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Using HttpURLConnection to POST in Java

I've read lots and tried lots relating to HTTP POSTS using HttpURLConnection and almost everything I come across has a similar structure which starts with these 3 lines:
url = new URL(targetURL);
connection = (HttpURLConnection)url.openConnection();
connection.setRequestMethod("POST");
When I try this I always get a "Connection Already Established" exception when calling setRequestMethod, which makes perfect sense as I'm clearly calling openConnection before setting the request type. Although reading the docs openConnection doesn't actually open the connection in theory.
There are several posts about this problem on SO such as this and this. I don't understand however why every piece of advice about how to write this code has these 3 lines in this order.
I'm guessing this code must work in most instances as someone must have tested it, so why doesn't this code work for me? How should I be writing this code?
I am aware these are other libraries I can use out there, I'm just wondering why this doesn't work.

Why the suspect code in the question has been duplicated all over the internet is something I can't answer. Nor can I answer why it seems to work for some people and not others. I can however answer the other question now, mainly thanks to this link that Luiggi pointed me to.
The key here is understanding the intricacies of the HttpURLConnection class. When first created the class defaults to a "GET" request method, so nothing needs to be changed in this instance. The following is rather unintuitive, but to set the request method to "POST" you should not call setRequestMethod("POST"), but rather setDoOutput(true) which implicitly sets the request method to post. Once you've done that you're good to go.
Below, I believe, is what a post method should look like. This is for posting json, but can obviously be altered for any other content type.
public static String doPostSync(final String urlToRead, final String content) throws IOException {
final String charset = "UTF-8";
// Create the connection
HttpURLConnection connection = (HttpURLConnection) new URL(urlToRead).openConnection();
// setDoOutput(true) implicitly set's the request type to POST
connection.setDoOutput(true);
connection.setRequestProperty("Accept-Charset", charset);
connection.setRequestProperty("Content-type", "application/json");
// Write to the connection
OutputStream output = connection.getOutputStream();
output.write(content.getBytes(charset));
output.close();
// Check the error stream first, if this is null then there have been no issues with the request
InputStream inputStream = connection.getErrorStream();
if (inputStream == null)
inputStream = connection.getInputStream();
// Read everything from our stream
BufferedReader responseReader = new BufferedReader(new InputStreamReader(inputStream, charset));
String inputLine;
StringBuffer response = new StringBuffer();
while ((inputLine = responseReader.readLine()) != null) {
response.append(inputLine);
}
responseReader.close();
return response.toString();
}

As per https://stackoverflow.com/a/3324964/436524, you need to call connection.setDoOutput(true) for it to expect a POST request.
This makes your code like this:
url = new URL(targetURL);
connection = (HttpURLConnection)url.openConnection();
connection.setDoOutput(true);

new to android - understanding the use of OutputStreamWriter in web service calls

I have been going over the following tutorial and came across this code which I do not understand the purpose of:
URLConnection conn = url.openConnection();
conn.setDoOutput(true);
OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
wr.write( data );
wr.flush();
I don't know what is the purpose of the above code and where is it writing this data to...
From what I could gather, the documentation states that it converts character to bytes... but then it writes it to some where... not sure why this is.

It is basically used for turning a character stream into a byte stream.
The byte streams and character streams are incompatible for linking as the first one operates on 8-bit ASCII characters and the other on 16-bit Unicode characters. To link them explicitly, two classes exist in java.io package, InputStreamReader and OutputStreamWriter.
InputStreamReader links a byte stream, with the character stream BufferedReader (on reading-side) .
Whereas with the OutputStreamWriter, the characters of 2-bytes are encoded (converted) into bytes of 1-byte (InputStreamReader does it other way – bytes to characters).
For a Java program to interact with a server-side process it simply must be able to write to a URL, thus providing data to the server. It can do this by following these steps:
1.Create a URL.
2.Retrieve the URLConnection object.
3.Set output capability on the URLConnection.
4.Open a connection to the resource.
5.Get an output stream from the connection.
6.Write to the output stream.
7.Close the output stream.
Now in the snippet you provided ,
OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
creates an output stream on the connection and opens an
OutputStreamWriter on it--Step 5 If the URL does support output, then
this method returns an output stream that is connected to the input
stream of the URL on the server side — the client's output is the
server's input.If the URL does not support output, getOutputStream
method throws an UnknownServiceException.
And
wr.write( data );
wr.close();
wr.flush();
It writes the required information to the output stream and closes the
stream. The data written to the output stream on the client side is
the input on the server side.

It's writing it to the output stream of the URLConnection - which is basically used for the body of an HTTP request (assuming it's an HTTP URL, of course).

Sending sms via java

I am going to send sms via java. The problem is the sms gateway ask me to send in this format
http://push1.maccesssmspush.com/servlet/com.aclwireless.pushconnectivity.listen
ers.TextListener?userId=xxxxx&pass=xxxx&appid=xxxx&subappid=xxxx&msgtyp
e=1&contenttype=1&selfid=true&to=9810790590,9810549717&from=ACL&dlrre
q=true&text=This+is+a+test+msg+from+ACL&alert=
The problem how to call this from a java application is it possible or does it need special libraries? IS it using HttpURLConnection will do the job? Thank you.
A Sample code I have done below is this correct.
URL sendSms1 = new URL("http://push1.maccesssmspush.com/servlet/com.aclwireless.pushconnectivity.listen
ers.TextListener?userId=xxxxx&pass=xxxx&appid=xxxx&subappid=xxxx&msgtyp
e=1&contenttype=1&selfid=true&to=9810790590,9810549717&from=ACL&dlrre
q=true&text=This+is+a+test+msg+from+ACL&alert=");
URLConnection smsConn1 =
sendSms1.openConnection();

It's just an HTTP call, you don't need anything special in Java (or any modern language, I expect). Just build up the string as appropriate*, then make an HTTP request to that URL.
Take a peek at the Sun tutorial Reading from and Writing to a URLConnection if you need to pick up the basics of how to do the request part in Java. This uses the built-in classes, I'm sure there are dozens of libraries that handles connections in funky and/or convenient ways too, so by all means use one of those if you're familiar with it.
*One potential gotcha which might not have occurred to you - your query string arguments will have to be URL-encoded. So the + characters for example in the text parameter, are encoded spaces (which would have a different meaning in the URL). Likewise, if you wanted to send a ? character in one of your parameters, it would have to appear as %3F. Have a look at the accepted answer to HTTP URL Address Encoding in Java for an example of how you might build the URL string safely.

