or
            (if (hasAnnotation(VOLATILE_ANNOTATION_FQ_NAME)) Opcodes.ACC_VOLATILE else 0) or
            (if (hasAnnotation(TRANSIENT_ANNOTATION_FQ_NAME)) Opcodes.ACC_TRANSIENT else 0) or
            // 如果有`JvmSynthetic`注解,返回`ACC_SYNTHETIC`标识
            (if (hasAnnotation(JVM_SYNTHETIC_ANNOTATION_FQ_NAME) ||
                isPrivateCompanionFieldInInterface(languageVersionSettings)
            ) Opcodes.ACC_SYNTHETIC else 0)

上述源码中 Opcodes 是字节码操作库 ASM 中的类

猜想 Kotlin 编译器也是使用 ASM 编译生成/修改Class文件

:ok:,知道了 JvmSynthetic 注解的实现原理,是不是可以仿照 JvmSynthetic 给构造方法也添加 ACC_SYNTHETIC 标识呢:question:

首先想到的就是利用 AGP Transform 进行字节码修改

AGP Transform

AGP Transform 的搭建、使用,网上有很多相关文章,此处不再描述,下图是本仓库的组织架构

这里简单说明下:

api-xxx

api-xxx模块中只有一个注解类 Hide

@Target({ElementType.FIELD, ElementType.CONSTRUCTOR, ElementType.METHOD})
@Retention(RetentionPolicy.CLASS)
public @interface Hide {
}
@Target(
    AnnotationTarget.FIELD,
    AnnotationTarget.CONSTRUCTOR,
    AnnotationTarget.FUNCTION,
    AnnotationTarget.PROPERTY_GETTER,
    AnnotationTarget.PROPERTY_SETTER,
)
@Retention(AnnotationRetention.BINARY)
annotation class Hide

kcp

kcp相关,下篇再讲

lib-xxx

lib-xxx模块中包含对注解api-xxx的测试,打包成SDK,供app模块使用

plugin

plugin模块包含AGP Transform

实现plugin模块

创建MaskPlugin

创建 MaskPlugin 类,实现 org.gradle.api.Plugin 接口

class MaskPlugin implements Plugin {

    @Override
    void apply(Project project) {
        // 输出日志,查看Plugin是否生效
        project.logger.error("Welcome to guodongAndroid mask plugin.")

        // 目前增加了限制仅能用于`AndroidLibrary`
        LibraryExtension extension = project.extensions.findByType(LibraryExtension)
        if (extension == null) {
            project.logger.error("Only support [AndroidLibrary].")
            return
        }

        extension.registerTransform(new MaskTransform(project))
    }
}

创建MaskTransform

创建 MaskTransform,继承 com.android.build.api.transform.Transform 抽象类,主要实现 transform 方法,以下为核心代码

class MaskTransform extends Transform {
    @Override
    void transform(TransformInvocation transformInvocation) throws TransformException, InterruptedException, IOException {
        long start = System.currentTimeMillis()
        logE("$TAG - start")

        TransformOutputProvider outputProvider = transformInvocation.outputProvider
        
        // 没有适配增量编译

        // 只关心本项目生成的Class文件
        transformInvocation.inputs.each { transformInput ->
            transformInput.directoryInputs.each { dirInput ->
                if (dirInput.file.isDirectory()) {
                    dirInput.file.eachFileRecurse { file ->
                        if (file.name.endsWith(".class")) {
                            // 使用ASM修改Class文件
                            ClassReader cr = new ClassReader(file.bytes)
                            ClassWriter cw = new ClassWriter(cr, ClassWriter.COMPUTE_MAXS)
                            ClassVisitor cv = new CheckClassAdapter(cw)
                            cv = new MaskClassNode(Opcodes.ASM9, cv, mProject)
                            int parsingOptions = 0
                            cr.accept(cv, parsingOptions)
                            byte[] bytes = cw.toByteArray()

                            FileOutputStream fos = new FileOutputStream(file)
                            fos.write(bytes)
                            fos.flush()
                            fos.close()
                        }
                    }
                }

                File dest = outputProvider.getContentLocation(dirInput.name, dirInput.contentTypes, dirInput.scopes, Format.DIRECTORY)
                FileUtils.copyDirectory(dirInput.file, dest)
            }

