Java Read File to String (with Examples)

During Java application development, we need to read the contents of a file into a string in many situations such as processing configuration files, parsing data, or handling text-based resources.

In this Java tutorial, we will explore different ways to read a text file into String in Java from traditional BufferedReader, new APIs in Java 8 and 11 to third-party libraries such as Apache Commons Lang and Guava.

1. Using Files.readString() – Java 11

With the new method readString() introduced in Java 11, it takes only a single line to read a file’s content into String using the UTF-8 charset.

• In case of any error during the read operation, this method ensures that the file is properly closed.
• It throws OutOfMemoryError if the file is extremely large, for example, larger than 2GB.

Path filePath = Path.of("c:/temp/demo.txt");

String content = Files.readString(fileName);

2. Using Files.lines() – Java 8

The lines() method reads all lines from a file into a Stream. The Stream is populated lazily when the stream is consumed.

• Bytes from the file are decoded into characters using the specified charset.
• The returned stream contains a reference to an open file. The file is closed by closing the stream.
• The file contents should not be modified during the reading process, or else the result is undefined.

Path filePath = Path.of("c:/temp/demo.txt");
StringBuilder contentBuilder = new StringBuilder();

try (Stream<String> stream = Files.lines(Paths.get(filePath), StandardCharsets.UTF_8)) {

  stream.forEach(s -> contentBuilder.append(s).append("\n"));
} catch (IOException e) {
  //handle exception
}

String fileContent = contentBuilder.toString();

3. Using Files.readAllBytes() – Java 7

The readAllBytes() method reads all the bytes from a file into a byte[]. Do not use this method for reading large files.

This method ensures that the file is closed when all bytes have been read or an I/O error, or other runtime exception, is thrown. After reading all the bytes, we pass those bytes to String class constructor to create a new String.

Path filePath = Path.of("c:/temp/demo.txt");
String fileContent = "";

try {

    byte[] bytes = Files.readAllBytes(Paths.get(filePath));
    fileContent = new String (bytes);
} catch (IOException e) {
    //handle exception
}

4. Using BufferedReader – Java 6

If you are still not using Java 7 or later, then use BufferedReader class. Its readLine() method reads the file one line at a time and returns the content.

Path filePath = Path.of("c:/temp/demo.txt");
String fileContent = "";
StringBuilder contentBuilder = new StringBuilder();

try (BufferedReader br = new BufferedReader(new FileReader(filePath))) {

    String sCurrentLine;
    while ((sCurrentLine = br.readLine()) != null) 
    {
        contentBuilder.append(sCurrentLine).append("\n");
    }
} catch (IOException e) {
    e.printStackTrace();
}

fileContent = contentBuilder.toString();

5. Commons IO’s FileUtils

We can use the utility classes provided by the Apache Commons IO library. The FileUtils.readFileToString() is an excellent way to read a whole file into a String in a single statement.

File file = new File("c:/temp/demo.txt");

String content = FileUtils.readFileToString(file, "UTF-8");

6. Guava’s Files

Guava also provides Files class that can be used to read the file content in a single statement.

File file = new File("c:/temp/demo.txt");

String content = com.google.common.io.Files.asCharSource(file, Charsets.UTF_8).read();

Use any of the above-given methods for reading a file into a string using Java.

Happy Learning !!

Source Code on Github