Android (Kotlin / Java) List<String> / Array<String> --> Rust Vec<String> conversion using JNI

Question

I have an Android app (written in Kotlin), with the processing code running in Rust. I'm trying to send List<String> data to the Rust back-end and interpret it as a Vec<String>. The size of the list is variable. I'm using JNI (Java Native Interface) for this.

Android (Kotlin)

On the Kotlin side I have the following function signature:

package eu.mypackage.rust

class SomeName {
    @Throws(IllegalArgumentException::class)
    external fun checkAnswer(answerInput: String, answerList: List<String>): Boolean
}

Which I call with:

val isCorrect = sn.checkAnswer(answerInput, answerList)

Rust

On the Rust side I have this draft function:

#[cfg(target_os = "android")]
#[allow(non_snake_case)]
pub mod android {
    extern crate jni;

    // This is the interface to the JVM  that we'll call the majority of our methods on.
    // @See https://docs.rs/jni/latest/jni/
    use self::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 self::jni::objects::{JClass, JString, JObject, JObjectArray};  // Not sure what is required

    // 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 self::jni::sys::{jstring, jboolean, jobjectArray, jsize};  // Not sure what is required

    fn kotlin_list_to_rust_vec_string(env: &JNIEnv, java_array: &jobjectArray) -> Vec<String> {
        // TODO function to convert
    }

    #[no_mangle] // This keeps Rust from "mangling" the name so it is unique (crate).
    pub extern "system" fn Java_eu_mycompany_rust_MyPackage_checkAnswer<'local>(
        mut env: JNIEnv<'local>,
        // 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<'local>,
        answerInput: JString<'local>,
        answerList: JObjectArray<'local>,  // JObjectArray correct?
    ) -> jboolean
    // Keep in mind that arrays of jboolean values should only ever hold values of 0 or 1 because any other value could lead to undefined behaviour within the JVM.
    // https://docs.rs/jni/latest/jni/struct.JNIEnv.html#jboolean-elements
    {
        let answer_input: String = env.get_string(&answerInput).expect("Couldn't get java string!").into();
        let answer_list = kotlin_list_to_rust_vec_string(&env, &answerList);

        // ... code to process answer_list
        
        return 1 // true or 0 for false
    }
}

Question

It seems there is no direct conversion for Java List<String> -> Rust Vec<String>, so a function like kotlin_list_to_rust_vec_string is required?
What code should be in kotlin_list_to_rust_vec_string? I assume env and answerList should be a reference to avoid screwing up the data on the Kotlin side? (The amount of data is small, so copying the data is fine)

Mark Rotteveel · Accepted Answer · 2025-03-09 09:37:10Z

0

Rust

With the help of a LLM, I was able to produce the following solution:

    fn kotlin_list_to_rust_vec_string(env: &mut JNIEnv, java_array: &JObjectArray) -> Vec<String> {
        let string_count = env.get_array_length(java_array).unwrap();

        let mut rust_strings = Vec::with_capacity(string_count as usize);

        for i in 0..string_count {
            let string_element = env.get_object_array_element(&java_array, i).unwrap();
            let java_string: JString = string_element.into();
            let rust_string = env.get_string(&java_string).unwrap().into();
            rust_strings.push(rust_string);
        }

        rust_strings
    }

The env variable had to be mutable for this to work though (not sure if this is a safe way of doing it?), so call the function with:

let answer_list = kotlin_list_to_rust_vec_string(&mut env, &answerList);

Android (Kotlin)

While the code compiled on the Rust side, my Android app crashed with the error:

java_vm_ext.cc:598] JNI DETECTED ERROR IN APPLICATION: jarray argument has non-array type: java.util.ArrayList
java_vm_ext.cc:598]     in call to GetArrayLength
java_vm_ext.cc:598]     from boolean eu.mypackage.rust.

The LLM explained the error as:

In Kotlin, when you pass a List<String> to a native method, it remains a java.util.ArrayList, which is not a JNI-compatible array. Solution: Convert the List<String> to a String[] before passing it to Rust

So updating the function signature:

    @Throws(IllegalArgumentException::class)
    // in Kotlin, when you pass a List<String> to a native method, it remains a java.util.ArrayList,
    // which is not a JNI-compatible array. Convert the List<String> to a String[]
    external fun checkAnswer(answerInput: String, answerList: Array<String>): Boolean  // List<String>

and adding .toTypedArray() to my function call:

val isCorrect = sn.checkAnswer(answerInput, answerList.toTypedArray())  // convert to Array

Solved the final issue.

edited Mar 9 at 9:37

Mark Rotteveel

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answered Mar 9 at 7:44

NumesSanguis

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2 Comments

Mark Rotteveel Mar 9 at 9:37

If you have follow up questions, ask them in a new question, not in an answer.

Edric Mar 10 at 5:44

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