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

Question

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

How can I do this quickly?

Answer 1

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.