Develop Reference

can I send same HTTP request with same HttpURLConnection multiple times?

In order to send a string data to the server once, I do as below:
Make “HttpURLConnection” to my URL address and open it
Set the required headers
for the connection I Set setDoOutput to True
new a DataOutputStream from my connection and finally write my string data to it.
HttpURLConnection myConn = (HttpURLConnection);
myUrl.openConnection();
myConn.setRequestProperty("Accept", "application/json, text/plain, */*");
myConn.setDoOutput(true);
DataOutputStream my_output = new DataOutputStream(myConn.getOutputStream());
my_output.write(myData.getBytes("UTF-8"));
But what to do if I want to send exactly the same data with same URl and headers multiple times?
Can I write to it multiple times?(I mean that is it possible to use the last line of code multiple times?) Or should I repeat the above steps and try it with a new connection?
And if yes should I wait for some second or millisecond before sending the next one?
I also searched for some other alternatives such as “HttpClient” Http API and making synchronous Http request which as far as I got can help me setting the headers only once.
At the end, I appreciate your help and support and any other alternatives would be welcome.
Thanks a million.

I understand that the question has be answered in the comments, but I am leaving this here so that future viewers can see it.
An HTTP request contains 3 main parts:
Request Line: Method, Path, Protocol
Headers: Key-Pairs
Body: Data
Running my_output.write() will just add bytes to the body until my_output.flush() has been executed. Flushing the stream will write the data to the server.
Because HTTP requests are usually closed by the server once all data has been sent/received, whether or not you create a new connection or just add on to the body depends on your intentions. Typically, clients will create a new connection for each request because each response should be handled individually, rather than sending a repetitive body. This will vary though because some servers choose to hold a connection (such as WebSockets).

If you are open to external libraries, you may find this chart insightful:
AsyncHttpClient would be a good fit for your intentions.
Alternatively, you can use cURL by running a terminal command with Runtime.getRuntime().exec(). More information about using cURL with POST Requests can be found here. While cURL is efficient, you have to depend on the fact that your OS supports the command (though usually all devices that can run Java have this command).

How to send info in body using post method in java class file

I requested to send some parameters from java file using post method. I did
String urlParameters = "param1=a&param2=b&param3=c";
URL url = new URL("http://testing/index.jsp");
URLConnection conn = url.openConnection();
conn.setDoOutput(true);
OutputStreamWriter writer = new OutputStreamWriter(conn.getOutputStream());
writer.write(urlParameters);
writer.flush();
But from receiver's end asks me to send it in body instead of url parameter. I am not sure what I am doing wrong. Please explain me how this code will work and what changes has to be done if I want to send info in request body.

i believe you either need to call the connect() method on the URLConnection at the end, or call a method that would cause the connect to be called for you, like fetching the resulting input stream.
Also you should think about what format the body should be in. Often people like to use standard formats like json, but you will have to decide that between you and the people implementing the server.

HTTPS Post request (not) using certificate/hash print?

