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

How can I do this quickly?

5

22 Answers

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.

35

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();

4

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);
}

6

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; }
}

2

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.

4

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.

8

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.

2

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.

0

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);
}

5

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.

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(); } }
}

You can use Scanner class

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

5

Java 9:

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

5

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

1

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,

2

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 use streams to do it more precisely:

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

1

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) { } }
}

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

<!-- -->
<dependency> <groupId>commons-io</groupId> <artifactId>commons-io</artifactId> <version>2.6</version>
</dependency>

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(); } }
}

1

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();
}

1

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());

1