Can get_byte_array_region() method have a new variant where I can pass the buff length instead of taking buf.len()?

[ghermaniov]

Hi,

I am currently porting some JNI code from C++ to RUST in an Android app.
In C++ I was using env->GetByteArrayRegion() to copy data into a local buffer array with a pre-allocated fixed size, for performance reasons. The code looked like this:

env->GetByteArrayRegion(byte_buffer, 0, bytesCount, local_byte_buffer);

byte_buffer and bytesCount are coming from Java code through a JNI function, so I know how many bytes I have.
As you can see, in Android's NDK, this method is implemented with 4 parameters but in RUST JNI, get_byte_array_region() (and all the others similar) have just 3 parameters and internally they are using buf.len() as the number of bytes to read.

This is forcing me to create a buffer array inside my JNI function with the size equals to bytes_count. I would like to be able to re-use a pre-allocated, static, buffer in each call for performance reasons.

In my C++ implementation I was using the JNI tips from Android developer docs:
https://developer.android.com/training/articles/perf-jni#region-calls

Is there a way for me to achieve this?
Thanks!

[argv-minus-one]

The buf parameter to JNIEnv::get_*_array_region is a slice reference, a data structure consisting of a pointer and length. That's why there's only the one parameter instead of two.

If you have a buffer and you want to copy into only part of it, take a slice of that part, using slice notation. Something like this:

// I assume you have these variables, or something similar, already.
let env: JNIEnv;
let mut local_byte_buffer: Vec<u8>;
let bytesCount: usize;
let byte_buffer: JByteArray;

// Take a slice of the first `bytesCount` bytes of `local_byte_buffer`.
let local_byte_buffer_slice = &mut local_byte_buffer[..bytesCount];

// Now, do the copying.
env.get_byte_array_region(&byte_buffer, 0, local_byte_buffer_slice)?;

This will copy into the first bytesCount bytes of local_byte_buffer.

The slice doesn't have to be at the beginning of the buffer:

// Take a slice that's `bytesCount` bytes long, starting at offset 42.
let local_byte_buffer_slice = &mut local_byte_buffer[42..(42 + bytesCount)];

// Now, do the copying.
env.get_byte_array_region(&byte_buffer, 0, local_byte_buffer_slice)?;

For more information about slices in general, see The Rust Programming Language chapter about slices.

[ghermaniov]

Hi,

I am new to Rust, learning as I go, and I completely missed the slices. I will try this.

Thank you very much!