java - Prevent file channel from closing after reading xml file

Question

For more detailed information regarding the motivation behind this goal (and my efforts to solve it) view my previous question. I decided to ask this as a new question entirely as I thought that it had evolved sufficiently to merit doing so. As a summary, I intend to use JDOM in combination with NIO in order to:

1. Gain an exclusive file lock on an xml file.
2. Read the file into a Document object.
3. Make arbitrary changes (with lock still active!).
4. Write the changes back to the xml file.
5. Release the file lock.

The issue which I am getting however is that the built-in code to read the xml file into a document object closes the channel (and therefore releases the lock), as seen below:

import java.io.*;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import javax.xml.parsers.*;
import org.w3c.dom.Document;
import org.xml.sax.SAXException;

public class Test4{ 
    String path = "Test 2.xml";
    private DocumentBuilderFactory dbFactory;
    private DocumentBuilder dBuilder;
    private Document doc;

    public Test4(){
        try (final FileChannel channel = new RandomAccessFile(new File(path), "rw").getChannel()) {
            dbFactory = DocumentBuilderFactory.newInstance();
            dBuilder = dbFactory.newDocumentBuilder();

            System.out.println(channel.isOpen());
            doc = dBuilder.parse(Channels.newInputStream(channel));
            System.out.println(channel.isOpen());

            channel.close();
        } catch (IOException | ParserConfigurationException | SAXException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args){
        new Test4();
    }
}

Output:

true
false

Having looked through the documentation and trawled the built in Java libraries, I am really struggling to even find where the channel is closed, let alone how to prevent it closing. Any pointers would be great! Thanks.

Answer 1

One clean way to do this is to create a FilterInputStream and override the close to do nothing:

public Test() {
    try {
        channel = new RandomAccessFile(new File(path), "rw").getChannel();
        dbFactory = DocumentBuilderFactory.newInstance();
        dBuilder = dbFactory.newDocumentBuilder();

        System.out.println(channel.isOpen());
        NonClosingInputStream ncis = new NonClosingInputStream(Channels.newInputStream(channel));
        doc = dBuilder.parse(ncis);
        System.out.println(channel.isOpen());
        // Closes here.
        ncis.reallyClose();
        channel.close(); //Redundant
    } catch (IOException | ParserConfigurationException | SAXException e) {
        e.printStackTrace();
    }
}

class NonClosingInputStream extends FilterInputStream {

    public NonClosingInputStream(InputStream it) {
        super(it);
    }

    @Override
    public void close() throws IOException {
        // Do nothing.
    }

    public void reallyClose() throws IOException {
        // Actually close.
        in.close();
    }
}