[java] How can I read a large text file line by line using Java?

I need to read a large text file of around 5-6 GB line by line using Java.

How can I do this quickly?

You can use streams to do it more precisely:

Files.lines(Paths.get("input.txt")).forEach(s -> stringBuffer.append(s);

For reading a file with Java 8

package com.java.java8;

import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Stream;

/**
 * The Class ReadLargeFile.
 *
 * @author Ankit Sood Apr 20, 2017
 */
public class ReadLargeFile {

    /**
     * The main method.
     *
     * @param args
     *            the arguments
     */
    public static void main(String[] args) {
        try {
            Stream<String> stream = Files.lines(Paths.get("C:\\Users\\System\\Desktop\\demoData.txt"));
            stream.forEach(System.out::println);
        }
        catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Java 9:

try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
    stream.forEach(System.out::println);
}

In Java 8, there is also an alternative to using Files.lines(). If your input source isn't a file but something more abstract like a Reader or an InputStream, you can stream the lines via the BufferedReaders lines() method.

For example:

try (BufferedReader reader = new BufferedReader(...)) {
  reader.lines().forEach(line -> processLine(line));
}

will call processLine() for each input line read by the BufferedReader.

I documented and tested 10 different ways to read a file in Java and then ran them against each other by making them read in test files from 1KB to 1GB. Here are the fastest 3 file reading methods for reading a 1GB test file.

Note that when running the performance tests I didn't output anything to the console since that would really slow down the test. I just wanted to test the raw reading speed.

1) java.nio.file.Files.readAllBytes()

Tested in Java 7, 8, 9. This was overall the fastest method. Reading a 1GB file was consistently just under 1 second.

import java.io..File;
import java.io.IOException;
import java.nio.file.Files;

public class ReadFile_Files_ReadAllBytes {
  public static void main(String [] pArgs) throws IOException {
    String fileName = "c:\\temp\\sample-1GB.txt";
    File file = new File(fileName);

    byte [] fileBytes = Files.readAllBytes(file.toPath());
    char singleChar;
    for(byte b : fileBytes) {
      singleChar = (char) b;
      System.out.print(singleChar);
    }
  }
}

2) java.nio.file.Files.lines()

This was tested successfully in Java 8 and 9 but it won't work in Java 7 because of the lack of support for lambda expressions. It took about 3.5 seconds to read in a 1GB file which put it in second place as far as reading larger files.

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.util.stream.Stream;

public class ReadFile_Files_Lines {
  public static void main(String[] pArgs) throws IOException {
    String fileName = "c:\\temp\\sample-1GB.txt";
    File file = new File(fileName);

    try (Stream linesStream = Files.lines(file.toPath())) {
      linesStream.forEach(line -> {
        System.out.println(line);
      });
    }
  }
}

3) BufferedReader

Tested to work in Java 7, 8, 9. This took about 4.5 seconds to read in a 1GB test file.

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

public class ReadFile_BufferedReader_ReadLine {
  public static void main(String [] args) throws IOException {
    String fileName = "c:\\temp\\sample-1GB.txt";
    FileReader fileReader = new FileReader(fileName);

    try (BufferedReader bufferedReader = new BufferedReader(fileReader)) {
      String line;
      while((line = bufferedReader.readLine()) != null) {
        System.out.println(line);
      }
    }
  }

You can find the complete rankings for all 10 file reading methods here.

FileReader won't let you specify the encoding, use InputStreamReaderinstead if you need to specify it:

try {
    BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(filePath), "Cp1252"));         

    String line;
    while ((line = br.readLine()) != null) {
        // process the line.
    }
    br.close();

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

If you imported this file from Windows, it might have ANSI encoding (Cp1252), so you have to specify the encoding.

In Java 8, you could do:

try (Stream<String> lines = Files.lines (file, StandardCharsets.UTF_8))
{
    for (String line : (Iterable<String>) lines::iterator)
    {
        ;
    }
}

Some notes: The stream returned by Files.lines (unlike most streams) needs to be closed. For the reasons mentioned here I avoid using forEach(). The strange code (Iterable<String>) lines::iterator casts a Stream to an Iterable.

I usually do the reading routine straightforward:

void readResource(InputStream source) throws IOException {
    BufferedReader stream = null;
    try {
        stream = new BufferedReader(new InputStreamReader(source));
        while (true) {
            String line = stream.readLine();
            if(line == null) {
                break;
            }
            //process line
            System.out.println(line)
        }
    } finally {
        closeQuiet(stream);
    }
}

static void closeQuiet(Closeable closeable) {
    if (closeable != null) {
        try {
            closeable.close();
        } catch (IOException ignore) {
        }
    }
}

What you can do is scan the entire text using Scanner and go through the text line by line. Of course you should import the following:

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
public static void readText throws FileNotFoundException {
    Scanner scan = new Scanner(new File("samplefilename.txt"));
    while(scan.hasNextLine()){
        String line = scan.nextLine();
        //Here you can manipulate the string the way you want
    }
}

Scanner basically scans all the text. The while loop is used to traverse through the entire text.

The .hasNextLine() function is a boolean that returns true if there are still more lines in the text. The .nextLine() function gives you an entire line as a String which you can then use the way you want. Try System.out.println(line) to print the text.

Side Note: .txt is the file type text.

