Java – Write a Reader to File

In this quick tutorial, we’re going to write the contents of a Reader to a File using plain Java, then Guava and finally the Apache Commons IO library.

This article is part of the “Java – Back to Basic” series here on VietMX’s Blog.

1. With Java

Let’s start with the simple Java solution:

@Test
public void givenUsingPlainJava_whenWritingReaderContentsToFile_thenCorrect() 
  throws IOException {
    Reader initialReader = new StringReader("Some text");

    int intValueOfChar;
    StringBuilder buffer = new StringBuilder();
    while ((intValueOfChar = initialReader.read()) != -1) {
        buffer.append((char) intValueOfChar);
    }
    initialReader.close();

    File targetFile = new File("src/test/resources/targetFile.txt");
    targetFile.createNewFile();

    Writer targetFileWriter = new FileWriter(targetFile);
    targetFileWriter.write(buffer.toString());
    targetFileWriter.close();
}

First – we’re reading the contents of the Reader into a String; then we’re simply writing the String to File.

2. With Guava

The Guava solution is simpler – we now have the API to deal with writing the reader to file:

@Test
public void givenUsingGuava_whenWritingReaderContentsToFile_thenCorrect() 
  throws IOException {
    Reader initialReader = new StringReader("Some text");

    File targetFile = new File("src/test/resources/targetFile.txt");
    com.google.common.io.Files.touch(targetFile);
    CharSink charSink = com.google.common.io.Files.
      asCharSink(targetFile, Charset.defaultCharset(), FileWriteMode.APPEND);
    charSink.writeFrom(initialReader);

    initialReader.close();
}

3. With Apache Commons IO

And finally, the Commons IO solution – also using higher level APIs to read data from the Reader and write that data to file:

@Test
public void givenUsingCommonsIO_whenWritingReaderContentsToFile_thenCorrect() 
  throws IOException {
    Reader initialReader = new CharSequenceReader("CharSequenceReader extends Reader");

    File targetFile = new File("src/test/resources/targetFile.txt");
    FileUtils.touch(targetFile);
    byte[] buffer = IOUtils.toByteArray(initialReader);
    FileUtils.writeByteArrayToFile(targetFile, buffer);

    initialReader.close();
}

And there we have it – 3 simple solutions for writing the contents of a Reader to File. Make sure to check out the sample over on GitHub.