一个关于JAVA调用数据库存储过程的有关问题

import java.awt.*;import java.awt.event.*;import java.sql.*;import javax.swing.*;import javax.swing.table.*;import java.util.*; public class storedprocedure extends JFrame implements ItemListener {             Connection conn;    CallableStatement cstm1;    CallableStatement cstm2;    ResultSet rs1;    ResultSet rs2;    JTable table;    DefaultTableModel dtm;    DefaultComboBoxModel dcm;    JComboBox comb;    JLabel label1 = new JLabel("请选择课程名称：");;    JLabel label2 = new JLabel("考试人数：");    JLabel label3 = new JLabel("平均分：");    Vector<String> title = new Vector<String>();             public storedprocedure (){                try{            Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");        }catch(Exception e1){JOptionPane.showMessageDialog(null,"驱动获取失败！");}        try{            Connection conn =DriverManager.getConnection("jdbc:sqlserver://localhost:1433;databaseName=练习","sa","");        }catch(SQLException e2){JOptionPane.showMessageDialog(null,"数据库连接失败！");}        try{            cstm1 = conn.prepareCall("{call getcoursename()}");            rs1 = cstm1.executeQuery();            dcm = new DefaultComboBoxModel();            comb = new JComboBox(dcm);            initcombobox();            cstm2 = conn.prepareCall("{?=call stat(?,?)}", ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_READ_ONLY);            cstm2.registerOutParameter(1, java.sql.Types.INTEGER);            cstm2.setString(2, (String)comb.getSelectedItem()+"%");            cstm2.registerOutParameter(3, java.sql.Types.FLOAT);            rs2 = cstm2.executeQuery();                        ResultSetMetaData dbmd = rs2.getMetaData();                        for(int i=1 ;i<=dbmd.getColumnCount();i++){                title.addElement(dbmd.getColumnName(i));            }            dtm = new DefaultTableModel(null,title);            table = new JTable(dtm);            inittable();            int count = cstm2.getInt(1);            Float avg = Math.round(cstm2.getFloat(3)*100f)/100.0f;            table.setRowHeight(20);            label2.setText("考试人数："+count);            label3.setText("平均分："+avg);                        JPanel pane = new JPanel();            pane.add(label1);pane.add(comb);pane.add(label2);pane.add(label3);            JScrollPane jp = new JScrollPane(table);            Container con = getContentPane();            con.setLayout(new BorderLayout());            con.add(pane,"North");            con.add(pane,"Center");            comb.addItemListener(this);        }catch(Exception e4){e4.printStackTrace();}        this.addWindowListener(new WindowAdapter(){            public void WindowClosing(WindowEvent e){                try{                    rs1.close();                    rs2.close();                    cstm1.close();                    cstm2.close();                    conn.close();                }catch(SQLException e6){e6.printStackTrace();};            }        });        setTitle("学生成绩浏览");        Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();        setLocation((screen.width-400)/2,(screen.height-400)/2);        setSize(600,400);        setVisible(true);    }         public void inittable(){                dtm.setRowCount(0);        try{            Vector<String> v = new Vector<String>();            rs2.beforeFirst();            while(rs2.next()){            for(int i=1;i<=title.size();i++){                v.addElement(rs2.getString(i));            }            dtm.addRow(v);            }        }catch(Exception e5){e5.printStackTrace();};    }     public void initcombobox(){        dcm.removeAllElements();        try{            while(rs1.next()){                dcm.addElement(rs1.getString(1).trim());            }                    }catch(Exception e3){e3.printStackTrace();}    }         public void itemStateChanged(ItemEvent e){                if(e.getStateChange()==ItemEvent.SELECTED){            try{            cstm2.registerOutParameter(1, java.sql.Types.INTEGER);            cstm2.setString(2, (String)comb.getSelectedItem()+"%");            cstm2.registerOutParameter(3, java.sql.Types.FLOAT);            rs2 = cstm2.executeQuery();            inittable();            int count = cstm2.getInt(1);            Float avg = Math.round(cstm2.getFloat(3)*100f)/100.0f;            table.setRowHeight(20);            label2.setText("考试人数："+count);            label3.setText("平均分："+avg);                        }catch(Exception e7){e7.printStackTrace();}        }    }        public static void main (String[] args){                new storedprocedure();    }}