Java Read a text file line by line

This article covers 3 ways to read a text file line by line :

1. Java NIO libraries – FileChannel to read the files. It is a non-blocking mode of reading files
2. BufferedReader – Which is a blocking operation i.e it blocks any other read/write request to the file until it has completed the task.
3. Apache Commons IO – FileUtils, simpler way to read files line by line. It is also a blocking operation.

The input file for each example is same as the one shown below:

1. Java NIO libraries

The example presents the simplest way of reading a small file. Since this is a small file, we directly allocate required memory to ByteBuffer using FileChannel size.

package com.mkyong;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
public class NIOFileReadExample {
    public static void main(String[] args) throws IOException {
        RandomAccessFile file = new RandomAccessFile("src/com/mkyong/data.txt", "r");
        FileChannel channel = file.getChannel();
        System.out.println("File size is: " + channel.size());
        ByteBuffer buffer = ByteBuffer.allocate((int) channel.size());
        channel.read(buffer);
        buffer.flip();//Restore buffer to position 0 to read it
        System.out.println("Reading content and printing ... ");
        for (int i = 0; i < channel.size(); i++) {
            System.out.print((char) buffer.get());
        channel.close();
        file.close();

On executing the code you get the below output:

File size is: 19
Reading content and printing ... 

2. BufferedReader

package com.mkyong;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
public class ReadTextFile {
    public static void main(String[] args) throws IOException {
        try {
            File f = new File("src/com/mkyong/data.txt");
            BufferedReader b = new BufferedReader(new FileReader(f));
            String readLine = "";
            System.out.println("Reading file using Buffered Reader");
            while ((readLine = b.readLine()) != null) {
                System.out.println(readLine);
        } catch (IOException e) {
            e.printStackTrace();

The output on executing this code will be as shown below:

Reading file using Buffered Reader

3. Apache Commons IO

  <dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.5</version>
  </dependency>

package com.mkyong;
import org.apache.commons.io.FileUtils;
import java.io.File;
import java.io.IOException;
import java.util.List;
public class ReadTextFile {
    public static void main(String[] args) throws IOException {
        try {
            File f = new File("src/com/mkyong/data.txt");
            System.out.println("Reading files using Apache IO:");
            List<String> lines = FileUtils.readLines(f, "UTF-8");
            for (String line : lines) {
                System.out.println(line);
        } catch (IOException e) {
            e.printStackTrace();

The output on executing above code will be:

Reading files using Apache IO:

References

1. More about BufferedReader
2. More about FileReader
3. BufferedReader examples
4. Java 8 stream to read file line by line
5. More about FileChannel functions
6. Difference between ByteBuffer flip() and rewind() methods
7. ByteBuffer javadocs
8. FileChannel Javadocs
9. Apache Commons IO user guide