Read/Write using FileChannel and FileInput/OutputStream-Java

Question

I am new to file handling. I tried reading a file using fileinputstream and file channel. I could not find out the bug in the following code. It runs successfully but file has not been transferred. New file is created with zero bytes. Please have a look at the code and check what went wrong

public class FileTest
{
    public static void main(String[] args)
    {   
        try {
            File file = new File("sss.jpg");
            FileChannel inChannel=new FileInputStream(file).getChannel();
            //FileChannel inChannel = in.getChannel();
            ByteBuffer buffer = ByteBuffer.allocate(1024);
            while(inChannel.read(buffer) > 0) {
                FileChannel outChannel=new FileOutputStream("sss1.jpg",true).getChannel();
                outChannel.write(buffer);
            }
        }
        catch(IOException ex) {}
    }
}

Achintha Gunasekara · Accepted Answer · 2015-11-09 03:56:53Z

1

I'd do something like this,

import java.io.IOException;
import java.nio.channels.FileChannel;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;

public class Test_Stuff {

    public static void main(String[] args) throws FileNotFoundException, IOException {

        String thisFile = "Test.java";
        FileInputStream source = new FileInputStream(thisFile);
        FileOutputStream destination = new FileOutputStream("Output.java");
        FileChannel sourceFileChannel = source.getChannel();
        FileChannel destinationFileChannel = destination.getChannel();
        long size = sourceFileChannel.size();
        sourceFileChannel.transferTo(0, size, destinationFileChannel);
    }
}

answered Nov 9, 2015 at 3:56

Achintha Gunasekara

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1 Comment

user207421 Over a year ago

transferTo() needs to be called in a loop. It isn't guaranteed to complete the entire transfer in a single operation.

user207421 · Accepted Answer · 2015-11-09 04:03:08Z

1

while(inChannel.read(buffer) > 0) {
    FileChannel outChannel=new FileOutputStream("sss1.jpg",true).getChannel();
    outChannel.write(buffer);
}

You're creating a new file each time around the loop. OK you're appending but this is not efficient, and you're never closing it.

You forgot to flip and compact the buffer.

FileChannel outChannel = new FileOutputStream("sss1.jpg").getChannel();
while(inChannel.read(buffer) > 0) {
    buffer.flip();
    outChannel.write(buffer);
    buffer.compact();
}
outChannel.close();
inChannel.close();

answered Nov 9, 2015 at 4:03

user207421

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3 Comments

Struse Over a year ago

Thank you. The buffer is bytebuffer but flip is the method of class buffer. Does it go well with bytebuffer class?. Is it even possible? can u explain?

djeikyb Over a year ago

@Struse look at the javadoc and/or the source code. there's a relationship between ByteBuffer and Buffer. look at the class signatures first.

Struse Over a year ago

@djeikyb I saw them. Sorry i missed the listed inherited method from buffer class right at the bottom. Thank you anyways

Community · Accepted Answer · 2017-05-23 10:27:17Z

0

You should absolutely follow EJP's or Achintha Gunasekara's answer. You are doing multiple things poorly (eg, creating many output channels, not using FileChannel::open, etc). But I believe the essential problem is not calling flip. From the javadoc:

Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded. After a sequence of channel-read or put operations, invoke this method to prepare for a sequence of channel-write or relative get operations. For example:
 buf.put(magic);    // Prepend header
 in.read(buf);      // Read data into rest of buffer
 buf.flip();        // Flip buffer
 out.write(buf);    // Write header + data to channel
This method is often used in conjunction with the compact method when transferring data from one place to another.

edited May 23, 2017 at 10:27

CommunityBot

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answered Nov 9, 2015 at 4:02

djeikyb

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2 Comments

user207421 Over a year ago

Nothing wrong with checking via > 0 in blocking mode: read() can't return zero unless there is no space in the buffer, which would be a programming error you wouldn't want to spin-loop on. Using the Files object isn't compulsory.

djeikyb Over a year ago

@EJP that's a fair point, hadn't thought of it, thanks. i wasn't thinking of it as an error, per se, more just that the javadoc says -1 signals end of stream, so should use that. but now i'll use > 0 with your comment in mind.

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