一个关于JAVA调用数据库存储过程的问题；

Question

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();
}}运行错误提示：java.lang.NullPointerException
at storedprocedure.<init>(storedprocedure.java:37)
at storedprocedure.main(storedprocedure.java:135)
也就是这一个地方有问题：cstm1 = conn.prepareCall("{call getcoursename()}");求解！！！！！！！！

Answer 1

 Connection conn =DriverManager.getConnection("jdbc:sqlserver://localhost:1433;databaseName=练习","sa","");
注意结构，
把Connection 往外面提出去，作用域就在那try catch里面，然后就消失了。
Connection conn = null;
 try{
        Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
     }catch(Exception e1){JOptionPane.showMessageDialog(null,"驱动获取失败！");}
        try{
            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()}");