Response outputstream content length?

You may consider writing to your own ByteArrayOutputStream and flush it to the response output stream at the very end.

The problem is that you must set the content length in the response header before you start writing any data to the output stream. So your options are:

1. Write the data to a byte[] array using ByteOutputStream and then copy that to the response output stream once you have the size of the data. However, if you're writing large files, this obviously isn't an option.
2. Write the data to a temp file and then copy that to the response output once you get the file size. Depending on what you're doing, this may have a performance penalty that is unacceptable.
3. Depending on how expensive it is to generate the data in the first place, you could generate it once, and throw it away to get the count and then generate it again. Guessing that this is unlikely to be a realistic solution.
4. Resign yourself to the fact that you won't be able to report the content length in the response header.

The easiest way is probably to wrap it in another OutputStream implementation which forwards on all the write requests, but keeps an internal counter. Then you just write to that instead. Shouldn't be too hard to implement - and indeed there may be one already.

EDIT: Just guessing at a sensible name (CountingOutputStream) came up with an implementation in Apache Commons IO.

EDIT: As noted elsewhere, if this is for HTTP and your client isn't already doing buffering of the full data (in which case I'd have thought it could work out the content length), you may have problems due to needing to write the length before writing the data. In some cases you may find that it will work up to a certain size (which the client buffers) and then fail. In that case, David's solutions will be appropriate.