NIO2 AsynchronousFileChannel

package com.java7developer.chapter2;

import java.nio.*;
import java.nio.channels.*;
import java.nio.file.*;
import java.io.IOException;

public class Nio2ReadLargeLogFile implements
        CompletionHandler<Integer, AsynchronousFileChannel> {

    // need to keep track of the next position.
    int pos = 0;
    AsynchronousFileChannel channel = null;
    ByteBuffer buffer = null;

    public void completed(Integer result, AsynchronousFileChannel attachment) {
//        System.out.println("bytes num " + result);
        // if result is -1 means nothing was read.
        if (result != -1) {
            pos += result; // don't read the same text again.
                            // your output command.
            //这里必须读取0,result直接的buffer，否则会读取到垃圾数据
            System.out.print(new String(buffer.array(),0,result));

//            buffer.clear(); // reset the buffer so you can read more.
            buffer.flip();
            attachment.read(buffer, pos, attachment, this);
        } else {
            return;
        }
        // initiate another asynchronous read, with this.

    }

    public void failed(Throwable exc, AsynchronousFileChannel attachment) {
        System.err.println("Error!");
        exc.printStackTrace();
    }

    public void doit() {
        Path file = Paths.get("c:/a");
        AsynchronousFileChannel channel = null;
        try {
            channel = AsynchronousFileChannel.open(file);
        } catch (IOException e) {
            System.err.println("Could not open file: " + file.toString());
            System.exit(1); // yeah. heh.
        }
        buffer = ByteBuffer.allocate(1000);

        // start off the asynch read.
        channel.read(buffer, pos, channel, this);
        // this method now exits, thread returns to main and waits for user
        // input.
    }

    public static void main(String[] args) {
        Nio2ReadLargeLogFile tn = new Nio2ReadLargeLogFile();
        tn.doit();
        // wait fur user to press a key otherwise java exits because the
        // asynch thread isn't important enough to keep it running.
        try {
            System.in.read();
        } catch (IOException e) {
        }
    }
}