Science With the Virtual Observatory 2006 Summer School

Adding Registry Support to the Cone Search Client: Hints

The stubbed portion of ConeCaller.java that we need to add to is in the main() method; it looks like this:

    if (query != null) {
        // search the registry for ConeSearch services


    }

We can follow a bit of the pattern we used in FindConeSearch.java's main() to query the Registry. So to start, we can insert into the above if-block the following:

  SimpleResource[] cs = FindConeSearch.keywordSearch(query);
  if (cs == null || cs.length == 0) {
      System.err.println("No matching cone search services found");
  }
  else {
      System.out.println("==================================");

      for(int i=0; i < cs.length; i++) {
          // print out the title, URL, & number of records
          // returned by the cone search

      }
  }

The for loop steps through the matched service descriptions from which we need to extract the baseURL. We need to remember to fix any messy URLs, we can insert as the first step within the loop, the fix:

   String baseURL =  
      FindConeSearch.correctBaseURL(cs[i].getServiceURL());

Next we'll print out some some info from the resource description:

   System.out.println("Title: " + cs[i].getTitle());
   System.out.println("Address: " + baseURL);

As for submitting a Cone Search and counting the results, let's put that into a separate function.

   int nr = countHits(ra,dec,sr, baseURL);

   System.out.println("Matching catalog records found: " +
                       nr);
   System.out.println("==================================");

Our first implementation of countHits() might look pretty simple, borrowing from the readVot() function:

public static int countHits(double ra, double dec, double sr,
                            String coneUrl) 
    throws Exception
{
   URL cone = new URL(coneUrl + "RA="+ra+"&DEC="+dec+"&SR="+sr);

   VOTWrap.VOTable vot = VOTWrap.createVOTable(cone.openStream());
   VOTWrap.TableData tdata = 
       vot.getResource(0).getTable(0).getTableData();
   return tdata.getTRCount();
}

However, there are lots of things that can go wrong, so we better try to catch some of the errors with a try-catch enclosure:

public static int countHits(double ra, double dec, double sr,
                            String coneUrl) 
    throws Exception
{
   try {
      URL cone = new URL(coneUrl + "RA="+ra+"&DEC="+dec+"&SR="+sr);

      VOTWrap.VOTable vot = VOTWrap.createVOTable(cone.openStream());
      VOTWrap.TableData tdata = 
          vot.getResource(0).getTable(0).getTableData();
      return tdata.getTRCount();
   }
   catch (  ) {

   }
}

We'll catch three possible errors. First, despite our effort to clean up the URL, it may still be erroneous:

   catch (MalformedURLException ex) {
       System.err.println("Problem with URL: " + coneUrl);
       return 0;
   }

Next, the server may be down...

   catch (ConnectException ex) {
       System.err.println("Service not responding.");
       return 0;
   }

Finally, we'll catch any other possible problems:

   catch (Exception ex) {
       System.err.println("Unknown service error.");
       return 0;
   }

The most common reason for this last error condition is due to the VOTable not having a TABLE in it (causing a NullPointerException). Instead the VOTable only contains an error message. There is a standard for encoding errors, so in principle we could (and should) pull this out and display it. Unfortunately, very few Cone Search implementations follow the standard correctly.

Our implementation is mostly complete. To get our countHits() function to compile not that we have put in the catching of exceptions, we need to remove the throws clause in the signiture:

public static int countHits(double ra, double dec, double sr, 
                            String coneUrl) 
{
    try {
        URL cone = new URL(coneUrl + "RA="+ra+"&DEC="+dec+"&SR="+sr);

        VOTWrap.VOTable vot = VOTWrap.createVOTable(cone.openStream());
        VOTWrap.TableData tdata = 
            vot.getResource(0).getTable(0).getTableData();
        return tdata.getTRCount();
    }
    catch (MalformedURLException ex) {
        System.err.println("Problem with URL: " + coneUrl);
        return 0;
    }
    catch (ConnectException ex) {
        System.err.println("Service not responding.");
        return 0;
    }
    catch (Exception ex) {
        System.err.println("Unknown service error.");
        return 0;
    }
}

Ahh. Don't you feel safer now?

The NVO Summer School is made possible through the support of the National Science Foundation and the National Aeronautics and Space Administration.