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

Question

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.

Answer 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);
    out.flush();
} 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.setContentType(mimeType);
response.setHeader("Content-Disposition", contentDisposition);
response.setContentLength(fileSize);

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