[+/-] : Secure File Copy in Java

This example is from the book _Java in a Nutshell_ by David Flanagan.

import java.io.*;

public class FileCopy {
  public static void copy(String source_name, String dest_name) 
    throws IOException
  {
    File source_file = new File(source_name);
    File destination_file = new File(dest_name);
    FileInputStream source = null;
    FileOutputStream destination = null;
    byte[] buffer;
    int bytes_read;
    
    try {
      /* First make sure the specified source file 
       exists, is a file, and is readable.*/
      if (!source_file.exists() || !source_file.isFile())
        throw new FileCopyException("FileCopy: no such source file: "
          + source_name);
      if (!source_file.canRead())
        throw new FileCopyException("FileCopy: source file "
          + "is unreadable: " + source_name);
      
      /* If the destination exists, make sure it is a writeable file
        and ask before overwriting it.  If the destination doesn't
        exist, make sure the directory exists and is writeable.*/
      if (destination_file.exists()) {
        if (destination_file.isFile()) {
          DataInputStream in = new DataInputStream(System.in);
          String response;
          
          if (!destination_file.canWrite())
            throw new FileCopyException("FileCopy: destination " 
              + "file is unwriteable: " + dest_name);
          
          System.out.print("File " + dest_name + 
               " already exists.  Overwrite? (Y/N): ");
          System.out.flush();
          response = in.readLine();
          if (!response.equals("Y") && !response.equals("y"))
            throw new FileCopyException("FileCopy: copy cancelled.");
        }
        else
          throw new FileCopyException("FileCopy: destination "
                  + "is not a file: " +  dest_name);
      }
      else {
        File parentdir = parent(destination_file);
        if (!parentdir.exists())
          throw new FileCopyException("FileCopy: destination "
                  + "directory doesn't exist: " + dest_name);
        if (!parentdir.canWrite())
          throw new FileCopyException("FileCopy: destination "
                  + "directory is unwriteable: " + dest_name);
      }
      
      /* If we've gotten this far, then everything is okay; we can
         copy the file.*/
      source = new FileInputStream(source_file);
      destination = new FileOutputStream(destination_file);
      buffer = new byte[1024];
      while(true) {
        bytes_read = source.read(buffer);
        if (bytes_read == -1) break;
        destination.write(buffer, 0, bytes_read);
      }
    }
    /* No matter what happens, always close any streams we've opened.*/
    finally {
      if (source != null) 
        try { source.close(); } catch (IOException e) { ; }
      if (destination != null) 
        try { destination.close(); } catch (IOException e) { ; }
    }
  }
  
  /* File.getParent() can return null when the file is specified without
     a directory or is in the root directory.  
     This method handles those cases.*/
  private static File parent(File f) {
    String dirname = f.getParent();
    if (dirname == null) {
      if (f.isAbsolute()) return new File(File.separator);
      else return new File(System.getProperty("user.dir"));
    }
    return new File(dirname);
  }
  
  public static void main(String[] args) {
    if (args.length != 2)
      System.err.println("Usage: java FileCopy " + 
             " ");
    else {
      try { copy(args[0], args[1]); }
      catch (IOException e) { System.err.println(e.getMessage()); }
    }
  }
}

class FileCopyException extends IOException { 
  public FileCopyException(String msg) { super(msg); }
}

...
Read more