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?

Solution 1

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.

Solution 2

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();

Solution 3

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);
}

Solution 4

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;
    }
}

Solution 5

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.

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