Java Reading from a File using FileChannel

Question

I've been getting some strange outputs from this code upon reading from a large file, the file was printed using a while loop to 99,999 digits however, upon reading the file and printing the contents it only outputs 99,988 lines. Also, is using a ByteBuffer the only option for reading back the file? I've seen some other code using a CharBuffer, but I'm not sure which one I should use, and in what cases I should use them. NOTE: filePath is a Path object pointing to a file on the disk.

    private void byteChannelTrial() throws Exception {
        try (FileChannel channel = (FileChannel) Files.newByteChannel(filePath, READ)) {
            ByteBuffer buffer = ByteBuffer.allocate(1024);
            String encoding = System.getProperty("file.encoding");
            while (channel.read(buffer) != -1) {
                buffer.rewind();
                System.out.print(Charset.forName(encoding).decode(buffer));
                buffer.clear();
            }
        }

That code should print the entire contents of the file correctly. Is it printing what it does print correctly? and have you considered using a BufferedReader wrapped around an InputStreamReader wrapped around a FileInputStream? — user207421
– user207421, Commented Mar 25, 2013 at 7:14
Are you counting digits or lines (you mention both)? Also, how are you determining the number of lines written? — Perception
– Perception, Commented Mar 25, 2013 at 7:15
Every new integer corresponds with a line. So the number count is the line count. — Sarah Szabo
– Sarah Szabo, Commented Mar 25, 2013 at 7:18

horaceman · Accepted Answer · 2013-03-25 08:45:19Z

3

Usually, flip() is called before buffer data is read. the rewind() method does bellowing works:

public final Buffer rewind() {
    position = 0;
    mark = -1;
    return this;
}

it does not set the 'limit' as flip() does:

public final Buffer flip() {
    limit = position;
    position = 0;
    mark = -1;
    return this;
}

So, take a tray using flip() instead of rewind() before reading.

answered Mar 25, 2013 at 8:45

horaceman

3851 silver badge5 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Evgeniy Dorofeev · Accepted Answer · 2013-03-25 07:17:11Z

0

For reading text BufferedReader is the best

    try (BufferedReader rdr = Files.newBufferedReader(Paths.get("path"),
            Charset.defaultCharset())) {
        for (String line; (line = rdr.readLine()) != null;) {
            System.out.println(line);
        }
    }

BTW

String encoding = System.getProperty("file.encoding");
Charset.forName(encoding);

is equivalent to

Charset.defaultCharset();

answered Mar 25, 2013 at 7:17

Evgeniy Dorofeev

137k31 gold badges209 silver badges288 bronze badges

Comments

Sarah Szabo · Accepted Answer · 2013-03-26 06:29:14Z

0

Well, it actually turns out that this combination works:

    private void byteChannelTrial() throws Exception {
        try (FileChannel channel = (FileChannel) Files.newByteChannel(this.filePath, READ)) {
            ByteBuffer buffer = ByteBuffer.allocate(1024);
            while (channel.read(buffer) != -1) {
                buffer.flip();
                System.out.print(Charset.defaultCharset().decode(buffer));
                buffer.clear();
            }
        }
    }

As to why it works, i am not quite certain.

answered Mar 26, 2013 at 6:29

Sarah Szabo

10.9k10 gold badges42 silver badges64 bronze badges

2 Comments

user207421 Over a year ago

It works because you are calling flip() instead of rewind(), as per @horaceman's answer.

bric3 Over a year ago

For reference this code may fail to decode multi-byte chars, or with combining chars (diacritics, emojis) on buffer limit because the bytes may be incomplete in the current buffer. This will typically happen with UTF-8 encoding.

Collectives™ on Stack Overflow

Java Reading from a File using FileChannel

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related