Why do you have to call URLConnection#getInputStream to be able to write out to URLConnection#getOutputStream?

Question

I\'m trying to write out to URLConnection#getOutputStream, however, no data is actually sent until I call URLConnection#getInputStream. Even if I set URLConnnection#doInput

Answer 1

Although the getInputStream() method can certainly cause a URLConnection object to initiate an HTTP request, it is not a requirement to do so.

Consider the actual workflow:

1. Build a request
2. Submit
3. Process the response

Step 1 includes the possibility of including data in the request, by way of an HTTP entity. It just so happens that the URLConnection class provides an OutputStream object as the mechanism for providing this data (and rightfully so for many reasons that aren't particularly relevant here). Suffice to say that the streaming nature of this mechanism provides the programmer an amount of flexibility when supplying the data, including the ability to close the output stream (and any input streams feeding it), before finishing the request.

In other words, step 1 allows for supplying a data entity for the request, then continuing to build it (such as by adding headers).

Step 2 is really a virtual step, and can be automated (like it is in the URLConnection class), since submitting a request is meaningless without a response (at least within the confines of the HTTP protocol).

Which brings us to Step 3. When processing an HTTP response, the response entity -- retrieved by calling getInputSteam() -- is just one of the things we might be interested in. A response consists of a status, headers, and optionally an entity. The first time any one of these is requested, the URLConnection will perform virtual step 2 and submit the request.

No matter if an entity is being sent via the connection's output stream or not, and no matter whether a response entity is expected back, a program will ALWAYS want to know the result (as provided by the HTTP status code). Calling getResponseCode() on the URLConnection provides this status, and switching on the result may end the HTTP conversation without ever calling getInputStream().

So, if data is being submitted, and a response entity is not expected, don't do this:

// request is now built, so...
InputStream ignored = urlConnection.getInputStream();

... do this:

// request is now built, so...
int result = urlConnection.getResponseCode();
// act based on this result