How to Read a File into a String in Java: Best Methods Explained

Reading a file’s contents into a string is a common task in Java programming. Whether you’re handling configuration files, parsing data, or processing text inputs, knowing the most efficient methods can enhance performance and code maintainability. This guide explores various approaches to reading a file into a Java string, using built-in Java APIs and popular libraries. We’ll also discuss memory management, character encoding, and best practices.

Why Read a File into a String?

Before diving into the code, let’s understand why this task is essential:

• Simplifies text processing – Enables easy manipulation (e.g., splitting, replacing, searching).
• Unified data handling – Converts files (CSV, JSON, logs) into strings for parsing.
• Cross-platform compatibility – Handles different line terminators (\n vs. \r\n) consistently.

Methods to Read a File into a String in Java

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

Java 11 introduced Files.readString(), a concise method for small files:

import java.nio.file.*;
import java.io.IOException;

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

Pros:

• Simple and readable.
• Preserves line terminators. Cons:
• Loads the entire file into memory, making it unsuitable for large files.

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

For Java 7 and later, Files.readAllBytes() combined with a String constructor is a viable alternative:

import java.nio.file.*;
import java.io.IOException;
import java.nio.charset.StandardCharsets;

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

Best for: Projects using Java 7–10. Note: Always specify character encoding to prevent platform-dependent issues.

3. Using Apache Commons IO (External Library)

If you prefer external libraries, Apache Commons IO simplifies file reading:

import org.apache.commons.io.FileUtils;
import java.io.File;
import java.io.IOException;

public class ReadFileToString {
    public static void main(String[] args) throws IOException {
        String content = FileUtils.readFileToString(new File("file.txt"), "UTF-8");
    }
}

Pros:

• Reduces boilerplate code.
• Automatically handles file closure. Cons:
• Requires an external dependency.

4. Using BufferedReader with StringBuilder (Java 6 and Older)

For legacy Java versions, BufferedReader is a memory-efficient choice:

import java.io.*;
import java.nio.charset.StandardCharsets;

public class ReadFileToString {
    static String readFile(String filePath) throws IOException {
        StringBuilder content = new StringBuilder();
        try (BufferedReader reader = new BufferedReader(
                new InputStreamReader(new FileInputStream(filePath), StandardCharsets.UTF_8))) {
            String line;
            while ((line = reader.readLine()) != null) {
                content.append(line).append(System.lineSeparator());
            }
        }
        return content.toString();
    }
}

Pros:

• Efficient for large files (processes line by line).
• Works in older Java versions. Cons:
• More verbose compared to newer alternatives.

5. Using Scanner (Alternative Approach)

The Scanner class can read an entire file using a delimiter:

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

public class ReadFileToString {
    public static void main(String[] args) throws IOException {
        try (Scanner scanner = new Scanner(new File("file.txt"), "UTF-8")) {
            String content = scanner.useDelimiter("\\A").next();
            System.out.println(content);
        }
    }
}

Note: The \\A regex pattern matches the start of input, reading all content at once.

Key Considerations When Reading Files

1. Character Encoding
   Always specify an encoding (e.g., UTF-8) to avoid compatibility issues across different systems.
2. Memory Management
   • Use Files.readString() or Files.readAllBytes() for small files.
   • Stream large files line-by-line using BufferedReader or Files.lines().
3. Error Handling
   • Use try-with-resources (Java 7+) to ensure automatic resource cleanup.

Comparison of Methods

Conclusion

The best method to read a file into a string depends on Java version, file size, and project requirements:

• Use Files.readString() for modern Java applications (11+).
• If using Java 7–10, Files.readAllBytes() is a good alternative.
• Apache Commons IO provides a convenient third-party option.
• For large files, use BufferedReader to avoid memory issues.

By following these best practices, you can write efficient, robust, and portable Java file-handling code.