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

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, u开发者_Python百科ntil 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.

---

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.

---

Apache Commons-IO:

IOUtils.copy(fileInputStream,outputStream);

JDK NIO

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

---

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(..))