3 Ways to Read Files – Java NIO

New I/O, usually called NIO, is a collection of APIs that offer additional capabilities for intensive I/O operations. It was introduced with the Java 1.4 release by Sun Microsystems to complement an existing standard I/O. The extended NIO that offers further new file system APIs, called NIO2, was released with Java SE 7 (“Dolphin”).

NIO related questions are very popular in java interviews now-a-days.

NIO2 provides two major methods of reading a file:

• Using buffer and channel classes
• Using Path and Files classes

In this post, I am showing a couple of ways to read a file from file system. So lets start them by first showing old famous approach first, so that we can see what really changed.

Old famous I/O way

This example shows how we have been reading a text file using old I/O library APIs. It uses a BufferedReader object for reading. Another way can be using InputStream implementation.

package com.howtodoinjava.test.nio;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class WithoutNIOExample
{
	public static void main(String[] args)
	{
		BufferedReader br = null;
		String sCurrentLine = null;
		try
		{
			br = new BufferedReader(
			new FileReader("test.txt"));
			while ((sCurrentLine = br.readLine()) != null)
			{
				System.out.println(sCurrentLine);
			}
		}
		catch (IOException e)
		{
			e.printStackTrace();
		}
		finally
		{
			try
			{
				if (br != null)
				br.close();
			} catch (IOException ex)
			{
				ex.printStackTrace();
			}
		}
	}
}

1) Read a small file in buffer of file size

package com.howtodoinjava.test.nio;

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

public class ReadFileWithFileSizeBuffer
{
	public static void main(String args[])
	{
		try 
		{
			RandomAccessFile aFile = new RandomAccessFile(
							"test.txt","r");
			FileChannel inChannel = aFile.getChannel();
			long fileSize = inChannel.size();
			ByteBuffer buffer = ByteBuffer.allocate((int) fileSize);
			inChannel.read(buffer);
			//buffer.rewind();
			buffer.flip();
			for (int i = 0; i < fileSize; i++)
			{
				System.out.print((char) buffer.get());
			}
			inChannel.close();
			aFile.close();
		} 
		catch (IOException exc)
		{
			System.out.println(exc);
			System.exit(1);
		}
	}
}

2) Read a large file in chunks with fixed size buffer

package com.howtodoinjava.test.nio;

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

public class ReadFileWithFixedSizeBuffer 
{
	public static void main(String[] args) throws IOException 
	{
		RandomAccessFile aFile = new RandomAccessFile
				("test.txt", "r");
	    FileChannel inChannel = aFile.getChannel();
	    ByteBuffer buffer = ByteBuffer.allocate(1024);
	    while(inChannel.read(buffer) > 0)
	    {
	    	buffer.flip();
	    	for (int i = 0; i < buffer.limit(); i++)
			{
				System.out.print((char) buffer.get());
			}
	    	buffer.clear(); // do something with the data and clear/compact it.
	    }
	    inChannel.close();
	    aFile.close();
	}
}

3) Faster file copy with mapped byte buffer

package com.howtodoinjava.test.nio;

import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

public class ReadFileWithMappedByteBuffer 
{
	public static void main(String[] args) throws IOException 
	{
		RandomAccessFile aFile = new RandomAccessFile
				("test.txt", "r");
	    FileChannel inChannel = aFile.getChannel();
	    MappedByteBuffer buffer = inChannel.map(FileChannel.MapMode.READ_ONLY, 0, inChannel.size());
	    buffer.load();	
    	for (int i = 0; i < buffer.limit(); i++)
		{
			System.out.print((char) buffer.get());
		}
	    buffer.clear(); // do something with the data and clear/compact it.
	    inChannel.close();
	    aFile.close();
	}
}

All above techniques will read the content of file and print it to console. You can do whatever you want once you have read it.

Happy Learning !!