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How to work with server response codes in java?

I am new to programming (especially in java) and I most likely lack knowledge with server work in java, my question is that I could send a request to the server and at the same time receive a response in the form of a response code, for example 404 (file not found), please someone tell me how to correctly implement this
the code we currently have
public static void Connection(int portNumber, String addr, String request) throws UnknownHostException, IOException {
URL url = new URL(addr);
String postData = request; // html request
int response = 0;
responses = response;
URLConnection conn = url.openConnection();
conn.setDoOutput(true);
conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
conn.setRequestProperty("Content-Length", Integer.toString(postData.length()));
//<-------------------------------------Add a response code------------------------------------->//
try (DataOutputStream dos = new DataOutputStream(conn.getOutputStream())) {
dos.writeBytes(postData);
}
try (BufferedReader bf = new BufferedReader(new InputStreamReader(
conn.getInputStream())))
{
String line;
while ((line = bf.readLine()) != null) {
System.out.println(line);
}
}
}
Honestly. I've been scouring the internet and trying to find this in java books, but I haven't been able to find a proper answer

If you just want to send Http request and receive the data back you can just use 3d party Http clients. The most popular are Apache Http Client with good tutorial - Apache HttpClient Tutorial and OK Http client with good tutorial - A Guide to OkHttp. However, If you want to learn how to use Java classes such as URLConnection so you can write your own code than I can offer you to look at source code of my own Http client that I wrote using those classes. This HttpClient can also be used as 3d party Http client (although it is a simplistic and not well-known as the 3d party clients I mentioned above), but also you can look at the source code that is not that big and (I hope) is well and clearly written. So it could be used as tutorial as well. This HttpClient comes as part of MgntUtils Open Source library written and maintained by me. Here is the source code of HttpClient. Here is its Javadoc. If you want the source code of the whole library you can get it on Github here, and just the library as Maven artifact is available from Maven Central here

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.

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);

Reading from a URLConnection

I have a php page in my server that accepts a couple of POST requests and process them. Lets say it's a simple page and the output is simply an echoed statement. With the URLConnection I established from a Java program to send the POST request, I tried to get the input using the input stream got through connection.getInputStream(). But All I get is the source of the page(the whole php script) and not the output it produces. We shall avoid socket connections here. Can this be done with Url connection or HttpRequest? How?
class htttp{
public static void main(String a[]) throws IOException{
URL url=new URL("http://localhost/test.php");
URLConnection conn = url.openConnection();
//((HttpURLConnection) conn).setRequestMethod("POST");
conn.setDoOutput(true);
conn.setDoInput(true);
OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
wr.write("Hello");
wr.flush();
wr.close();
InputStream ins = conn.getInputStream();
InputStreamReader isr = new InputStreamReader(ins);
BufferedReader in = new BufferedReader(isr);
String inputLine;
String result = "";
while( (inputLine = in.readLine()) != null )
result += inputLine;
System.out.print(result);
}
}
I get the whole source of the webpage test.php in result. But I want only the output of the php script.

The reason you get the PHP source itself, rather than the output it should be rendering, is that your local HTTP server - receiving your request targeted at http://localhost/test.php - decided to serve back the PHP source, rather than forward the HTTP request to a PHP processor to render the output.
Why this happens? that has to do with your HTTP server's configuration; there might be a few reasons for that. For starters, you should validate your HTTP server's configuration.
Which HTTP server are you using on your machine?
What happens when you browse http://localhost/test.php through your browser?

The problem here is not the Java code - the problem lies with the web server. You need to investigate why your webserver is not executing your PHP script but sending it back raw. You can begin by testing using a simple PHP scipt which returns a fixed result and is accessed using a GET request (from a web browser). Once that is working you can test using the one that responds to POST requests.

Charset specific chars work in IDE, but not on live

So heres my problem. I'm reading a json from web using httpurlconnection. That json contains german special chars (äöü). Inside NetBeans, everything is fine. When I build the jar an run it, "Silberanhänger" changes to "Silberanhänger". Heres the code, nothing special inside
URL url = new URL("jsonUrl);
HttpURLConnection con = (HttpURLConnection) url.openConnection();
con.setUseCaches(false);
con.setRequestProperty("Accept-Language","de-de,de;q=0.8,en-us;q=0.5,en;q=0.3");
con.setRequestProperty("Cookie","s="+session);
try (BufferedReader bf = new BufferedReader(new InputStreamReader(
con.getInputStream()))) {
jsonRepresentation = bf.readLine(); //only 1 line
}
con.disconnect();
System.out.println(jsonRepresentation) // "ä" in IDE, "ä" in Live

Setting -Dfile.encoding=UTF8 is a hack that will have side-effects on all code run on that JVM. A better hack would be to specify the charset in the InputStreamReader's constructor
new InputStreamReader(con.getInputStream(), "UTF-8")
However this might still fail if the HTTP server on the other end changes its encoding. You would be better off using a HTTP library such as Apache HTTPComponents to parse the HTTP response into a String. It will read the encoding from the HTTP header and do the right thing in all circumstances.

Set jvm encoding with -Dfile.encoding=UTF8