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JAVA – WRITE TO FILE



Last updated: December 1, 2023

Written by: Eugen Paraschiv
 * Java IO

 * Java FileWriter

Course – LS – All


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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.


FURTHER READING:


JAVA – APPEND DATA TO A FILE

A quick and practical guide to appending data to files.
Read more →


FILENOTFOUNDEXCEPTION IN JAVA

A quick and practical guide to FileNotFoundException in Java.
Read more →


HOW TO COPY A FILE WITH JAVA

Take a look at some common ways of copying files in Java.
Read more →


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();
}Copy

The output in the file will be:



HelloCopy

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();
}Copy

The file will then be:

Hello WorldCopy


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();
}Copy

The resulting file will contain:

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

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();
}Copy

The output in the file will of course be:

HelloCopy


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);
}Copy


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();
}Copy

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;
}Copy

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));
}Copy


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();
}Copy


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);
}Copy


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();
}Copy

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();
}Copy

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.



The implementation of all these examples and code snippets can be found over on
GitHub.

Course – LS – All


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