code: public static void appendFile(File sourceFile, File destinationFile) {
          ValidationUtils.assertNotNull(destinationFile, "destFile");
          ValidationUtils.assertNotNull(sourceFile, "sourceFile");
          if (!FileLocks.lock(sourceFile)) {
              throw new FileLockException("Fail to lock " + sourceFile);
          }
          if (!FileLocks.lock(destinationFile)) {
              throw new FileLockException("Fail to lock " + destinationFile);
          }
  
          FileChannel in = null;
          FileChannel out = null;
          try {
              in = new FileInputStream(sourceFile).getChannel();
              out = new FileOutputStream(destinationFile, true).getChannel();
              final long size = in.size();
              // In some Windows platforms, copying large files fail due to insufficient system resources.
              // Limit copy size to 32 MB in each transfer
              final long count = 32 * MB;
              long position = 0;
  
              while (position < size) {
                  position += in.transferTo(position, count, out);
              }
          } catch (IOException e) {
              throw new SdkClientException("Unable to append file " + sourceFile.getAbsolutePath()
                      + "to destination file " + destinationFile.getAbsolutePath() + "\n" + e.getMessage(), e);
          } finally {
              closeQuietly(out, LOG);
              closeQuietly(in, LOG);
              FileLocks.unlock(sourceFile);
              FileLocks.unlock(destinationFile);
              try {
                  if (!sourceFile.delete()) {
                      LOG.warn("Failed to delete file " + sourceFile.getAbsolutePath());
                  }
              } catch (SecurityException exception) {
                  LOG.warn("Security manager denied delete access to file " + sourceFile.getAbsolutePath());
              }
          }
      }
docs: Append the data in sourceFile to destinationFile.
  
  Note that the sourceFile is deleted after appending the data.
  
  @param sourceFile
  The file that is to be appended.
  @param destinationFile
  The file to append to.