Java – Write to File

1. Overview

In this tutorial, we’ll explore different ways to write to a file using Java. We’ll make use of BufferedWriter, PrintWriter, FileOutputStream, DataOutputStream, RandomAccessFile, FileChannel, and the Java 7 Files utility class.

We’ll also look at locking the file while writing and discuss some final takeaways on writing to file.

This tutorial is part of the Java “Back to Basics” series here on Baeldung.

2. Write With BufferedWriter

Let’s start simple and use BufferedWriter to write a String to a new file:

public void whenWriteStringUsingBufferedWritter_thenCorrect() 
  throws IOException {
    String str = "Hello";
    BufferedWriter writer = new BufferedWriter(new FileWriter(fileName));
    writer.write(str);
    
    writer.close();
}

The output in the file will be:

Hello

We can then append a String to the existing file:

@Test
public void whenAppendStringUsingBufferedWritter_thenOldContentShouldExistToo() 
  throws IOException {
    String str = "World";
    BufferedWriter writer = new BufferedWriter(new FileWriter(fileName, true));
    writer.append(' ');
    writer.append(str);
    
    writer.close();
}

The file will then be:

Hello World

3. Write With PrintWriter

Next, let’s see how we can use PrintWriter to write formatted text to a file:

@Test
public void givenWritingStringToFile_whenUsingPrintWriter_thenCorrect() 
  throws IOException {
    FileWriter fileWriter = new FileWriter(fileName);
    PrintWriter printWriter = new PrintWriter(fileWriter);
    printWriter.print("Some String");
    printWriter.printf("Product name is %s and its price is %d $", "iPhone", 1000);
    printWriter.close();
}

The resulting file will contain:

Some String
Product name is iPhone and its price is 1000$

Note how we’re not only writing a raw String to a file, but also some formatted text with the printf method.

We can create the writer using FileWriter, BufferedWriter, or even System.out.

4. Write With FileOutputStream

Let’s now see how we can use FileOutputStream to write binary data to a file.

The following code converts a String into bytes and writes the bytes to a file using FileOutputStream:

@Test
public void givenWritingStringToFile_whenUsingFileOutputStream_thenCorrect() 
  throws IOException {
    String str = "Hello";
    FileOutputStream outputStream = new FileOutputStream(fileName);
    byte[] strToBytes = str.getBytes();
    outputStream.write(strToBytes);

    outputStream.close();
}

The output in the file will of course be:

Hello

5. Write With DataOutputStream

Next, let’s take a look at how we can use DataOutputStream to write a String to a file:

@Test
public void givenWritingToFile_whenUsingDataOutputStream_thenCorrect() 
  throws IOException {
    String value = "Hello";
    FileOutputStream fos = new FileOutputStream(fileName);
    DataOutputStream outStream = new DataOutputStream(new BufferedOutputStream(fos));
    outStream.writeUTF(value);
    outStream.close();

    // verify the results
    String result;
    FileInputStream fis = new FileInputStream(fileName);
    DataInputStream reader = new DataInputStream(fis);
    result = reader.readUTF();
    reader.close();

    assertEquals(value, result);
}

6. Write With RandomAccessFile

Let’s now illustrate how to write and edit inside an existing file rather than just writing to a completely new file or appending to an existing one. Simply put: We need random access.

RandomAccessFile enables us to write at a specific position in the file given the offset — from the beginning of the file — in bytes.

This code writes an integer value with offset given from the beginning of the file:

private void writeToPosition(String filename, int data, long position) 
  throws IOException {
    RandomAccessFile writer = new RandomAccessFile(filename, "rw");
    writer.seek(position);
    writer.writeInt(data);
    writer.close();
}

If we want to read the int stored at a specific location, we can use this method:

private int readFromPosition(String filename, long position) 
  throws IOException {
    int result = 0;
    RandomAccessFile reader = new RandomAccessFile(filename, "r");
    reader.seek(position);
    result = reader.readInt();
    reader.close();
    return result;
}

To test our functions, let’s write an integer, edit it, and finally read it back:

