posting xml to a URL

You need to have some active enitity behind that URL which would read your data - it should be servlet or PHP script or CGI, or something which should be a process ready to accept your data

Then you indeed acn setDoOutput(true), etc. like in this example:

http://www.exampledepot.com/egs/java.net/post.html

It usually is more than just address and port because the URL mentions that active entity which accepts your data. I don't think it is an absolute law, I guess you can invenet something to make this script accept your data by deafult on certain address and port, but as a rule after address and port you mention the name of that script which accepts your data, name of servlet or PHP ascript or something like that

If I create a URLConnection and open it, but do not open an outputStream on it yet, will I see activity on the server side socket or is it not until the stream is opened?

I am not sure how you mean to see activity on the server side socket until the stream is opened.

If I am polling the socket on the URL's code will something be sent when the line
URLConnection con = url.openConnection(); is invoked, or is nothing sent until
con.getOutputStream() line in executed?

Ok...so if the connection is refused upon opening the outputStream, a response or exception will be raised as long as the conn.getOutputStream() is in a try block,
or do I also need to open an inputStream to receive the potential reject?

Also: for inputStreams I would expect to see the server side socket get hit once with a GET query, whereas should I expect the outputStream to hit the socket twice, ie.. once with a POST and no data, and the the data from the actual write()?

>or do I also need to open an inputStream to receive the potential reject?

I don't think there could be an universal answer to that question.
If you get no response say because you have incorrect address, then it makes no sense to open input stream. If you are rejected by the script in your URL which analyzed the data and prepared some srror message - then maybe inputStream will be necessary

i think it is better to deal with this situation using very concrete example - what kind of URL you are contacting and what you want to achieve. There are many variants of different types and situations of failures and we'll not predict all cases theoretically

I will need to ask the url developer I am working with this question.
When you say "what type of URL"?  What are you asking for exactly?

All I know at this moment is it is a http url to which I want to post XML data to.