如何将文件流转换成byte[]数组

InputStream is = new FileInputStream(new File("D://a.txt")); 
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
byte[] bytes = new byte[1024];
int temp;
while ((temp = is.read(bytes)) != -1) {
     outputStream.write(bytes, 0, temp);
}
//转换后的byte[]
byte[] finalBytes = outputStream.toByteArray();

ObsObject obsObject = obsClient.getObject(bucketName, objectName);
        InputStream inputStream = obsObject.getObjectContent();
        byte[] bytes = null;
        try {
            bytes = IOUtils.toByteArray(inputStream);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            inputStream.close();
        }
        return bytes;

  InputStream input = obsObject.getObjectContent();
        // 读取对象内容
        byte[] b = new byte[1024];
        OutputStream bos = response.getOutputStream();
        int len;
        while ((len = input.read(b)) != -1) {
            bos.write(b, 0, len);
        }

  XSSFClientAnchor anchor = new XSSFClientAnchor(0, 0, 0, 0,
                    startColumn, startRow, startColumn + column + 1, startRow + row + 1);
            drawing.createPicture(anchor, wb.addPicture(bytes, XSSFWorkbook.PICTURE_TYPE_PNG));

byte[] bytes = DatatypeConverter.parseBase64Binary("base64字符串");

String base64Str = DatatypeConverter.printBase64Binary(bytes);

String str = new String(Base64.encode(baos.toByteArray()));