Java how to read part of file from specified position of bytes?

Question

I have a situation when I need to read only part of file, beginning from specified byte position.

I try with next :

protected void writePartToStream(final InputStream in, final OutputStream out, long startBytes) {
        final byte[] b = new byte[BUFFER_SIZE];
        int count = 0;
        amountWritten = startBytes;


        // skip logic
        // how to skip???

        do {
            // write to the output stream
            try {


                out.write(b, 0, count);
                out.flush();
            } catch (IOException e) {
                e.printStackTrace();
            }

            amountWritten += count;
            System.out.println("Amount writtent=" + amountWritten);
            // read more bytes from the input stream
            try {
                count = in.read(b, (int) amountWritten, b.length);
            } catch (IOException e) {
            }
        } while (count != -1 && !getStatus().equals(Status.cancelled));

        // the connection was likely terminated abrubtly if these are not equal
        if (!getStatus().equals(Status.cancelled) && getError() == Error.none
                && amountWritten != fileSize) {
            setStatus(Status.error);
            this.error = Error.connection;
        }
    }

But I never working with I/O, so I don't have any idea how to start read file, from specified position.

FileInputStream has method read(byte[] b, int off, int len) where you can specify start position. — callOfCode
– callOfCode, Commented Mar 2, 2016 at 11:14

Denis Lukenich · Accepted Answer · 2016-03-02 11:16:26Z

9

Another chance is to use the Channel of the file to read values directly from any position as shown in the following example:

int amountBytesToRead = 1337;
int positionToRead = 4211;
FileInputStream fis = new FileInputStream("test.txt");

//A direct ByteBuffer should be slightly faster than a 'normal' one for IO-Operations
ByteBuffer bytes = ByteBuffer.allocateDirect(amountBytesToRead);
fis.getChannel().read(bytes, positionToRead);

byte[] readBytes = bytes.array();
//Handle Bytes

answered Mar 2, 2016 at 11:16

Denis Lukenich

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1 Comment

Prabhakaran M Over a year ago

Thank you. But I got java.lang.UnsupportedOperationException at java.nio.ByteBuffer.array when i used allocateDirect method. It runs fine when allocate method is used.

Eli Acherkan · Accepted Answer · 2016-03-02 11:10:47Z

2

Take a look at java.io.RandomAccessFile.seek(). It allows reading a file from an arbitrary position, but its constructor requires a filename or a File object, not just an input stream as your method currently receives.

answered Mar 2, 2016 at 11:10

Eli Acherkan

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1 Comment

Sergey Shustikov Over a year ago

Looks like that what I'm looking for. I will try with it and if it helps - I will approve it.

fabian · Accepted Answer · 2016-03-02 11:25:08Z

1

You can set the position in the file for a FileInputStream using FileChannel.position(long) of it's FileChannel:

FileInputStream fis = ...
long offset = ...
fis.getChannel().position(offset);

// read data from fis

answered Mar 2, 2016 at 11:25

fabian

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Comments

Yessy · Accepted Answer · 2018-04-25 04:36:34Z

using FileInputStream.getChannel().read(ByteBuffer, long posotion) instead

this method don't modify the file current position

static class FileInputStreamNoModifyCurrentPosition extends InputStream {
    private long mCurrentPosition;
    private long mRemaining;
    private final FileInputStream mFileInputStream;

    public FileInputStreamNoModifyCurrentPosition(FileInputStream fileInputStream, long offset, long length) throws IOException {
        mFileInputStream = fileInputStream;
        mCurrentPosition = offset;
        mRemaining = length;
    }

    @Override
    public int available() throws IOException {
        return (int) mRemaining;
    }

    @Override
    public int read() throws IOException {
        byte[] buffer = new byte[1];
        if (read(buffer) > 0) {
            return (int) buffer[0] & 0xFF;
        } else {
            return -1;
        }
    }

    @Override
    public int read(byte[] buffer, int offset, int count) throws IOException {
        if (mRemaining >= 0) {
            int readCount = mFileInputStream.getChannel().read(ByteBuffer.wrap(buffer, offset, (int) Math.min(count, mRemaining)), mCurrentPosition);
            if (readCount > 0) {
                skip(readCount);
            }
            return readCount;
        }
        return -1;
    }

    @Override
    public int read(byte[] buffer) throws IOException {
        return read(buffer, 0, buffer.length);
    }

    @Override
    public long skip(long count) throws IOException {
        mCurrentPosition += count;
        mRemaining -= count;
        return count;
    }

    @Override
    public void mark(int readlimit) {
        super.mark(readlimit);
    }

    @Override
    public boolean markSupported() {
        return super.markSupported();
    }

    @Override
    public synchronized void reset() throws IOException {
        super.reset();
    }
}

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