使用NIO FileChannel将文本文件读取到字符串列表

Question

I'm trying to read an entire text file into list of strings using the non-blocking FileChannel. Using regular IO it's as simple as:

List<String> lines = null;
    try {
        lines = Files.readAllLines(FILE);
    } catch (IOException ioe) {
        ioe.printStackTrace();
    }

My attempt using NIO and FileChannel looks something like this:

List<String> words = null;

try {
    FileInputStream stream = new FileInputStream(FILE);
    FileChannel channel = stream.getChannel();
    ByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, (int) channel.size());
    buffer.flip();
    byte[] bytes = new byte[buffer.remaining()];
    buffer.get(bytes);
    BufferedReader reader = new BufferedReader(new InputStreamReader(new ByteArrayInputStream(bytes)));
    for (String line = reader.readLine(); line != null; line = reader.readLine())
        words.add(line);
    reader.close();
    buffer.clear();
    channel.close();
    stream.close();
    for (String word : words)
        System.out.println(word);
} catch (IOException ioe) {
    ioe.printStackTrace();
}

I map the entire file to a ByteBuffer, read the ByteBuffer into a byte array, then try to read the byte array using a BufferedReader, but when I try to print the contents of the List i get a NullPointerException.

What is the proper way to go about doing this?

Answer 1

Besides @Slaw 's point that this code is not using non-blocking IO, JavaDoc for FileChannel.map states:

The mapped byte buffer returned by this method will have a position of zero and a limit and capacity of size; its mark will be undefined.

After commenting the flip operation and initializing the words list, the code worked as I expected.

public static void main(String[] args) {
    List<String> words = new ArrayList<>();
    File FILE = new File("SOME_FILE_OF_LINES");

    try {
        FileInputStream stream = new FileInputStream(FILE);
        FileChannel channel = stream.getChannel();
        ByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, (int) channel.size());
//          buffer.flip();
        byte[] bytes = new byte[buffer.remaining()];
        buffer.get(bytes);
        BufferedReader reader = new BufferedReader(new InputStreamReader(new ByteArrayInputStream(bytes)));
        for (String line = reader.readLine(); line != null; line = reader.readLine())
            words.add(line);
        reader.close();
        buffer.clear();
        channel.close();
        stream.close();
        for (String word : words)
            System.out.println(word);
    } catch (IOException ioe) {
        ioe.printStackTrace();
    }
}