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JDBC driver, getTable Method

Posted on 2004-04-22
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Last Modified: 2008-03-04
i am  writing a jdbc driver for one of the application. while writing a code for method getTables in DatabaseMetadata.java, i am getting some weird exception while traversing the resultset. (on further debuging, it is coming from next method in Resultset class.)

this is a part of client application which is using JDBC driver.
--------------------------------------------
DatabaseMetaData md = conn.getMetaData();
      System.out.println("TEST-2");

      ResultSet rs1 = md.getTables(null, null, null, null);
      System.out.println("TEST3");
      //System.out.println(rs1.size());
       while ( rs1.next() )
      {
            System.out.println(rs1.getString(1));
      }
    } catch(Exception ex) {
      System.out.println(ex.getMessage());
     
    }
--------------------------------------------

this is a getTable Method
--------------------------------------------
public ResultSet getTables(String catalog, String schemaPattern, String tableNamePattern, String[] types) throws SQLException {
    // the field descriptors for the new ResultSet
System.out.println("in gettable-1");
    Field f[] = new Field[4]; // = new Vector();
    Vector field1_ = new Vector();
    ResultSet r;      // ResultSet for the SQL query that we need to do
    Vector v = new Vector();            // The new ResultSet tuple stuff
// FIELD_TYPE_VAR_STRING = 253 in DBQDefs.java
    f[0] = new Field("FILE_ALIAS", 253, 32);
    f[1] = new Field("USER_EXIT", 253 ,32);
    f[2] = new Field("FILE_INDEX", 253, 32);
    f[3] = new Field("FILE_NAME", 253 ,256);

field1_.addElement(f[0]);
field1_.addElement(f[1]);
field1_.addElement(f[2]);
field1_.addElement(f[3]);

    // Now form the query
    StringBuffer sql = new StringBuffer("disp files");
System.out.println("in gettable-2");
    java.sql.Statement stmt = _conn.createStatement();       
     //Statement stmt = new com.appres.sgi.Statement(_conn, _database);
    r = stmt.executeQuery(sql.toString());

      while (r.next())
      {
            byte[][] tuple = new byte[4][0];
            tuple[0] = r.getString(1).getBytes();
            tuple[1] = r.getString(2).getBytes();
            tuple[2] = r.getString(3).getBytes();
                tuple[3] = r.getString(4).getBytes();
            // remove after success
            System.out.print(r.getString(1));
            v.addElement(tuple);
      }
      r.close();
      System.out.println("in gettable-3");
      return new com.appres.sgi.ResultSet(field1_, v);
  }
--------------------------------------------

and this is a part of Resultset.java part from where it throws the exception.
-----------------------------------------------------
public boolean next() throws java.sql.SQLException
      {

          // debug
          //System.out.println("in Resultset Next" + rows_.size());
          if (rows_.size() == 0) return false;
          if (current_row_num_ + 1 >= rows_.size()) return false;

          current_row_num_++;
          try {
          current_row_ = (Vector) rows_.elementAt(current_row_num_);
          } catch(RuntimeException ex2){
              System.out.println(ex2.getMessage());
              throw ex2;
          }
          catch(Exception ex1){
              System.out.println(ex1.getMessage()); throw ex1;

          }
          return true;
      }
-------------------------------------------------
exception is thrown at rows_.elementAt(current_row_num_);

thanks in advance for any help.

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Question by:mkums00
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19 Comments
 
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by:CEHJ
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>>current_row_ = (Vector) rows_.elementAt(current_row_num_);

Make sure you're not going beyond the max index of the Vector
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by:CEHJ
Comment Utility
i.e.

current_row_num_++;
if (current_row_num >= rows_.size()) {
    return false;
}
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Author Comment

by:mkums00
Comment Utility
CEHJ,
It is giving error on very first iteration. i added the line u mentioned, same result.
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Expert Comment

by:CEHJ
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Sorry - i wasn't concentrating - you're doing that anyway. Please post full stack trace
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Author Comment

by:mkums00
Comment Utility
here is the output of ex2.printStackTrace(); in Resultset.java

