Question

I was working on a servlet that will generate a unique code and update that in a mySQL database.
Now, in that, I want to catch any exception thrown in case that unique code already exists in the mySQL table and generate a new code and try updating the database. The problem is I want to do this WITHIN the for loop itself. The code is as follows:

    try
    {
    connection = datasource.getConnection();
    SQLUpdate = "INSERT INTO Voucher_dump VALUES( '"+unique_code+"','08-10-2011 04:48:48','0')";
      PreparedStatement ps1 = connection.prepareStatement(SQLUpdate);
      ps1.executeUpdate();
      ResultSet r = ps1.getResultSet(); // this is where I'm checking if it's a duplicate
      if(r==null)
          out.println("This is a duplicate");
      else out.println("Updated");
      trial12= "08-10-2011 04:48:480.03999855056924717a";
      SQLUpdate = "INSERT INTO Voucher_dump VALUES( '"+trial12+"','08-10-2011 04:48:48','0')";

      ps1 = connection.prepareStatement(SQLUpdate);
      ps1.executeUpdate();
      r = ps1.getResultSet();
      if(r==null)
          out.println("This is a duplicate");
      else out.println("Updated");

    }
    catch (SQLException sqle)
    {
        sqle.printStackTrace();
    }

I don’t want to wait till the end of the entire loop to catch the SQLException (I have already defined this key in mySQL as primary). The moment, the result comes back as a duplicate entry, I want to re-generate this key and attempt the update again.My output for this particular code is coming blank on my output page (all other parameters are showing correctly). Neither is “This is a duplicate” displayed nor is “Updated”. Maybe, ResultSet is not the best way to do it. Could you guys give me some advice on what would be the best way forward ?

Answer 1

Some advice in no particular order:

1. Close the connection in a finally block.
2. Close statements individually if you’ll be creating many of them before closing the connection. (“Many” is defined by your DBAs.)
3. Format your code.
4. Don’t use stdout and/or stderr from real code. Pick a logging framework.
5. Consider using some helper classes to simplify (and correct) your database access, like Spring’s JdbcTemplate.
6. Make sure to include relevant context when you post example code.

Due to #6, I don’t know what out is, but I suspect the reason you’re not seeing anything is that you’re inserting a duplicate value with the first statement, which will cause a SQLException from that line, not at getResultSet(), where you seem to expect it. Since the error is written to stdout, it’ll show up in your server logs somewhere, but nothing will be written to out. I’m not sure why you think getResultSet() will return null or not null depending on whether there was a constraint violation. Take a look at the javadoc for that method.

Update: 7. As BalusC points out, never, ever concatenate a string directly into a JDBC Statment. Use PreparedStatment’s placeholders and set* methods. For info on SQL injection, see Wikipedia and XKCD.