Update WireHelpers.java to improve data-blob copying by bulk put

[stepeos]

Hello maintainers!
For context: I am using capnproto-java for image processing pipelines in kotlin on android. For this matter, there is a lot of ByteArrays for data buffers of image blobs.

Now while profiling I saw that the data copying has some nasty CPU-usage:

Almost a third of the copy operation is checkIndex().

And in the source code I saw this TODO.

I have seen that bulk ByteBuffer.put(byte[], int, int) and ByteBuffer.put(ByteBuffer serc) is available since Java 8. Is there a reason not to for minimal Java version?

I see the same issue in the code with getWritableDataPointer without taking a closer look.

Cheers
Steve

[stepeos]

i was thinking of using put(byte[] int, int length) instead, since the buffers' positions will be changed.
However, both methods act on the position so there is no thread safe way to do this, or am I wrong?
I don't have the time to dig deeper here, and for my usecase; my change already fixes my issue, but I'll leave this as a thought.

[dwrensha]

Looks good to me. Thanks!

[stepeos]

@dwrensha what about the other functions with the same code?
I pointed out getWritableDataPointer for instance. Want me to PR or are you doing that?

Also, is it thread safe? Because I am changing the ByteBuffer position?

[dwrensha]

I opened #144 for the other TODOs.

[stepeos]

perf, thanks!

[dwrensha]

Hm... setTextPointer uses duplicate():

capnproto-java/runtime/src/main/java/org/capnproto/WireHelpers.java

Lines 785 to 787 in 08009f3

	ByteBuffer slice = value.buffer.duplicate();
	slice.position(value.offset);
	slice.limit(value.offset + value.size);

I suppose that is more thread-safe?

[dwrensha]

I just pushed 28ab5ce

[dwrensha]

I added a test in cfea947.

The size member of Data.Reader sometimes is smaller than the limit of the buffer member. This is another reason to use ByteBuffer.duplicate().

[stepeos]

Ah thanks 👍, I tried avoiding the duplicate copying but it makes sense I see!

[dwrensha]

For the record, duplicate() does not copy the buffer's content. It just creates a new wrapper for it: https://docs.oracle.com/javase/8/docs/api/java/nio/ByteBuffer.html#duplicate--