How to correctly handle null values passed as method argumets?

[uniqx]

Hi, I need to process a JString value passed as method argument, which may or may not be null. Here's a rough outline of how I imagine my code might look like. (Although the unwrap call crashes when the passed value is null.)

#[no_mangle]
#[allow(non_snake_case)]
pub extern "system" fn Java_com_example_MyClass_myMethod<'local>(
    mut env: JNIEnv<'local>,
    _class: JClass<'local>,
    myValue: JString<'local>,
) -> jstring {

    let myValue: String = env
        .get_string(&myValue)
        .unwrap()
        .to_string_lossy()
        .into_owned();

    // if myvalue is null
    handleValue(None);
    // else
    handleValue(Some(myValue));
}

I couldn't find any documentation on how jni-rs suggests to deal handling null values.

[rib]

You can use JObject::null() for a null object reference.

There has been discussion in the past around whether to use Option::None for null but considering how common it is to pass null object references in Java and considering needing to map from the sys types to jni references I think it would be more awkward (or at least a lot more verbose) to have to wrap all object references in an Option.

Here's one of the older discussions I was thinking of: #148

[uniqx]

I ended up unwrapping the JString variable into and Option:

    let myValue: Option<String> = match env.get_string(&myValue) {
        Ok(v) => Some(v.to_string_lossy().into_owned()),
        Err(_) => None,
    };

So when my java method gets called with the String parameter set to null: (new MyClass()).myMethod(null) it'll convert to Option::None on the rust side.

[argv-minus-one]

That will yield None if any error occurs in getting the string's value. That may or may not be what you really want. If you only want to have None if the Java String is null, then like @rib said, call is_null on the JString. Perhaps something like this:

if myValue.is_null() {
    let myValue: String = env
        .get_string(&myValue)
        .unwrap()
        .to_string_lossy()
        .into_owned();
    handleValue(Some(myValue));
}
else {
    handleValue(None);
}