How can I write a byte array to a file in Java?

Question

How to write a byte array to a file in Java?

Answer 1

To write a byte array to a file use the method

public void write(byte[] b) throws IOException

from BufferedOutputStream class.

java.io.BufferedOutputStream implements a buffered output stream. By setting up such an output stream, an application can write bytes to the underlying output stream without necessarily causing a call to the underlying system for each byte written.

For your example you need something like:

String filename= "C:/SO/SOBufferedOutputStreamAnswer";
BufferedOutputStream bos = null;
try {
//create an object of FileOutputStream
FileOutputStream fos = new FileOutputStream(new File(filename));

//create an object of BufferedOutputStream
bos = new BufferedOutputStream(fos);

KeyGenerator kgen = KeyGenerator.getInstance("AES"); 
kgen.init(128); 
SecretKey key = kgen.generateKey(); 
byte[] encoded = key.getEncoded();

bos.write(encoded);

} 
// catch and handle exceptions...

Answer 2

You can use IOUtils.write(byte[] data, OutputStream output) from Apache Commons IO.

KeyGenerator kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
SecretKey key = kgen.generateKey();
byte[] encoded = key.getEncoded();
FileOutputStream output = new FileOutputStream(new File("target-file"));
IOUtils.write(encoded, output);

Answer 3

No need for external libs to bloat things - especially when working with Android. Here is a native solution that does the trick. This is a pice of code from an app that stores a byte array as an image file.

    // Byte array with image data.
    final byte[] imageData = params[0];

    // Write bytes to tmp file.
    final File tmpImageFile = new File(ApplicationContext.getInstance().getCacheDir(), "scan.jpg");
    FileOutputStream tmpOutputStream = null;
    try {
        tmpOutputStream = new FileOutputStream(tmpImageFile);
        tmpOutputStream.write(imageData);
        Log.d(TAG, "File successfully written to tmp file");
    }
    catch (FileNotFoundException e) {
        Log.e(TAG, "FileNotFoundException: " + e);
        return null;
    }
    catch (IOException e) {
        Log.e(TAG, "IOException: " + e);
        return null;
    }
    finally {
        if(tmpOutputStream != null)
            try {
                tmpOutputStream.close();
            } catch (IOException e) {
                Log.e(TAG, "IOException: " + e);
            }
    }

Answer 4

Apache Commons IO Utils has a FileUtils.writeByteArrayToFile() method. Note that if you're doing any file/IO work then the Apache Commons IO library will do a lot of work for you.

Answer 5

As Sebastian Redl points out the most straight forward now java.nio.file.Files.write. Details for this can be found in the Reading, Writing, and Creating Files tutorial.

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Old answer: FileOutputStream.write(byte[]) would be the most straight forward. What is the data you want to write?

The tutorials for Java IO system may be of some use to you.

Answer 6

As of Java 1.7, there's a new way: java.nio.file.Files.write

import java.nio.file.Files;
import java.nio.file.Paths;

KeyGenerator kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
SecretKey key = kgen.generateKey();
byte[] encoded = key.getEncoded();
Files.write(Paths.get("target-file"), encoded);

Java 1.7 also resolves the embarrassment that Kevin describes: reading a file is now:

byte[] data = Files.readAllBytes(Paths.get("source-file"));