I admit there is a possibility that I am not well informed about the subject, but I've done a LOADS of reading and I still can't get answer to my question.
From what I have learnt, to make communication secure with HTTPS I need to be using some sort of public key (reminds me of pgp-encryption).
My goal is to make a secured POST request from my java application (which I, in the moment it starts working, will rewrite to Android app, if it matters) to a php application accessible via https address.
Naturally I did some Google research on the topic and I got a lot of results how to make ssl connection. Non of those results used any sort of certificate/hash prints. They just use HttpsURLConnection instead of HttpURLConnection, everything else is almost identical.
Right now, almost copy paste of something I found here is this:
String httpsURL = "https://xx.yyyy.zzz/requestHandler.php?getParam1=value1&getParam2=value2";
String query = "email=" + URLEncoder.encode("abc#xyz.com", "UTF-8");
query+="&";
query+="password="+URLEncoder.encode("tramtarie","UTF-8");
URL myurl = new URL(httpsURL);
HttpsURLConnection con = (HttpsURLConnection) myurl.openConnection();
con.setRequestMethod("POST");
con.setRequestProperty("Content-length",String.valueOf(query.length()));
con.setRequestProperty("Content-Type","application/x-www-form-urlencoded");
con.setRequestProperty("User-Agent","Mozilla/4.0 (compatible; MSIE 5.0;Windows98;DigExt)");
con.setDoOutput(true);
con.setDoInput(true);
DataOutputStream output = new DataOutputStream(con.getOutputStream());
output.writeBytes(query);
output.close();
DataInputStream input = new DataInputStream(con.getInputStream());
for(
int c = input.read();
c!=-1;c=input.read())
System.out.print((char)c);
input.close();
System.out.println("Resp Code:"+con.getResponseCode());
System.out.println("Resp Message:"+con.getResponseMessage());
Which sadly does not work and ends up with this exception:
Exception in thread "main" javax.net.ssl.SSLHandshakeException: java.security.cert.CertificateException: No subject alternative DNS name matching app.elessy.cz found
This probably means that it checks the certificate and finds out that the certificate I am using does not match domain name for which is registered (it is webhosting certificate, registered for webhosting domain, not the domain I own, the only reason I am using https is to secure data for internal purposes, I do not want this site to be visited by users from outside, so this certificate should be ok).
There are two things that I just don't get about the code and everything.
No code I have been able to find use MD5/SHA-1 (supposedly the public keys for message encryption?) prints or
certificate, they just somehow automatically connect to https
website and should work. Doesn't work for me though.
Do I really need those md5/sha-1 prints that are provided to me? Or at least, what in the given context do those prints mean?
Edit:
Following the given answer and duplicate mark, I managed to get it working - in the meaning that I can communicate with application behind https.
But I didnt have to use any sort of md5/sha1 print. How do I know now that it is safe? Does this protocol on his own? Like that communication is secured either way, when I use built-in java classes to connect to app behind https?
I probably do not seek for precise technical explanation, but more for an assurance that yes - the communication is safe even though I do not use (knowingly) certificate/servers public key to encrypt my messages. That it does the ssl connection for me.

Getting HTTP response code in Java

I need to find the HTTP response code of URLs in java. I know this can be done using URL & HTTPURLConnection API and have gone through previous questions like this
and this.
I need to do this on around 2000 links so speed is the most required attribute and among those I already have crawled 150-250 pages using crawler4j and don't know a way to get code from this library (due to which I will have to make connection on those links again with another library to find the response code).

In Crawler4J, the class WebCrawler has a method handlePageStatusCode, which is exactly what you are looking for and what you would also have found if you had looked for it. Override it and be happy.

The answer behind your first link contains everything you need:
How to get HTTP response code for a URL in Java?
URL url = new URL("http://google.com");
HttpURLConnection connection = (HttpURLConnection)url.openConnection();
connection.setRequestMethod("GET");
connection.connect();
int code = connection.getResponseCode();
The response code is the HTTP code returned by the server.

Trying to GET a Google Spreadsheet in Java is returning HTTP error 405

Been working on this all day and have gotten no where with it.
My Java code looks like this:
final URL url = new URL(String.format("https://spreadsheets.google.com/feeds/download/spreadsheets/Export?key=%s&exportFormat=tsv&gid=0", spreadsheetId));
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestProperty("Authorization", "GoogleLogin auth=" + wiseAuth.getAuthToken());
conn.setRequestProperty("GData-Version", "3.0");
conn.setRequestMethod("GET");
conn.setDoOutput(true); // trouble here, see below
conn.setInstanceFollowRedirects(true);
conn.connect();
I always get a FileNotFound error when attempting to do conn.getInputStream(). I narrowed it down to being that the response code is 405 Method Not Allowed. The exception is returning me my URL and I can access the page just fine in my browser.
It was then that I discovered that setDoOutput(true) executes a POST internally. But if I remove that line, conn.getInputStream() is null, and conn.getOutputStream() appears to return nothing--though maybe I am setting it up wrong?

I don't recommend you to do it like this, even if you get it working now you cannot ensure you will get it working in the future if Google started changing it.
Instead, consider using Google Spreadsheet API. The provided Java examples are pretty straightforward and you should able to accomplish what you want.

I would recommend using a web debugger like Fiddler to see what exactly your application is sending in the GET request and compare it to your browser. You might be missing an important header or something, and Fiddler makes it really easy to slowly strip down your browser's request to the essential elements (just drag a request to clone it, then take out headers).