Re: NullPointer error returned from ResultSet.java

Dave,

The driver does support having multiple result sets open at the same
time, and I do it frequently in my code.

Having two result sets open lets you nest queries as follows so that you
can use the results for one as input into the second:

PreparedStatement stmt1 = conn.prepareStatement("select bar1 from bar");
PreparedStatement stmt2 = conn.prepareStatement("select foo1 from foo "+
                                                 "where bar1 = ?");
ResultSet rset1 = stmt1.executeQuery();
while (rset1.next()) {
   //for each row in bar, update foo
   stmt2.setInt(1, rset1.getInt(1));
   ResultSet rset2 = stmt2.executeQuery();
   while (rset2.next()) {
     //do something useful with the results
   }
   rset2.close();
}
rset1.close();
stmt1.close();
stmt2.close();

thanks,
--Barry
Dave Cramer wrote:
> Jeff,
>
> You can't have two result sets open on the same connection.
>
> Dave
> On Mon, 2002-10-07 at 11:57, Jeff Duffy wrote:
>
>>Greetings,
>>
>> I have a JSP page with a rather large scriptlet that uses multiple nested
>>statements and ResultSets. Under very specific conditions I am encountering
>>the following error:
>>
>>java.lang.NullPointerException
>> at org.postgresql.jdbc2.ResultSet.next(ResultSet.java:113)
>>
>>
>>Here's one pertinent snippet of my code:
>>( This code begins inside a while(rs.next()) loop for ResultSet rs,
>>using Statement stmt)
>>
>>
>>sql.append("AN SQL QUERY");
>>rs2 = stmt2.executeQuery(sql.toString());
>>
>>while(rs2.next()){
>>     String courseId = rs2.getString(1);
>>     String roleName = rs2.getString(2);
>>
>>     rs3 = stmt3.executeQuery("SELECT nextval('assignment_id')");
>>     rs3.next();
>>     assignmentId = rs3.getString(1);
>>
>>    sql.delete(0, sql.length());
>>    sql.append("AN SQL QUERY");
>>    stmt3.executeUpdate(sql.toString());
>>}
>>
>> Through debugging it appears that rs2 is throwing the error; the message
>>states the error is in method next(), but if I have only zero records or
>>only one record in rs2, there is no error. It's only when I have multiple
>>records that the error is thrown the second time round the while loop, so I
>>suspect it may be thrown at the assignment of courseId.
>>
>> This error is reproduced given the same conditions (nested ResultSets with
>>the innermost ResultSet executing an Update inside a while(ResultSet.next())
>>loop.  It almost seems that the call to rs3.executeUpdate() is closing rs2,
>>but I cannot see why that would happen. This is occurring in at least six
>>different source files.
>>
>> If I create a new Vector and stuff the contents of rs2 into it, then
>>iterate over the vector and use rs3 as shown above, no error is thrown.
>>
>> Updating to the 7.3beta1 JDBC driver changes the error message to:
>>
>>javax.servlet.ServletException: postgresql.con.closed
>>
>> which is even more puzzling.
>>
>>
>> Notes about my coding style that may be relevant:
>>- I use one StringBuffer for all sql queries and clear it for each new query
>>for efficiency. This hasn't ever caused problems elsewhere.
>>
>>- I predeclare all Statement and ResultSet objects at the outermost block of
>>code (the 'top') like so:
>>
>>Statement stmt   = conn.createStatement();
>>Statement stmt2  = conn.createStatement();
>>Statement stmt3  = conn.createStatement();
>>ResultSet rs, rs2, rs3;
>>
>>Other data:
>>
>>- The connections are obtained from a custom class that uses the jdbcpool
>>pool manager.
>>- I'm using the Sun JDK v1.4.0 for Linux (it happens on Win32 as well).
>>- The JDBC library is v7.2 (the pgjdbc2.jar binary download).
>>- PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC 2.96
>>
>> Any flashes of insight?
>>
>>Thanks
>>
>>Jeff Duffy
>>jeff@alanne.com
>>
>>
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