Java AsynchronousFileChannel and ByteBuffer

Question

I try to use AsynchronousFileChannel to implement copying file. The AsynchronousFileChannel objects for read and write are declare as

AsynchronousFileChannel asyncRead = AsynchronousFileChannel.open(sourcePath);
AsynchronousFileChannel asyncWrite = AsynchronousFileChannel.open(targetPath, StandardOpenOption.WRITE, StandardOpenOption.CREATE);

The CompletionHandler for read looks like

CompletionHandler<Integer, ByteBuffer> handlerRead = new CompletionHandler<Integer, ByteBuffer>() {

        @Override
        public void completed(Integer arg0, ByteBuffer arg1) {
            System.out.println("finished read ...");

            // question line
            asyncWrite.write(ByteBuffer.wrap(arg1.array()), 0, null, handlerWrite);
        }

        @Override
        public void failed(Throwable arg0, ByteBuffer arg1) {
            System.out.println("failed to read ...");
        }
    };

Then I start file read by

asyncRead.read(buffer, 0, buffer, handlerRead);

The question is, after the read is complete, If I write file (please see the the comment "question line" to see where it is called)

// no output
asyncWrite.write(arg1, 0, null, handlerWrite);

there will be no content written out. I have to wrap the buffer again

// works fine
asyncWrite.write(ByteBuffer.wrap(arg1.array()), 0, null, handlerWrite);

in order to see the content written out

My question is, what is the reason I have to use the ByteBuffer to wrap the content of another ByteBuffer?

user207421 · Accepted Answer · 2014-11-10 02:50:04Z

3

what is the reason I have to use the ByteBuffer to wrap the content of another ByteBuffer?

You don't. You should have flipped the original ByteBuffer instead. You got a similar effect by calling wrap(), which sets the position of the newly wrapped ByteBuffer to zero.

answered Nov 10, 2014 at 2:50

user207421

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4 Comments

Dino Tw Over a year ago

I tried asyncWrite.write((ByteBuffer)arg1.flip(), 0, null, handlerWrite);, it also works. Thank you for the hint, although I still don't know what's happened under the hood yet.

user207421 Over a year ago

So have a look the Javadoc for flip(). Its purpose is to put a Buffer into a writable state. Normally they are in a readable state. You should compact() or clear() afterwards if you're going to read into it again.

Dino Tw Over a year ago

Yep, I read the doc already, but don't digest it yet. From what you said, it sounds like an iterator that I have to do something to move the pointer to the head if I want to iterate the list for the 2nd time. Is my understanding correct?

user207421 Over a year ago

No, it's not an iterator. NIO Buffers exist in two states: ready for read and ready for write. Unfortunately. You have to flip() to get from read-ready to write-ready, and you have to compact() or clear() to get back again. Poor design IMHO, especially the misnamed flip() operation, which isn't reversible despite its name.

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