It looks like a simple GET request, you can use Apache HttpClient libarary for executing such a request. Have a look into a tutorial by Vogella here: http://www.vogella.de/articles/ApacheHttpClient/article.html for sample source code and explanations.

You can try to use java.net.URL library。
like this
// at this before you need to generate the urlString as "http://push1.maccesssmspush.com/servlet/com.aclwireless.pushconnectivity.listen
ers.TextListener?userId=xxxxx&pass=xxxx&appid=xxxx&subappid=xxxx&msgtyp
e=1&contenttype=1&selfid=true&to=9810790590,9810549717&from=ACL&dlrre
q=true&text=This+is+a+test+msg+from+ACL&alert="
URL url = new URL(urlString);
// send sms
URLConnection urlConnection = url.openConnection();// open the url
// and you, also can get the feedback if you want
BufferedReader br = new BufferedReader(new InputStreamReader(
urlConnection.getInputStream()));

URL url = new URL("http://smscountry.com/SMSCwebservice.asp");
HttpURLConnection urlconnection = (HttpURLConnection) url.openConnection();
[Edit]
urlconnection.setRequestMethod("POST");
urlconnection.setRequestProperty("Content-Type","application/x-www-form-urlenc‌​oded");
urlconnection.setDoOutput(true);
OutputStreamWriter out = new OutputStreamWriter(urlconnection.getOutputStream());
out.write(postData);
out.close();
BufferedReader in = new BufferedReader(new InputStreamReader(urlconnection.getInputStream()));
String decodedString;
while ((decodedString = in.readLine()) != null) {
retval += decodedString;
}

Trying to read from a URL(in Java) produces gibberish on certain occaisions

I'm trying to read from a URL, and then print the result.
BufferedReader in = new BufferedReader(
new InputStreamReader(new URL("http://somesite.com/").openStream(), "UTF-8"));
String s = "";
while ((s=in.readLine())!=null) System.out.println(s);
in.close();
It works great most of the time, and prints the website's source.
However, my problem is, on specific websites, instead of the source code, it will print out gibberish, such as symbols and other unusual characters.
Is there some property that varies from website to website that would affect how it is read? The page loads just fine in Firefox, and I can view the source there with no problem. If firefox can access the source, I should be able to as well; I'm just not sure why it isn't working...
EDIT: added "UTF-8" to InputStreamReader. All of the strange characters are now question marks...still not working...

So after much searching I found the answer to this. The xml is read as gibberish because it is Gzip compressed. The way to read this is by using the GZIPInputStream. This is because the XML is compressed differently.
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestProperty("Accept-Encoding", "gzip");
InputStreamReader in = new InputStreamReader (new GZIPInputStream(connection.getInputStream()));
String str;
while (true) {
int ch = in.read();
if (ch==-1) {
break;
}

You're probably running into a character encoding issue.
There should be an HTTP header like the following in the response:
Content-Type: text/html; charset=UTF-8

Try using telnet to diagnose what's coming over the wire. It may not be textual data. For example, what happens when yo do this?
telnet somesite.com 80
GET / HTTP/1.0
Host: somesite.com
(two carriage returns required after last line)
This should allow you to see the headers and content coming in and should give you a better clue as to what's going on.

I had the same issue until I used HttpURLConnection with setChunkedStreamingMode set.
HttpURLConnection connection = (HttpURLConnection)serverAddress.openConnection();
connection.setRequestMethod("GET");
connection.setDoOutput(true);
connection.setReadTimeout(2000);
connection.setChunkedStreamingMode(0);
connection.connect();
BufferedReader rd = new BufferedReader(new InputStreamReader(connection.getInputStream()));
String line = "";
while ((line = rd.readLine()) != null)
{
sb.append(line);
}
System.out.println(sb.toString());

URL parameter encoding/receiving problem

I use the below code to send data to a servlet:
When encoding = "UTF-8" or "GBK", the data is received correctly.
But when encoding = "UTF-16", The receiver receives null. WHY??
The Sender:
URL url = new URL(notifyURL);
HttpURLConnection connection = (HttpURLConnection)url.openConnection();
connection.setDoOutput(true);
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded; charset=" + encoding);
OutputStreamWriter out = new OutputStreamWriter(connection.getOutputStream());
out.write("notify_id=" + URLEncoder.encode("123", encoding) + "&notify_type=" + URLEncoder.encode("any", encoding));
out.flush();
out.close();
connection.connect();
The receiver servlet:
log.info(request.getParameter("notify_type")); //print null

According to the Javadocs for URLEncoder, you should only use UTF-8 because other encodings may cause problems. They link directly to the W3C spec from the Javadocs.

You have 2 issues,
UTF-16 is not supported by lots of web servers. Some URLDecoder can only handle single byte encoding (ASCII, Latin-1) and UTF-8.
You are using mixed encoding if your default encoding is not UTF-16. UTF-8 and GBK are both ASCII compatible (ASCII is encoded as itself) so you can mix with ASCII but you can't do that with UTF-16.