Cargo new rust_lib --lib

├── app
│   ├── build
│   ├── build.gradle
│   ├── libs
│   ├── proguard-rules.pro
│   └── src
├── build
│   └── kotlin
├── build.gradle
├── gradle
│   └── wrapper
├── gradle.properties
├── gradlew
├── gradlew.bat
├── local.properties
├── rust_lib //位置在這
│   ├── Cargo.lock
│   ├── Cargo.toml
│   ├── src
│   └── target
└── settings.gradle

[lib]
name = "rust_lib"
crate-type = ["cdylib"]
[dependencies]
jni = "0.20.0"  

[target.aarch64-linux-android]
linker = "/Users/android-sdk-macosx/ndk-bundle/toolchains/llvm/prebuilt/darwin-x86_64/bin/aarch64-linux-android21-clang++"
[target.armv7-linux-androideabi]
linker = "/Users/android-sdk-macosx/ndk-bundle/toolchains/llvm/prebuilt/darwin-x86_64/bin/armv7a-linux-androideabi21-clang++"

class RustGreetings {
    fun sayHello(to: String): String {
        return greeting(to)
    }
    companion object {
        @JvmStatic external fun greeting(pattern: String): String
    }
}

use jni::JNIEnv;
// These objects are what you should use as arguments to your native
// function. They carry extra lifetime information to prevent them escaping
// this context and getting used after being GC'd.
use jni::objects::{JClass, JString};
// This is just a pointer. We'll be returning it from our function. We
// can't return one of the objects with lifetime information because the
// lifetime checker won't let us.
use jni::sys::jstring;
// This keeps Rust from "mangling" the name and making it unique for this
// crate.
#[no_mangle]
pub extern "system" fn Java_com_android_integratingrust_RustGreetings_greeting(
    env: JNIEnv,
    // This is the class that owns our static method. It's not going to be used,
    // but still must be present to match the expected signature of a static
    // native method.
    class: JClass,
    input: JString,
) -> jstring {
    // First, we have to get the string out of Java. Check out the `strings`
    // module for more info on how this works.
    let mut input: String = env
        .get_string(input)
        .expect("Couldn't get java string!")
        .into();
    input = append_string(&input);
    // Then we have to create a new Java string to return. Again, more info
    // in the `strings` module.
    let output = env
        .new_string(format!("Hello, {}!", input))
        .expect("Couldn't create java string!");
    // Finally, extract the raw pointer to return.
    output.into_raw()
}
//============== rust code ===============
fn append_string(value: &str) -> String {
    let mut origin = String::from(value);
    origin.push_str("this is Rust");
    return origin;
}
 

aarch64-linux-android
arm-linux-androideabi
armv7-linux-androideabi
i686-linux-android
thumbv7neon-linux-androideabi
x86_64-linux-android

rustup target add aarch64-linux-android armv7-linux-androideabi

cargo build --target aarch64-linux-android --release

.
├── CACHEDIR.TAG
├── aarch64-linux-android
│   ├── CACHEDIR.TAG
│   └── release
├── armv7-linux-androideabi
│   ├── CACHEDIR.TAG
│   └── release
├── debug
│   ├── build
│   ├── deps
│   ├── examples
│   └── incremental
└── release
    ├── build
    ├── deps
    ├── examples
    └── incremental

    sourceSets {
        main {
            jniLibs.srcDirs = ['src/main/libs']
        }
    }