Reading a File into a Java String: Modern Techniques

Java provides multiple ways to read the entire contents of a file into a single String. From traditional methods using BufferedReader to modern approaches with NIO utilities, there’s a technique to suit different Java versions and project requirements. Here’s a guide to some of the most popular methods, including options for Java 7 and newer, as well as some third-party alternatives.

1. Using Files.readString() in Java 11+

For Java 11 and later, the Files.readString() method offers a simple and efficient way to read the entire content of a file into a String. This method preserves line terminators.

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.charset.StandardCharsets;

public class FileReaderExample {
    public static void main(String[] args) throws IOException {
        String content = Files.readString(Path.of("example.txt"), StandardCharsets.UTF_8);
        System.out.println(content);
    }
}

This approach is concise and avoids handling byte arrays manually.

2. Using Files.readAllBytes() (Java 7+)

In Java 7 and later, Files.readAllBytes() provides a way to read a file’s content as a byte array, which can then be converted to a String.

import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.charset.StandardCharsets;

public class FileReaderExample {
    public static String readFile(String filePath) throws IOException {
        byte[] bytes = Files.readAllBytes(Paths.get(filePath));
        return new String(bytes, StandardCharsets.UTF_8);
    }

    public static void main(String[] args) throws IOException {
        String content = readFile("example.txt");
        System.out.println(content);
    }
}

This method is highly compatible and straightforward but doesn’t include line terminators.

3. Using Files.lines() with Java Streams (Java 8+)

Java 8 introduced the Files.lines() method, which returns a Stream<String>. This allows line-by-line processing and can be useful when the file is too large to hold in memory as a single String.

import java.nio.file.Files;
import java.nio.file.Path;
import java.io.IOException;
import java.util.stream.Collectors;

public class FileReaderExample {
    public static String readFile(String filePath) throws IOException {
        try (var lines = Files.lines(Path.of(filePath))) {
            return lines.collect(Collectors.joining(System.lineSeparator()));
        }
    }

    public static void main(String[] args) throws IOException {
        String content = readFile("example.txt");
        System.out.println(content);
    }
}

Using Stream for file reading ensures that the resources are managed automatically and allows for custom processing of each line.

4. Using BufferedReader with StringBuilder

For more control, you can use BufferedReader and StringBuilder to read a file line-by-line. This method is compatible with all Java versions and allows appending line terminators explicitly.

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

public class FileReaderExample {
    public static String readFile(String filePath) throws IOException {
        StringBuilder content = new StringBuilder();
        try (BufferedReader reader = new BufferedReader(new FileReader(filePath))) {
            String line;
            while ((line = reader.readLine()) != null) {
                content.append(line).append(System.lineSeparator());
            }
        }
        return content.toString();
    }

    public static void main(String[] args) throws IOException {
        String content = readFile("example.txt");
        System.out.println(content);
    }
}

This method is versatile and compatible with older Java versions, though slightly more verbose than Files methods.

5. Using Apache Commons IO

If your project includes Apache Commons IO, FileUtils from this library provides a convenient one-liner for reading files.

import org.apache.commons.io.FileUtils;
import java.io.File;
import java.io.IOException;
import java.nio.charset.StandardCharsets;

public class FileReaderExample {
    public static String readFile(String filePath) throws IOException {
        return FileUtils.readFileToString(new File(filePath), StandardCharsets.UTF_8);
    }

    public static void main(String[] args) throws IOException {
        String content = readFile("example.txt");
        System.out.println(content);
    }
}

Using Apache Commons IO simplifies file operations but requires adding an external dependency.

6. Using Scanner with Delimiter

The Scanner class offers another way to read a file in a single line of code, by setting a delimiter to match the entire file.

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

public class FileReaderExample {
    public static String readFile(String filePath) throws IOException {
        try (Scanner scanner = new Scanner(new File(filePath), "UTF-8")) {
            return scanner.useDelimiter("\\A").next();
        }
    }

    public static void main(String[] args) throws IOException {
        String content = readFile("example.txt");
        System.out.println(content);
    }
}

Using Scanner is efficient for small files, but it’s important to handle closing properly, either with try-with-resources or explicitly.

Each of these methods offers different benefits, depending on your Java version and specific requirements:

• Files.readString() (Java 11+): Simple and effective for small to medium files.
• Files.readAllBytes() (Java 7+): Flexible and compatible across many versions.
• Files.lines() (Java 8+): Efficient for large files or stream-based processing.
• BufferedReader: Provides control and works with older Java versions.
• Apache Commons IO: Quick one-liner if you’re using external libraries.
• Scanner: Convenient but best suited for small files.

Choose the approach that best fits your needs and Java environment to streamline file reading in your applications.