Java NIO Read File Example

Learn to read a file from the filesystem using the Java NIO APIs i.e using buffer, channel, and path classes.

1. FileChannel and ByteBuffer to read small files

Use this technique to read a small file where all the file content is fits into the buffer, and the file can be read in a single operation.

Example 1: Java read small file using ByteBuffer and RandomAccessFile

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.flip();

			//Verify the file content
			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. FileChannel and ByteBuffer to read large files

Use this technique to read the large file where all the file content will not fit into the buffer at a time, the buffer size will be needed of some very big number. In this case, we can read the file in chunks with a fixed size small buffer.

Example 2: Java 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. Read a file using MappedByteBuffer

MappedByteBuffer extends the ByteBuffer class with operations that are specific to memory-mapped file regions.

Example 3: Reading a file using memory-mapped files in Java

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 the above techniques will read the content of the file and print it to the console.

Happy Learning !!