问题

最近在用kotlin写jni，但是生成头文件的时候遇到了些问题。
首先 javah 在java >= 1.9 就被取消用javac -h代替，但是javac对kotlin不适用，kotlinc也没有 -h 这个生成头文件的选项。

解决方法

在stackoverflow论坛找到了个解决方案，那个人提供了一个gradle task，大概原理是先用complieKotlin任务(或kotlinc)生成class字节码，再用javac编译回java文件，然后再调用javac -h 针对那个java文件生成jni头文件，我稍微修复了下，然后迁移到gradle kotlin dsl(就kts脚本)，代码在下面。

使用方法

复制到build.gradle.kts的最外层就可以了，然后sync一下gradle，然后在gradle task里的build分类下就有generate jniheader这个任务了，StackOverflow原贴地址https://stackoverflow.com/a/65661275/14646226

需要改下代码里的inputs.dir("src/main/kotlin")到你的kt源码文件夹, outputs.dir("src/main/generated/jni")到你想的输出文件夹

代码

val generateJniHeaders: Task by tasks.creating {
    group = "build"
    dependsOn(tasks.getByName("compileKotlin"))

    // For caching
    inputs.dir("src/main/kotlin")
    outputs.dir("src/main/generated/jni")

    doLast {
        val javaHome = org.gradle.internal.jvm.Jvm.current().javaHome
        val javap = javaHome.resolve("bin").walk().firstOrNull { it.name.startsWith("javap") }?.absolutePath ?: error("javap not found")
        val javac = javaHome.resolve("bin").walk().firstOrNull { it.name.startsWith("javac") }?.absolutePath ?: error("javac not found")
        val buildDir = file("build/classes/kotlin/main")
        val tmpDir = file("build/tmp/jvmJni").apply { mkdirs() }

        val bodyExtractingRegex = """^.+\Rpublic \w* ?class ([^\s]+).*\{\R((?s:.+))\}\R$""".toRegex()
        val nativeMethodExtractingRegex = """.*\bnative\b.*""".toRegex()

        buildDir.walkTopDown()
            .filter { "META" !in it.absolutePath }
            .forEach { file ->
                if (!file.isFile) return@forEach

                val output = com.gradle.publish.plugin.dep.org.apache.commons.io.output.ByteArrayOutputStream().use {
                    project.exec {
                        commandLine(javap, "-private", "-cp", buildDir.absolutePath, file.absolutePath)
                        standardOutput = it
                    }.assertNormalExitValue()
                    it.toString()
                }

                val (qualifiedName, methodInfo) = bodyExtractingRegex.find(output)?.destructured ?: return@forEach

                val lastDot = qualifiedName.lastIndexOf('.')
                val packageName = qualifiedName.substring(0, lastDot)
                val className = qualifiedName.substring(lastDot+1, qualifiedName.length)

                val nativeMethods =
                    nativeMethodExtractingRegex.findAll(methodInfo).mapNotNull { it.groups }.flatMap { it.asSequence().mapNotNull { group -> group?.value } }.toList()
                if (nativeMethods.isEmpty()) return@forEach

                val source = buildString {
                    appendLine("package $packageName;")
                    appendLine("public class $className {")
                    for (method in nativeMethods) {
                        if ("()" in method) appendLine(method)
                        else {
                            val updatedMethod = StringBuilder(method).apply {
                                var count = 0
                                var i = 0
                                while (i < length) {
                                    if (this[i] == ',' || this[i] == ')') insert(i, " arg${count++}".also { i += it.length + 1 })
                                    else i++
                                }
                            }
                            appendLine(updatedMethod)
                        }
                    }
                    appendLine("}")
                }
                val outputFile = tmpDir.resolve(packageName.replace(".", "/")).apply { mkdirs() }.resolve("$className.java").apply { delete() }.apply { createNewFile() }
                outputFile.writeText(source)

                project.exec {
                    commandLine(javac, "-h", "src/main/generated/jni", outputFile.absolutePath)
                }.assertNormalExitValue()
            }
    }
}

gradle task位置截图