How to read text file contents line by line?

In the following code example you will learn how to read file contents line by line using the Apache Commons FileUtils.lineIterator() method. Reading file contents one line at a time, do some processing, and release it from memory immediately will lower the memory consumption used by your program.

The snippet below give you the basic usage of the FileUtils.lineIterator() method. You pass the file to read and the encoding to use. An Iterator of the lines in the file will be returned. Use the hasNext() method to see if there are lines to read from the iterator. The nextLine() method will give you the next line from the file.

When we finished with the iterator we need to close it using the LineIterator.close() or LineIterator.closeQuietly() method.

package org.kodejava.example.commons.io;

import org.apache.commons.io.FileUtils;
import org.apache.commons.io.LineIterator;

import java.io.File;
import java.net.URL;
import java.util.Objects;

public class ReadFileLineByLine {
    public static void main(String[] args) throws Exception {
        // Load file from resource directory.
        ClassLoader classLoader = ReadFileLineByLine.class.getClassLoader();
        URL url = Objects.requireNonNull(classLoader.getResource("data.txt"),
                "Resource could not be found.");

        File file = new File(url.getFile());
        LineIterator iterator = FileUtils.lineIterator(file, "UTF-8");
        try {
            while (iterator.hasNext()) {
                String line = iterator.nextLine();
                System.out.println("line = " + line);
            }
        } finally {
            LineIterator.closeQuietly(iterator);
        }
    }
}

In the example above we load the file from a resource directory. That’s why we use the ClassLoader.getResource() method. If you want to load a file from an absolute path you can simply create a File object and pass the absolute path to the file.

Maven Dependency

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

How do I create File object from URL?

The code snippet below use the FileUtils.toFile(URL) method that can be found in the commons-io library to convert a URL into a File. The url protocol should be file or else null will be returned.

package org.kodejava.example.commons.io;

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.net.URL;

public class URLToFileObject {
    public static void main(String[] args) throws Exception {
        // FileUtils.toFile(URL url) convert from URL the File.
        String data = FileUtils.readFileToString(FileUtils.toFile(
                URLToFileObject.class.getResource("/data.txt")));
        System.out.println("data = " + data);

        // Creates a URL with file protocol and convert it into File object.
        File file = FileUtils.toFile(new URL("file://D:/demo.txt"));
        data = FileUtils.readFileToString(file);
        System.out.println("data = " + data);
    }
}

Maven Dependencies

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.4</version>
</dependency>

How do I copy a URL into a file?

The code snippet below show you how to use the FileUtils.copyURLToFile(URL, File) method of the Apache Commons IO library to help you to copy the contents of a URL directly into a file.

package org.kodejava.example.commons.io;

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.io.IOException;
import java.net.URL;

public class URLToFile {
    public static void main(String[] args) {
        try {
            URL url = new URL("//www.google.com");
            File destination = new File("google.html");

            // Copy bytes from the URL to the destination file.
            FileUtils.copyURLToFile(url, destination);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.4</version>
</dependency>

How do I create a human-readable file size?

This example demonstrate how to create a human-readable file size using the FileUtils class of the Apache Commons IO library. The byteCountToDisplaySize() method take the file size in bytes and return a human-readable display of the file size in bytes, kilobytes, megabytes, gigabytes, etc.

package org.kodejava.example.commons.io;

import org.apache.commons.io.FileUtils;

import java.io.File;

public class ReadableFileSize {
    public static void main(String[] args) {
        File file = new File("/Users/kodejava/Downloads/Java/JDK/jdk-6u7-linux-i586.bin");

        long size = file.length();
        String display = FileUtils.byteCountToDisplaySize(size);

        System.out.println("Name    = " + file.getName());
        System.out.println("Size    = " + size);
        System.out.println("Display = " + display);
    }
}

Here are the result of our program:

Name    = jdk-6u7-linux-i586.bin
Size    = 78482905
Display = 74 MB

How do I create a copy of a file?

This example shows how we can use the Apache Commons IO library to simplify a file copy process.

package org.kodejava.example.commons.io;

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.io.IOException;

public class FileCopyExample {
    public static void main(String[] args) {
        // The source file name to be copied.
        File source = new File("january.doc");

        // The target file name to which the source file will be copied.
        File target = new File("january-backup.doc");

        // A temporary folder where we are gonna copy the source file to.
        // Here we use the temporary folder of the OS, which can be
        // obtained using java.io.tmpdir property.
        File targetDir = new File(System.getProperty("java.io.tmpdir"));

        try {
            // Using FileUtils.copyFile() method to copy a file.
            System.out.println("Copying " + source + " file to " + target);
            FileUtils.copyFile(source, target);

            // To copy a file to a specified folder we can use the
            // FileUtils.copyFileToDirectory() method.
            System.out.println("Copying " + source + " file to " + targetDir);
            FileUtils.copyFileToDirectory(source, targetDir);
        } catch (IOException e) {
            // Errors will be reported here if any error occurs during 
            // copying the file
            e.printStackTrace();
        }
    }
}