Does closing a BufferedOutputStream also close the underlying OutputStream?

Question

I am streaming binary data (a CSV file extracted from the database as a Clob) to the browser by calling response.getOutputStream() and would normally wrap the OutputStream in a BufferedOutputStream when copying the data.

Should I close the BufferedOutputStream or will this also close the underlying OutputStream?

[Extra question: Do I need to use a BufferedOutputStream in this case or is the response already buffered?]

Answer 1

Yes, it closes it. As for whether you should close it - are you expecting to write anything else to the response stream? If not, I think it's fine to close it. If you don't close it, you should obviously flush it instead - but I suspect you could figure that bit out for yourself :)

The behaviour is actually inherited from FilterOutputStream. The Javadocs for for FilterOutputStream.close state:

The close method of FilterOutputStream calls its flush method, and then calls the close method of its underlying output stream.

As for whether you should buffer it - I'm not sure that this is well defined. It may be buried in the servlet spec somewhere - and it may even be configurable (sometimes you really don't want buffering, but if you can buffer the whole response it means you can serve a nicer error page if things go wrong after you've started writing).

Answer 2

Closing the BufferedOutputStream will also close the underlying OutputStream. You should close the BufferedOutputStream so that it flushes its contents before closing the underlying stream. See the implementation of FilterOutputStream.close() (from which BufferedOutputStream extends) to convince yourself.

I guess that whether or not the response stream given to your servlet is buffered or not depends on the implementation of your Servlet Container. FWIW I know that Tomcat does buffer its servlet response streams by default, in order to attempt to set the content-length HTTP header.