Use C++ array directly in Java API requiring byte[]

Question

As it stands, I have a C++ app that has a unsigned char* buffer containing PCM audio data. I need to call the Android API method AudioTrack.write() on an instance of AudioTrack over JNI (from C++ to Java) with this data, and I would like to avoid making an extra copy in doing so. Can I do this?

AudioTrack accepts as one of its arguments a Java byte[], the argument that should correspond to my PCM data (unsigned char*).

Sorry if this is a duplicate... it's hard to effectively search for this kind of thing.

You could play with JNI's GetPrimitiveArrayCritical() and ReleasePrimitiveArrayCritical(), but I wouldn't recommend it. A better idea: Ask the Android team to support NIO buffers instead of (or in addition to) arrays... — Samuel Audet
– Samuel Audet, Commented Feb 23, 2012 at 8:57
Even better is the native support for audio in later versions of Android. The frustrating thing is that once I kick this over to Java, it kicks it back to native code again. — notlesh
– notlesh, Commented Feb 23, 2012 at 17:13
Sigh... sometimes I wished Google included a decent JIT compiler so we could all simply start using Java with OpenCL, OpenGL, OpenSL, etc. instead of fooling around with native APIs like that... :( But then again, I guess this is Google's business model, give away broken software and rule the world, where did we see that happen before hey? — Samuel Audet
– Samuel Audet, Commented Feb 25, 2012 at 9:54
@samual-audet I don't want to use their Java API's. Then I would have to rebuild my game from scratch to support other platforms. By writing 99% of my game using standard C++, I can support numerous platforms with the same codebase. Also, why would you call any of this broken? Most of the Android API that I've worked with has been nothing but a pleasure (so long as it does what I want.) — notlesh
– notlesh, Commented Feb 25, 2012 at 16:45
Hum, "buggy and nonoptimal" would have been a better description than "broken"... The more I deal with Android, the less I wish I had :( — Samuel Audet
– Samuel Audet, Commented Feb 26, 2012 at 2:52

bobby · Accepted Answer · 2012-02-23 08:21:15Z

1

Something like this should do.
I haven't compiled this, and it would be wise to check the syntax also with the specs.

jbyteArray byteArray; byteArray = env->NewByteArray(audioDataLength);
env->SetByteArrayRegion(byteArray, 0, audioDataLength , (jbyte*) audioData);

Where audioDataLength is the length of the char* audioData

answered Feb 23, 2012 at 8:21

bobby

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3 Comments

notlesh Over a year ago

Unfortunately, that makes a copy. [from the SetByteArrayRegion documentation]: A family of functions that copies back a region of a primitive array from a buffer.

bobby Over a year ago

The java byte[] can be accessed from JNI through the API 'GetByteArrayElements'. But that will mean you will have to allocate the array in java, and try and access from JNI. You should be able to use that as a char* in jni, but I suspect some form of copy will be needed. BTW, is the requirement for not making an extra copy for memory conservation or performance?

notlesh Over a year ago

It's both. I'm writing a game and I'm already hitting limits in both areas.

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