How to use java.nio.channels.FileChannel to read to ByteBuffer achieve similiar behavior like BufferedReader#readLine()

Question

I want to use java.nio.channels.FileChannel to read from a file, but I want to read line per line like BufferedReader#readLine() does. The reason why I need to use java.nio.channels.FileChannel instead of java.io is because I need to put a lock on a file, and read line by line from that lock file. So I am force to use java.nio.channels.FileChannel. Please help

EDIT Here is my code trying to use FileInputStream to get the FileChannel

public static void main(String[] args){
    File file = new File("C:\\dev\\harry\\data.txt");
    FileInputStream inputStream = null;
    BufferedReader bufferedReader = null;
    FileChannel channel = null;
    FileLock lock = null;
    try{
        inputStream = new FileInputStream(file);
        channel  = inputStream.getChannel();
        lock = channel.lock();
        bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
        String data;
        while((data = bufferedReader.readLine()) != null){
            System.out.println(data);
        }
    }catch(IOException e){
        e.printStackTrace();
    }finally{
        try {
            lock.release();
            channel.close();
            if(bufferedReader != null) bufferedReader.close();
            if(inputStream != null) inputStream.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

when the code is here lock = channel.lock();, it is immediately go to the finally and lock is still null, so lock.release() generate NullPointerException. I am not sure why.

Suraj Chandran · Accepted Answer · 2011-06-14T16:09:09.540

1

The reason is that you need to use FileOutpuStream instead of FileInputStream. Please try this code:

        FileOutputStream outStream = null;
        BufferedWriter bufWriter = null;
        FileChannel channel = null;
        FileLock lock = null;
        try{
            outStream = new FileOutputStream(file);
            channel  = outStream.getChannel();
            lock = channel.lock();
            bufWriter = new BufferedWriter(new OutputStreamWriter(outStream));
        }catch(IOException e){
            e.printStackTrace();
        }

This code works fine for me.

The NUllPointerException is actually hiding the real exception i.e. NotWritableChannelException. For locking i think we need to use OutputStream instead of InputStream.

edited Jun 14 '11 at 16:09

answered Jun 14 '11 at 15:51

Suraj Chandran

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I try that, for some reason, it does not work well when I try to lock the file with `FileInputStream`. Not sure why, – Thang Pham Jun 14 '11 at 15:54
i remeber i have used this before without any problems..can u tell me what is nto working – Suraj Chandran Jun 14 '11 at 15:55
@Suraj: I have update my post, with my code using `FileInputStream`, can you take a look? – Thang Pham Jun 14 '11 at 16:01
The reason is that you need to use OutputStream instead of INputStream, I will update the answer – Suraj Chandran Jun 14 '11 at 16:07
But I try to read from the file, not write. I cant read using OutputStream, or am I missing something here? – Thang Pham Jun 14 '11 at 16:29
usually locking is done during modification.....but anyway, even if you want to do that just lock it using outpustream, then open a inputstream and read, then later when you are done release the outputstream lock, i have seen this before – Suraj Chandran Jun 14 '11 at 16:33

How to use java.nio.channels.FileChannel to read to ByteBuffer achieve similiar behavior like BufferedReader#readLine()

1 Answers1