How to break a file into pieces using Java?

Read portions of bytes to byte array and store them in new files when buffer is full or it is end of file.

For example (code is not perfect, but it should help understanding the process)

class FileSplit {
    public static void splitFile(File f) throws IOException {
        int partCounter = 1;//I like to name parts from 001, 002, 003, ...
                            //you can change it to 0 if you want 000, 001, ...

        int sizeOfFiles = 1024 * 1024;// 1MB
        byte[] buffer = new byte[sizeOfFiles];

        String fileName = f.getName();

        //try-with-resources to ensure closing stream
        try (FileInputStream fis = new FileInputStream(f);
             BufferedInputStream bis = new BufferedInputStream(fis)) {

            int bytesAmount = 0;
            while ((bytesAmount = bis.read(buffer)) > 0) {
                //write each chunk of data into separate file with different number in name
                String filePartName = String.format("%s.%03d", fileName, partCounter++);
                File newFile = new File(f.getParent(), filePartName);
                try (FileOutputStream out = new FileOutputStream(newFile)) {
                    out.write(buffer, 0, bytesAmount);
                }
            }
        }
    }

    public static void main(String[] args) throws IOException {
        splitFile(new File("D:\\destination\\myFile.mp4"));
    }
}

myFile.mp4 size=12,7 MB

After split I had 13 files

• myFile.mp4.001 – myFile.mp4.012 with size 1 MB
• myFile.mp4.013 with size 806 KB

---

If you want to merge these files you can use

public static void mergeFiles(List<File> files, File into)
        throws IOException {
    try (FileOutputStream fos = new FileOutputStream(into);
         BufferedOutputStream mergingStream = new BufferedOutputStream(fos)) {
        for (File f : files) {
            Files.copy(f.toPath(), mergingStream);
        }
    }
}

You can also create some additional methods to make your life easier. For instance method which will create list of files containing separated parts based on name (and location) of one of these files.

public static List<File> listOfFilesToMerge(File oneOfFiles) {
    String tmpName = oneOfFiles.getName();//{name}.{number}
    String destFileName = tmpName.substring(0, tmpName.lastIndexOf('.'));//remove .{number}
    File[] files = oneOfFiles.getParentFile().listFiles(
            (File dir, String name) -> name.matches(destFileName + "[.]\\d+"));
    Arrays.sort(files);//ensuring order 001, 002, ..., 010, ...
    return Arrays.asList(files);
}

With that method we can overload mergeFiles method to use only one of the files File oneOfFiles instead of whole list List<File> (we will generate that list based on one of the files)

public static void mergeFiles(File oneOfFiles, File into)
        throws IOException {
    mergeFiles(listOfFilesToMerge(oneOfFiles), into);
}

You can also overload these methods to use String instead of File (we will wrap each String in File when needed)

public static List<File> listOfFilesToMerge(String oneOfFiles) {
    return listOfFilesToMerge(new File(oneOfFiles));
}

public static void mergeFiles(String oneOfFiles, String into) throws IOException{
    mergeFiles(new File(oneOfFiles), new File(into));
}