@Test
public void whenWritingToSpecificPositionInFile_thenCorrect() 
  throws IOException {
    int data1 = 2014;
    int data2 = 1500;
    
    writeToPosition(fileName, data1, 4);
    assertEquals(data1, readFromPosition(fileName, 4));
    
    writeToPosition(fileName2, data2, 4);
    assertEquals(data2, readFromPosition(fileName, 4));
}

7. Write With FileChannel

If we are dealing with large files, FileChannel can be faster than standard IO. The following code writes String to a file using FileChannel:

@Test
public void givenWritingToFile_whenUsingFileChannel_thenCorrect() 
  throws IOException {
    RandomAccessFile stream = new RandomAccessFile(fileName, "rw");
    FileChannel channel = stream.getChannel();
    String value = "Hello";
    byte[] strBytes = value.getBytes();
    ByteBuffer buffer = ByteBuffer.allocate(strBytes.length);
    buffer.put(strBytes);
    buffer.flip();
    channel.write(buffer);
    stream.close();
    channel.close();

    // verify
    RandomAccessFile reader = new RandomAccessFile(fileName, "r");
    assertEquals(value, reader.readLine());
    reader.close();
}

8. Write With Files Class

Java 7 introduces a new way of working with the filesystem, along with a new utility class: Files.

Using the Files class, we can create, move, copy, and delete files and directories. It can also be used to read and write to a file:

@Test
public void givenUsingJava7_whenWritingToFile_thenCorrect() 
  throws IOException {
    String str = "Hello";

    Path path = Paths.get(fileName);
    byte[] strToBytes = str.getBytes();

    Files.write(path, strToBytes);

    String read = Files.readAllLines(path).get(0);
    assertEquals(str, read);
}

9. Write to a Temporary File

Now let’s try to write to a temporary file. The following code creates a temporary file and writes a String to it:

@Test
public void whenWriteToTmpFile_thenCorrect() throws IOException {
    String toWrite = "Hello";
    File tmpFile = File.createTempFile("test", ".tmp");
    FileWriter writer = new FileWriter(tmpFile);
    writer.write(toWrite);
    writer.close();

    BufferedReader reader = new BufferedReader(new FileReader(tmpFile));
    assertEquals(toWrite, reader.readLine());
    reader.close();
}

As we can see, it’s just the creation of the temporary file that is interesting and different. After that point, writing to the file is the same.

10. Lock File Before Writing

Finally, when writing to a file, we sometimes need to make extra sure that no one else is writing to that file at the same time. Basically, we need to be able to lock that file while writing.

Let’s make use of FileChannel to try locking the file before writing to it:

@Test
public void whenTryToLockFile_thenItShouldBeLocked() 
  throws IOException {
    RandomAccessFile stream = new RandomAccessFile(fileName, "rw");
    FileChannel channel = stream.getChannel();

    FileLock lock = null;
    try {
        lock = channel.tryLock();
    } catch (final OverlappingFileLockException e) {
        stream.close();
        channel.close();
    }
    stream.writeChars("test lock");
    lock.release();

    stream.close();
    channel.close();
}

Note that if the file is already locked when we try to acquire the lock, an OverlappingFileLockException will be thrown.

11. Notes

After exploring so many methods of writing to a file, let’s discuss some important notes:

• If we try to read from a file that doesn’t exist, a FileNotFoundException will be thrown.
• If we try to write to a file that doesn’t exist, the file will be created first and no exception will be thrown.
• It is very important to close the stream after using it, as it is not closed implicitly, to release any resources associated with it.
• In output stream, the close() method calls flush() before releasing the resources, which forces any buffered bytes to be written to the stream.

Looking at the common usage practices, we can see, for example, that PrintWriter is used to write formatted text, FileOutputStream to write binary data, DataOutputStream to write primitive data types, RandomAccessFile to write to a specific position, and FileChannel to write faster in larger files. Some of the APIs of these classes do allow more, but this is a good place to start.

12. Conclusion

This article illustrated the many options of writing data to a file using Java.