java.nio Code Samples

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How do I use a FileChannel to read data into a Buffer?

The example below show how to use a FileChannel to read some data into a Buffer. We create a FileChannel from a FileInputStream instance. Because a channel reads data into buffer we need to create a ByteBuffer and set its capacity. Read data from a channel into buffer using the FileChannel.read() method.

To read out the data from the buffer we need to flip the buffer first using the Buffer.flip() method. The method will change the buffer from writing-mode into reading-mode. After the entire buffer is read clear the buffer using the clear() method call.

package org.kodejava.example.nio;

import java.io.FileInputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class FileRead {
    public static void main(String[] args) {
        try {
            String path = "C:\\Temp\\source.txt";
            FileInputStream fis = new FileInputStream(path);
            FileChannel fileChannel = fis.getChannel();

            ByteBuffer buffer = ByteBuffer.allocate(64);

            int bytesRead = fileChannel.read(buffer);
            while (bytesRead != -1) {
                System.out.println("Read: " + bytesRead);
                buffer.flip();

                while (buffer.hasRemaining()) {
                    System.out.print((char) buffer.get());                    
                }
                
                buffer.clear();
                bytesRead = fileChannel.read(buffer);
            }

            fis.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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