Is there a better way to write the full contents of a file to an OutputStream?


Solution 1

For Java 1.7+ you can use the Files.copy(Path, OutputStream), e.g.

HttpServletResponse response = // ...
File toBeCopied = // ...

try (OutputStream out = response.getOutputStream()) {
    Path path = toBeCopied.toPath();
    Files.copy(path, out);
} catch (IOException e) {
    // handle exception

Note, since you are dealing with HttpServletResponse is is also a good idea to set correct response headers. Add the following lines before you copy the actual file data to the response:

String mimeType = URLConnection.guessContentTypeFromName(toBeCopied.getName());
String contentDisposition = String.format("attachment; filename=%s", toBeCopied.getName());
int fileSize = Long.valueOf(toBeCopied.length()).intValue();

response.setHeader("Content-Disposition", contentDisposition);

Note, the encoding of the file name passed to the content disposition is important, see this question.

Solution 2

Apache Commons-IO:



new FileInputStream(file).getChannel().transferTo(otherChannel);

Solution 3

With commons-io you have a one-line solution:

IOUtils.copy(yourFileInputStream, outputStream);

Note that you'd have to close your streams manually (or by IOUtils.closeQuitely(..))


Related videos on Youtube

Author by


Freelance software developer/architect/integration specialist, working in and around Germany. My open source-software is published under Jackadull.

Updated on October 31, 2020


  • Madoc
    Madoc over 3 years

    When I want to write the full contents of a file into an OutputStream, I usually allocate a buffer as a byte[], then make a for loop to read data from the file's InputStream into the buffer and write the buffer contents into the OutputStream, until the InputStream has no more bytes available.

    This seems rather clumsy to me. Is there a better way to do this?

    Also, I am always unsure about the buffer size. Usually, I am allocating 1024 bytes, because it just feels good. Is there a better way to determine a reasonable buffer size?

    In my current case, I want to copy the full contents of a file into the output stream that writes the contents of an HTTP response. So, this is not a question about how to copy files on the file system.

  • rfeak
    rfeak over 13 years
    This would be my suggestion as well. Though what it does is the same thing.
  • Madoc
    Madoc over 13 years
    That looks really nice. But for this simple task, it seems a little overdone to me to include another library. Of course, if there are more reasons to include Commons-IO, then this will be the solution of choice.
  • Madoc
    Madoc over 13 years
    Wow. I gave a short comment regarding Commons-IO at Bozho's answer. But the channel solution is exactly what I was looking for. Thanks a lot, all of NIO channels passed me by, because I had already learned Java IO before NIO existed. And, by the way: 9 minutes! That's almost as fast as lightning.
  • yegor256
    yegor256 almost 12 years
    don't forget to close the FileInputStream