How do I read a file into a string in Java?

General Discussions
1

I’ve been using the following approach to read a file into a string in Java:

private String readFile(String file) throws IOException {
    BufferedReader reader = new BufferedReader(new FileReader(file));
    StringBuilder stringBuilder = new StringBuilder();
    String line;
    String ls = System.getProperty("line.separator");

    try {
        while ((line = reader.readLine()) != null) {
            stringBuilder.append(line).append(ls);
        }
        return stringBuilder.toString();
    } finally {
        reader.close();
    }
}

Is there a better or more efficient way to read file to string in Java, possibly using modern APIs?

2

If you’re using Java 11 or later, the cleanest way to read a file into a string is with Files.readString(). It does everything in a single line and handles the file encoding properly:

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

public class FileReaderExample {
    public static String readFile(String filePath) throws IOException {
        return Files.readString(Path.of(filePath));
    }
}

:white_check_mark: Why is this better?

  • No need to manually read line by line.
  • It automatically closes the file.
  • It’s concise and efficient for modern Java applications.

:rotating_light: Caveat:

Files.readString() reads the entire file into memory, so it may not be ideal for very large files.

3

If you want more control over memory usage, you can use Files.readAllLines(). This method reads the file line by line and stores it in a List before joining it into a single string:

import java.nio.file.Files;
import java.nio.file.Path;
import java.io.IOException;
import java.util.List;

public class FileReaderExample {
    public static String readFile(String filePath) throws IOException {
        List<String> lines = Files.readAllLines(Path.of(filePath));
        return String.join(System.lineSeparator(), lines);
    }
}

:white_check_mark: Why use this?

It doesn’t require a loop.

Keeps lines stored in a list, allowing further processing if needed.

:rotating_light: Caveat:

Like Files.readString(), this method loads everything into memory, so for huge files, a streaming approach is better.

4

For reading large files while keeping memory usage low, you can use Scanner. It reads line by line, which avoids storing the whole file in memory at once:

import java.io.File;
import java.io.IOException;
import java.util.Scanner;

public class FileReaderExample {
    public static String readFile(String filePath) throws IOException {
        StringBuilder content = new StringBuilder();
        try (Scanner scanner = new Scanner(new File(filePath))) {
            while (scanner.hasNextLine()) {
                content.append(scanner.nextLine()).append(System.lineSeparator());
            }
        }
        return content.toString();
    }
}

:white_check_mark: Why use this?

  • More memory-efficient than Files.readString().
  • Works well for very large files.
  • Auto-closes the scanner using the try-with-resources approach.

:rotating_light: Caveat:

  • Doesn’t handle different character encodings as easily as Files.readString().
  • Doesn’t include empty lines by default—Scanner.nextLine() skips them unless handled explicitly.