casting typed array to java byte[] is it possible?

[Paulo Lopes]

Hi Sundar,

Yes indeed that helps and it really gets close to my needs, I notice
however, if I do that, then:

var ia = new Int8Array(ab)
ia.length

will never have a valid value, it will be zero regardless of the
underlying ArrayBuffer, since the property is read only I cannot assign
it myself (since I know the size at construction time) either.

Trying to specify the size:

var bb = java.nio.ByteBuffer.allocateDirect(12)
var ab = new ArrayBuffer(bb)
var ia = new Int8Array(ab, 0, 12)

it's all 8 bit ints so (bytes) fails with:

java.lang.RuntimeException: byteOffset + byteLength out of range,
byteOffset=0, elementLength=12, bytesPerElement=1

Is this expected? Or can I specify the length somewhere?

Also ab.byteLength prints 0 instead of 12...


On Fri, 2018-10-12 at 17:34 +0530, Sundararajan Athijegannathan wrote:
> Hi Paulo Lopes,
> 
> ArrayBuffer can be created using a nio ByteBuffer.
> 
> // you could create a ByteBuffer in Java & pass
> var bb = java.nio.ByteBuffer.allocateDirect(100)
> 
> // create JS ArrayBuffer backed by Java ByteBuffer instance
> var ab = new ArrayBuffer(bb)
> 
> var ia = new Int8Array(ab)
> ia[0] = 10
> print(bb.get(0))
> 
> Hope this helps,
> -Sundar
> 
> On 12/10/18, 5:04 PM, Paulo Lopes wrote:
> > Hi Hannes,
> > 
> > Thanks for the tip, knowing that the convention isn't possible as
> > is, are there any alternatives to get the JVM buffer that backs a
> > typed array?
> > 
> > I wanted to perform some work on a given buffer and then pass it to
> > the JVM for further processing. Having to copy the buffer byte by
> > byte will introduce a penalty that kind of defeats the whole
> > purpose of using typed arrays to start with.
> > 
> > 
> > Paulo Lopes
> > Principal Software Engineer
> > 
> > 
> >    Original Message  
> > From: hannes.wallnoefer at oracle.com
> > Sent: October 12, 2018 10:10 AM
> > To: pmartins at redhat.com
> > Cc: nashorn-dev at openjdk.java.net
> > Subject: Re: casting typed array to java byte[] is it possible?
> > 
> > Hi Paulo,
> > 
> > Java.to() would be the way to go, but as you found out it does not
> > support typed arrays.
> > 
> > What works is to convert the typed array to an ordinary JS array
> > and convert to byte[] from there:
> > 
> >    Java.to(Array.prototype.slice.call(arr), 'byte[]‘);
> > 
> > That’s obviously not very elegant nor efficient, but it’s the only
> > workaround I can think of.
> > 
> > Hannes
> > 
> > 
> > > Am 11.10.2018 um 20:21 schrieb Paulo Lopes<pmartins at redhat.com>:
> > > 
> > > Hi,
> > > 
> > > I'm trying to handle a case where a Uint8Array is being passed to
> > > a
> > > method, that has the signature:
> > > 
> > > String encode(byte[]);
> > > 
> > > Sadly nashorn fails with:
> > > 
> > > java.lang.ClassCastException: Cannot cast
> > > jdk.nashorn.internal.objects.NativeUint8Array to [B
> > > 
> > > And trying to help the cast with:
> > > 
> > > javaObj.encode(Java.to(arr, 'byte[]'));
> > > 
> > > Does not help either. The documentation on typed arrays is quite
> > > scarce
> > > so I cannot see how to handle this, does anyone have a clue?
> > > 
> > > Thanks!
> > > Paulo
> > >