基于android-activity的Rust安卓应用如何访问ActivityCompat类？

我有一个基于android-activity的Rust原生安卓应用，想要调用ActivityCompat.requestPermission()，但调用jni_env.find_class("androidx/core/app/ActivityCompat")时出现异常，而jni_env.find_class("java/lang/String")能正常返回结果。我猜测ActivityCompat类所在的jar包未被包含在android-activity包装器中。

代码文件 #

Cargo.toml

[package]
name = "check2"
version = "0.1.0"
edition = "2021"

[dependencies]
android-activity = { version = "*", features = ["native-activity"] }
android_logger = "*"
jni="0.19.0"
ndk-context="*"
log="*"

[lib]
crate-type = ["lib", "cdylib"]

[package.metadata.android]
apk_label = "demonstration of missing java class"
target_sdk_version = 29
min_sdk_version = 29
#runtime_libs = "libs"

[[package.metadata.android.application.activity.intent_filter]]
actions = ["android.intent.action.MAIN"]
categories = [
    "android.intent.category.LAUNCHER",
]

lib.rs

use android_activity::AndroidApp;
use jni::errors::Error;
use jni::JavaVM;
use log::debug;
use std::ffi::c_void;

pub fn adapt_android_env() -> Result<(JavaVM, *mut c_void), Error> {
    let ctx = ndk_context::android_context();
    let vm = unsafe { jni::JavaVM::from_raw(ctx.vm().cast()) }?;
    let activity = ctx.context();

    Ok((vm, activity))
}

#[no_mangle]
fn android_main(_android_app: AndroidApp) {
    android_logger::init_once(
        android_logger::Config::default().with_max_level(log::LevelFilter::Trace),
    );

    debug!("application starting");

    let (jvm, _activity) = adapt_android_env().unwrap();
    let jni_env = jvm.attach_current_thread().unwrap();

    //

    {
        let x = jni_env.find_class("java/lang/String");
        debug!("java class string is {:?}", x);
    }
    {
        let x = jni_env.find_class("androidx/core/app/ActivityCompat");
        debug!("java class ActivityCompat is {:?}", x);
    }

    debug!("exception state is {:?}", jni_env.exception_check());
    debug!("exception clear is {:?}", jni_env.exception_clear());

    debug!("application ending");
}

1. 添加AndroidX Core依赖 #

android-activity默认仅依赖系统原生Activity相关库，AndroidX Core属于额外依赖，需要在Cargo.toml的安卓元数据中配置引入：

[package.metadata.android]
apk_label = "demonstration of missing java class"
target_sdk_version = 29
min_sdk_version = 29
# 添加AndroidX Core依赖
android_gradle_plugins = [
    "androidx.core:core:1.12.0"
]

该配置会让构建工具自动引入AndroidX Core库，确保ActivityCompat类可被加载。

2. 通过Activity的ClassLoader加载类 #

直接调用find_class可能因类加载器范围问题失败，可借助已有的Activity实例获取其类加载器来加载目标类：
修改lib.rs中的代码：

use android_activity::AndroidApp;
use jni::errors::Error;
use jni::{JavaVM, objects::JObject};
use log::debug;

pub fn adapt_android_env() -> Result<(JavaVM, JObject), Error> {
    let ctx = ndk_context::android_context();
    let vm = unsafe { JavaVM::from_raw(ctx.vm().cast()) }?;
    // 将原始指针转为JObject
    let activity = unsafe { JObject::from_raw(ctx.context().cast()) };

    Ok((vm, activity))
}

#[no_mangle]
fn android_main(_android_app: AndroidApp) {
    android_logger::init_once(
        android_logger::Config::default().with_max_level(log::LevelFilter::Trace),
    );

    debug!("application starting");

    let (jvm, activity) = adapt_android_env().unwrap();
    let jni_env = jvm.attach_current_thread().unwrap();

    // 获取Activity的ClassLoader
    let class_loader = jni_env.call_method(
        activity,
        "getClassLoader",
        "()Ljava/lang/ClassLoader;",
        &[]
    )?.l()?;

    // 使用ClassLoader加载ActivityCompat
    {
        let x = jni_env.find_class_with_loader("androidx/core/app/ActivityCompat", class_loader);
        debug!("java class ActivityCompat is {:?}", x);
    }

    {
        let x = jni_env.find_class("java/lang/String");
        debug!("java class string is {:?}", x);
    }

    debug!("exception state is {:?}", jni_env.exception_check());
    debug!("exception clear is {:?}", jni_env.exception_clear());

    debug!("application ending");
}

Activity的类加载器包含了AndroidX依赖的加载路径，以此加载类成功率更高。

3. 更新构建工具 #

确保cargo-apk为最新版本，旧版本对AndroidX依赖支持可能不完善：

cargo install cargo-apk --force

内容的提问来源于stack exchange，提问作者Mutant Bob