------------
java.lang.ClassCastException: [[B
        at com.appres.sgi.ResultSet.next(ResultSet.java:83)
        at HelloDBQ.main(HelloDBQ.java:48)

---------------------
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Expert Comment

by:CEHJ
Comment Utility
Tell me what this prints. Instead of

>>current_row_ = (Vector) rows_.elementAt(current_row_num_);

do



Obejct o = rows_.elementAt(current_row_num_);
System.out.println("Current row is of type " + o);
current_row_ = (Vector)o ;
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Expert Comment

by:CEHJ
Comment Utility
Or better:

System.out.println("Current row is of type " + o.getClass());
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Author Comment

by:mkums00
Comment Utility
here it is
---------------
Current row is of type class [[B
[[B
[[B
java.lang.ClassCastException: [[B
        at com.appres.sgi.ResultSet.next(ResultSet.java:86)
        at HelloDBQ.main(HelloDBQ.java:48)
--------------------------------

thanks
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Accepted Solution

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CEHJ earned 100 total points
Comment Utility
OK - so what's happening is that you don't have a Vector of Vectors, but a Vector of byte arrays. So you can't cast the row to the expected type (Vector). If you want to be able to have the Vector hold rows of any type, you need to make current_row_ of type Object
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Expert Comment

by:seanrowen
Comment Utility
"[[B" indicates that it's actually a two-dimensional array of bytes -- a byte[][].

I can see that your ResultSet has byte[][]s as its "rows", but you are later treating them as Vectors. This is the problem -- do you mean to treat the rows as byte[][] everywhere?
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by:CEHJ
Comment Utility
How does that differ from what i've already said seanrowen?
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Author Comment

by:mkums00
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CEHJ,
thanks for the insight. I can't change the type of curr_row_ , but will try to do something diffrent and let you know.
thanks
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by:CEHJ
Comment Utility
OK
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by:seanrowen
Comment Utility
CEHJ -- you said that the type of the Object in question was a Vector whose elements are arrays of bytes. The output that mkums00 posted shows that it is actually a byte[][], which is of course not the same thing.

Not a big deal, but I thought that might clarify things. You can see in the code where this result set is filled with byte[][], so maybe that makes things make more sense for him/her.
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Expert Comment

by:rama_krishna580
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try this example it will help you..

http://cswww.essex.ac.uk/TechnicalGroup/TechnicalHelp/databaseMetaData.htm

best of luck

R.K
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by:CEHJ
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>>you said that the type of the Object in question was a Vector whose elements are arrays of bytes

That's right - that is the situation. 'rows_' is of type Vector and contains arrays of bytes.
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by:seanrowen
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OK, I understand what you were referring to. rows_ is a Vector of byte[][].
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Author Comment

by:mkums00
Comment Utility
Thanks everyone for help. CEHJ was the first to provide the clue about where i was mistaken, I changed the code in getTables and it is working now.

instead of byte[][] , i changed it to be a vector.
----------------
 StringBuffer sql = new StringBuffer("disp files");
    java.sql.Statement stmt = _conn.createStatement();       
     //Statement stmt = new com.appres.sgi.Statement(_conn, _database);
    r = stmt.executeQuery(sql.toString());

      while (r.next())
      {
          /**
            byte[][] tuple = new byte[4][0];
            tuple[0] = r.getString(1).getBytes();
            tuple[1] = r.getString(2).getBytes();
            tuple[2] = r.getString(3).getBytes();
                tuple[3] = r.getString(4).getBytes();
            // remove after success
            System.out.print(r.getString(1));***/
            //v.addElement(tuple);
                row = new Vector(); // could be a memory leak???
                row.addElement(r.getString(1));
                row.addElement(r.getString(2));
                row.addElement(r.getString(3));
                row.addElement(r.getString(4));
                //v.addElement(row);
                v.add(row);
      }
      //v.add(row);
      r.close();
      System.out.println("in gettable-3");
      return new com.appres.sgi.ResultSet(field1_, v);
----------------

Thanks Again
M KUMAR
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by:CEHJ
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;-) No problem - don't forget to close the question
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