The clear way to achieve this,

For example:

If you have dataFile.txt on your current directory

import java.io.*;
import java.util.Scanner;
import java.io.FileNotFoundException;

public class readByLine
{
    public readByLine() throws FileNotFoundException
    {
        Scanner linReader = new Scanner(new File("dataFile.txt"));

        while (linReader.hasNext())
        {
            String line = linReader.nextLine();
            System.out.println(line);
        }
        linReader.close();

    }

    public static void main(String args[])  throws FileNotFoundException
    {
        new readByLine();
    }
}

The output like as below,

BufferedReader br;
FileInputStream fin;
try {
    fin = new FileInputStream(fileName);
    br = new BufferedReader(new InputStreamReader(fin));

    /*Path pathToFile = Paths.get(fileName);
    br = Files.newBufferedReader(pathToFile,StandardCharsets.US_ASCII);*/

    String line = br.readLine();
    while (line != null) {
        String[] attributes = line.split(",");
        Movie movie = createMovie(attributes);
        movies.add(movie);
        line = br.readLine();
    }
    fin.close();
    br.close();
} catch (FileNotFoundException e) {
    System.out.println("Your Message");
} catch (IOException e) {
    System.out.println("Your Message");
}

It works for me. Hope It will help you too.

You can also use Apache Commons IO:

File file = new File("/home/user/file.txt");
try {
    List<String> lines = FileUtils.readLines(file);
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

You can use Scanner class

Scanner sc=new Scanner(file);
sc.nextLine();

Once Java 8 is out (March 2014) you'll be able to use streams:

try (Stream<String> lines = Files.lines(Paths.get(filename), Charset.defaultCharset())) {
  lines.forEachOrdered(line -> process(line));
}

Printing all the lines in the file:

try (Stream<String> lines = Files.lines(file, Charset.defaultCharset())) {
  lines.forEachOrdered(System.out::println);
}

You need to use the readLine() method in class BufferedReader. Create a new object from that class and operate this method on him and save it to a string.

BufferReader Javadoc

You can use this code:

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/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();
        }

    }

}

In Java 7:

String folderPath = "C:/folderOfMyFile";
Path path = Paths.get(folderPath, "myFileName.csv"); //or any text file eg.: txt, bat, etc
Charset charset = Charset.forName("UTF-8");

try (BufferedReader reader = Files.newBufferedReader(path , charset)) {
  while ((line = reader.readLine()) != null ) {
    //separate all csv fields into string array
    String[] lineVariables = line.split(","); 
  }
} catch (IOException e) {
    System.err.println(e);
}

By using the org.apache.commons.io package, it gave more performance, especially in legacy code which uses Java 6 and below.

Java 7 has a better API with fewer exceptions handling and more useful methods:

LineIterator lineIterator = null;
try {
    lineIterator = FileUtils.lineIterator(new File("/home/username/m.log"), "windows-1256"); // The second parameter is optionnal
    while (lineIterator.hasNext()) {
        String currentLine = lineIterator.next();
        // Some operation
    }
}
finally {
    LineIterator.closeQuietly(lineIterator);
}

Maven

<!-- https://mvnrepository.com/artifact/commons-io/commons-io -->
<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.6</version>
</dependency>

Look at this blog:

• Java Read File Line by Line - Java Tutorial

The buffer size may be specified, or the default size may be used. The default is large enough for most purposes.

// Open the file
FileInputStream fstream = new FileInputStream("textfile.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(fstream));

String strLine;

//Read File Line By Line
while ((strLine = br.readLine()) != null)   {
  // Print the content on the console
  System.out.println (strLine);
}

//Close the input stream
fstream.close();

You can read file data line by line as below:

String fileLoc = "fileLocationInTheDisk";

List<String> lines = Files.lines(Path.of(fileLoc), StandardCharsets.UTF_8).collect(Collectors.toList());

A common pattern is to use

try (BufferedReader br = new BufferedReader(new FileReader(file))) {
    String line;
    while ((line = br.readLine()) != null) {
       // process the line.
    }
}

You can read the data faster if you assume there is no character encoding. e.g. ASCII-7 but it won't make much difference. It is highly likely that what you do with the data will take much longer.

EDIT: A less common pattern to use which avoids the scope of line leaking.

try(BufferedReader br = new BufferedReader(new FileReader(file))) {
    for(String line; (line = br.readLine()) != null; ) {
        // process the line.
    }
    // line is not visible here.
}

UPDATE: In Java 8 you can do

try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
        stream.forEach(System.out::println);
}

NOTE: You have to place the Stream in a try-with-resource block to ensure the #close method is called on it, otherwise the underlying file handle is never closed until GC does it much later.

Here is a sample with full error handling and supporting charset specification for pre-Java 7. With Java 7 you can use try-with-resources syntax, which makes the code cleaner.

If you just want the default charset you can skip the InputStream and use FileReader.

InputStream ins = null; // raw byte-stream
Reader r = null; // cooked reader
BufferedReader br = null; // buffered for readLine()
try {
    String s;
    ins = new FileInputStream("textfile.txt");
    r = new InputStreamReader(ins, "UTF-8"); // leave charset out for default
    br = new BufferedReader(r);
    while ((s = br.readLine()) != null) {
        System.out.println(s);
    }
}
catch (Exception e)
{
    System.err.println(e.getMessage()); // handle exception
}
finally {
    if (br != null) { try { br.close(); } catch(Throwable t) { /* ensure close happens */ } }
    if (r != null) { try { r.close(); } catch(Throwable t) { /* ensure close happens */ } }
    if (ins != null) { try { ins.close(); } catch(Throwable t) { /* ensure close happens */ } }
}

Here is the Groovy version, with full error handling:

File f = new File("textfile.txt");
f.withReader("UTF-8") { br ->
    br.eachLine { line ->
        println line;
    }
}