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Java - send HTTP POST request without downloading all the content

Is it possible to send HTTP POST request to a webserver and retrieve just headers of response or read just few bytes of the body, so the rest won't be downloaded at all (so it won't consume traffic)? If yes, how?
I know that there is a HEAD method for this, but I need to achieve it by POST method .. well, I am not sure if I need the POST method, I just need to post the data. Maybe if the webserver isn't secured well enough (it doesn't check what method it's used - it's just directly access the post data), is it possible to send "post data" by HEAD request?

There is no built-in HTTP mechanism for this, and HTTP HEAD requests do not allow content in the body. If however you are the one writing the server code then anything is possible.
If this is the case, I would suggest a URL parameter that triggers this behavior. For example:
POST /myURL - This would return the whole response
POST /myURL?body=minimal - Returns the reduced size response that you are looking for.
And you would have to code your server method to construct and return the appropriate response based on the URL parameter.

how to remove a header from URLConnection

I am talking to a file upload service that accepts post data, not form data. By default, java's HttpURLConnection sets the Content-Type header to application/x-www-form-urlencoded. this is obviously wrong if i'm posting pure data.
I (the client) don't know the content type. I don't want the Content-Type header set at all. the service has a feature where it will guess at the content type (based on the file name, reading some data from the file, etc).
How do I unset a header? There's no remove header, and setting it to null doesn't change the value and setting it to the empty string results in the header being set with no value.

I haven't tested this approach but you can try this:
Extend HttpURLConnection and try by overriding its getContentHandler() and setContentHandler(...) methods. Most probably this should work as, you will take a look at code of getContentHandler().

Use Apache HttpClient instead of URLConnection
Use fluent Request to generate your request
use removeHeader()

What do you mean "i don't want the Content-Type header to set at all"?
The browser (or other http client) sends your post request to the server, so it has to inform the server which way it encoded the parameters.
If the Content-Type header is not set, on the server side you (= your server) won't be able to understand how to parse the received data.
If you didn't set Content-Type, the default value will be used.
You browser (or other http client) MUST do two things:
Send key/value pairs.
Inform the server how the key/value pairs were encoded.
So, it is impossible to completely get rid of this header.

I just accomplished this by setting the header to null.
connection.setRequestProperty(MY_HEADER, null);

How can the servlet determine when an upload has been cancelled?

Here is some code on the javascript side for form-based uploads:
iframe.setAttribute('src', 'javascript:false;');
I'm using the code above to cancel an in-progress upload associated with an input element placed in an iframe.
I'm using the code below to cancel an in-progress upload sent via XHR:
myxhr.abort();
In both cases, no more bytes are sent to the servlet. The part I'm struggling with is on the servlet side. Currently, I can't figure out a way for the servlet instance to determine if the user has cancelled the upload. This is critical, otherwise the servlet will go on and process the partially uploaded file as if it is valid.
How can I determine, via the HttpServletRequest, if a user has cancelled the upload?

The POST request with the data contains the Content-Length header which tells you the size of the data that is going to be uploaded.
So when the data stops coming to your server and the size of the data received is less than expected - it would mean that the user (or some network glitch) has canceled the upload.

If the upload has been cancelled the browser will close the connection, resulting in a an IO exception on the servlet side. For example, in Tomcat it will say "Connection reset by peer" and this is a ClientAbortException. Other servers wrap the IOException differently. Point is just catch the IOException and you should be able to handle it as you wish.
Using content-length is not reliable because the HTTP spec does not require content-length headers for POSTs - or for GETs for the matter. Point is, unless you are sure your javascript XHR sets the header explicitly, this method won't work.
Alternatively you could calculate it yourself and set it to be sure, or even better append your own character stream to the end of a the posted data in the XHR, some unique string of characters, e.g. "jh923k49sk$2#%'. In the servlet, snip off the last 14 characters of the inbound message and check it against the string. If it is the same you know they didn't cancel.

I don't see how you can tell, just because a request stream has ended, whether it's done or cancelled. There would have to be a separate HTTP request to indicate cancellation that would have to include some token or ID associated with the upload, since HTTP is stateless and idempotent.

Does the server send response only when its HTTP 200?

im writing a java application that sends a post request to a server and expect a json from the server. Now when i need to get the response from the server do i only need to get it from the inputStream when the http code is 200 (HTTP OK) or is there any other cases ? , example :
//...
if (urlConn.getResponseCode() == HttpURLConnection.HTTP_OK) {
// only here try to get the response
}
//...

It depends on how the server is implemented. Check the API, if the server has one. If it's internal, ask your server guy.
Generally speaking, if your response code is either 2xx or 3xx, I would check the response anyway...

If the server your communicating with is following the spec then either 200 or 201 responses are valid to contain an entity. A 204 response is successful but has no data in the response.
See section 9.5 here: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.5 for details of acceptable responses to a POST. Extract below:
The action performed by the POST method might not result in a resource
that can be identified by a URI. In this case, either 200 (OK) or 204
(No Content) is the appropriate response status, depending on whether
or not the response includes an entity that describes the result.
If a resource has been created on the origin server, the response
SHOULD be 201 (Created) and contain an entity which describes the
status of the request and refers to the new resource, and a Location
header (see section 14.30).

There are three things to consider:
All 2xx codes denote success of some sort. But depending on the exact code, your reading code might be different. (204 for example means success but no content.)
There are redirecting codes (3xx). These are usually automatically followed by the http client library but you can also set them not to, in which case you need to have custom code that handles these cases.
There can be valuable information returned in the stream even if you get a code that denotes an error. Whether you want to process it depends on your exact needs.

Java AddRequestProperty while already connected?

Is there a way to use addRequestProperty if you have already connected to the URLConnection?
I need to do this cause I first have to connect to get the headerfields for the first Cookie, but later I have to send that first Cookie in order to get headerfields where a second set-cookie field is defined.

That won't work. The HTTP protocol conveys those properties as header items at the beginning of the request data. You'll need multiple requests.