How to Implement HTTP Request Status Code Processing

Question

I actually have two questions. First, I wondering how to add multiple status code processing to an http request. Secondly, I was wondering how to go about using alternate http requests to different servers in case the primary server is down. What kind of parameter would the program use to determine that the server is unavailable and switch to another server??
Currently, the program I've written calls an rdf server (http://www.rdfabout.com/sparql) using a sparql query,
the server returns an xml string, the program parses it, and calculates numbers
from the string. The program works, but the problem is that the server is down occasionally.
When the server is down, we need to add calls to another server to
increase reliability. So, the next task is to call this server:
http://www.melissadata.com/lookups/ZipDemo2000.asp
I need to do exactly the same things I did with the rdf server. The
difference will be constructing a request and a bit different parsing of
the response.
current SPARQL query is defined as follows:
PREFIX dc:  <http://purl.org/dc/elements/1.1/>
PREFIX census: <http://www.rdfabout.com/rdf/schema/census/>
PREFIX census1: <tag:govshare.info,2005:rdf/census/details/100pct/>
DESCRIBE ?table WHERE {
<http://www.rdfabout.com/rdf/usgov/geo/census/zcta/90292> census:details
?details .
?details census1:totalPopulation ?table .
?table dc:title "SEX BY AGE (P012001)" .
}current HTTP Request is defined as follows:
import java.net.*;
import java.net.URL;
import java.net.URLConnection;
import java.io.*;
import java.io.DataOutputStream;
import java.io.BufferedReader;
import java.io.StringReader;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.Arrays; 
public class MyConnection
     static Scanner sc = new Scanner(System.in);//allows user to input zipcode
    public static void main(String[] args) throws Exception
         int zip;//zipcode is declared as integer format
        //User defines zip through input
        //proceed to put SPARQL query into string, which is then used to call the server
        String requestPart1 =
        "query=PREFIX+dc%3A++%3Chttp%3A%2F%2Fpurl.org%2Fdc%2Felements%2F1.1%2F%3E+%0D%0APREFIX+census%3A+%3Chttp%3A%2F%2Fwww.rdfabout.com%2Frdf%2Fschema%2Fcensus%2F%3E+%0D%0APREFIX+census1%3A+%3Ctag%3Agovshare.info%2C2005%3Ardf%2Fcensus%2Fdetails%2F100pct%2F%3E+%0D%0A%0D%0ADESCRIBE+%3Ftable+WHERE+%7B+%0D%0A+%3Chttp%3A%2F%2Fwww.rdfabout.com%2Frdf%2Fusgov%2Fgeo%2Fcensus%2Fzcta%2F";
        String requestPart2 = "" + zip; // zipcode is transformed from int to string format and plugged into SPARQL query here
        String requestPart3 =
        "%3E+census%3Adetails+%3Fdetails+.+%0D%0A+%3Fdetails+census1%3AtotalPopulation+%3Ftable+.+%0D%0A+%3Ftable+dc%3Atitle+%22SEX+BY+AGE+%28P012001%29%22+.+%0D%0A%7D%0D%0A&outputMimeType=text%2Fxml";
        String response = "";
        URL url = new URL("http://www.rdfabout.com/sparql");//designates server to connect to
        URLConnection conn = url.openConnection();//opens connection to server
        // Set connection parameters.
        conn.setDoInput (true);
        conn.setDoOutput (true);
        conn.setUseCaches (false);
        // Make server believe we are form data…
        conn.setRequestProperty("Content-Type","application/x-www-form-urlencoded");
        DataOutputStream out = new DataOutputStream (conn.getOutputStream ());
        // Write out the bytes of the content string to the stream.
        out.writeBytes(requestPart1 + requestPart2 + requestPart3);
        out.flush ();
        out.close ();
        // Read response from the input stream.
        BufferedReader in = new BufferedReader (new InputStreamReader(conn.getInputStream ()));
        String temp;
        while ((temp = in.readLine()) != null)
             response += temp + "\n";
        temp = null;
        in.close ();
        //parsing stuff is taken care of after here
}What remains now is to:
1) add status code processing: notify if the server is not available, ect.
2) add ability to connect to additional server if primary server is down.
I'm thinking an if/else statement, which I've tried a few different ways,
but I don't quite know how to implement that...Also trying to add the
status code processing/error handling, but I'm not sure how to do that
for multiple/different errors, such as 404, 503, 504, ect.. try/catch statements?
So yeah, just been scratching my head on this trying to figure out how to work it..
If you can help me out on this, I've been going nuts trying to figure this out...

Answer 1

I think your issue comes form the fact that you are not casting URLConnection to HttpURLConnection.
Doing the cast would allow you to use getResponseCode() - among other methods - and test for a response different than 200.
Read: [http://mindprod.com/jgloss/urlconnection.html|http://mindprod.com/jgloss/urlconnection.html]