            // 不关心第三方Jar中的Class文件
            transformInput.jarInputs.each { jarInput ->
                String jarName = jarInput.name
                String md5Name = DigestUtils.md5Hex(jarInput.file.getAbsolutePath())
                if (jarName.endsWith(".jar")) {
                    jarName = jarName.substring(0, jarName.length() - 4)
                }
                File dest = outputProvider.getContentLocation(jarName + md5Name, jarInput.contentTypes, jarInput.scopes, Format.JAR)
                FileUtils.copyFile(jarInput.file, dest)
            }
        }

        long cost = System.currentTimeMillis() - start
        logE(String.format(Locale.CHINA, "$TAG - end, cost: %dms", cost))
    }

    private void logE(String msg) {
        mProject.logger.error(msg)
    }
}

创建MaskClassNode

创建 MaskClassNode,继承 org.objectweb.asm.tree.ClassNode,主要实现 visitEnd 方法

class MaskClassNode extends ClassNode {

    private static final String TAG = MaskClassNode.class.simpleName

    // api-java中`Hide`注解的描述符
    private static final String HIDE_JAVA_DESCRIPTOR = "Lcom/guodong/android/mask/api/Hide;"
    
    // api-kt中`Hide`注解的描述符
    private static final String HIDE_KOTLIN_DESCRIPTOR = "Lcom/guodong/android/mask/api/kt/Hide;"

    private static final Set HIDE_DESCRIPTOR_SET = new HashSet<>()

    static {
        HIDE_DESCRIPTOR_SET.add(HIDE_JAVA_DESCRIPTOR)
        HIDE_DESCRIPTOR_SET.add(HIDE_KOTLIN_DESCRIPTOR)
    }

    private final Project project

    MaskClassNode(int api, ClassVisitor cv, Project project) {
        super(api)
        this.project = project
        this.cv = cv
    }

    @Override
    void visitEnd() {

        // 处理Field
        for (fn in fields) {
            boolean has = hasHideAnnotation(fn.invisibleAnnotations)
            if (has) {
                project.logger.error("$TAG, before --> typeName = $name, fieldName = ${fn.name}, access = ${fn.access}")
                // 修改字段的访问标识
                fn.access += Opcodes.ACC_SYNTHETIC
                project.logger.error("$TAG, after --> typeName = $name, fieldName = ${fn.name}, access = ${fn.access}")
            }
        }

        // 处理Method
        for (mn in methods) {
            boolean has = hasHideAnnotation(mn.invisibleAnnotations)
            if (has) {
                project.logger.error("$TAG, before --> typeName = $name, methodName = ${mn.name}, access = ${mn.access}")
                // 修改方法的访问标识
                mn.access += Opcodes.ACC_SYNTHETIC
                project.logger.error("$TAG, after --> typeName = $name, methodName = ${mn.name}, access = ${mn.access}")
            }
        }

        super.visitEnd()

        if (cv != null) {
            accept(cv)
        }
    }

    /**
     * 是否有`Hide`注解
     */
    private static boolean hasHideAnnotation(List annotationNodes) {
        if (annotationNodes == null) return false
        for (node in annotationNodes) {
            if (HIDE_DESCRIPTOR_SET.contains(node.desc)) {
                return true
            }
        }
        return false
    }
}

使用Transform

build.gradle - project level

buildscript {
    ext.plugin_version = 'x.x.x'
    dependencies {
        classpath "com.guodong.android:mask-gradle-plugin:${plugin_version}"
    }
}

build.gradle - module level

# lib-kotlin
plugins {
    id 'com.android.library'
    id 'kotlin-android'
    id 'kotlin-kapt'
    id 'maven-publish'
    id 'com.guodong.android.mask'
}

lib-kotlin

interface InterfaceTest {

    // 使用api-kt中的注解
    @Hide
    fun testInterface()
}
class KotlinTest(a: Int) : InterfaceTest {

    // 使用api-kt中的注解
    @Hide
    constructor() : this(-2)

    companion object {

        @JvmStatic
        fun newKotlinTest() = KotlinTest()
    }

    private val binding: LayoutKotlinTestBinding? = null

    // 使用api-kt中的注解
    var a = a
        @Hide get
        @Hide set

    fun getA1(): Int {
        return a
    }

    fun test() {
        a = 1000
    }

    override fun testInterface() {
        println("Interface function test")
    }
}

app

# MainActivity.java

private void testKotlinLib() {
    // 创建对象时不能访问无参构造方法,可以访问有参构造方法或访问静态工厂方法
    KotlinTest test = KotlinTest.newKotlinTest();
    // 调用时不能访问`test.getA()`方法,仅能访问`getA1()方法
    Log.e(TAG, "testKotlinLib: before --> " + test.getA1());
    test.test();
    Log.e(TAG, "testKotlinLib: after --> " + test.getA1());
    
    
    test.testInterface();
    
    InterfaceTest interfaceTest = test;
    // Error - cannot resolve method 'testInterface' in 'InterfaceTest'
    interfaceTest.testInterface();
}

happy:happy:

参考文档


  1. 正确地使用 Kotlin 的 internal ↩ ↩

  2. Support more targets for @JvmSynthetic : KT-24981 (jetbrains.com) ↩

  3. Support 'constructor' target for JvmSynthetic annotation : KT-50609 (jetbrains.com) ↩

  4. Chapter 4. The class File Format (oracle.com) ↩

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