JAVA数据库编程中java.lang.NullPointerException异常

JAVA中是不是没有全局变量的概念，但定义在所有方法外的变量是什么变量 ？
import java.sql.*;
public class DataBaseManager {
    Connection con=null;
    ResultSet rs;
    Statement stmt;
    public DataBaseManager(){
        try{
            Class.forName("com.mysql.jdbc.Driver");
            con=DriverManager.getConnection("jdbc:mysql://localhost:3306/liu","root","yes");
            stmt=con.createStatement();
            con.setAutoCommit(false);
        }
        catch(ClassNotFoundException cnfe){
            System.out.println("Failed to load JDBC/ODBC driver.");
            cnfe.printStackTrace();
            System.exit(1);
        }
        catch(SQLException sqle){
            System.out.println(sqle.toString());
        }
    }
   
    public ResultSet getResult(String strSQL){
        try{
            rs=stmt.executeQuery(strSQL);
            return rs;
        }
        catch(SQLException sqle){
            System.out.println(sqle.toString());
            return null;
        }
    }
    public boolean updateSql(String strSQL){
        try{
            stmt.executeUpdate(strSQL);
            con.commit();
            return true;
        }
        catch(SQLException sqle){
            System.out.println(sqle.toString());
            return false;
        }
    }
    public void closeConnection(){
        try{
            rs.close();
            stmt.close();
            con.close();
        }
        catch(SQLException sqle){
            System.out.println(sqle.toString());
        }
    }
}




借阅类：
import java.sql.*;
public class BorrowBook {
    DataBaseManager db=new DataBaseManager();
    void borrow(String sno,String bno,String bdate,String rdate){
            String strSQL="insert into BookBrowse(studentname,bookname,borrowdate,returndate) values('"+sno+"','"+bno+"','"+bdate+"','"+rdate+"')";
            if(db.updateSql(strSQL)){
                System.out.println("借阅完成！");
                db.closeConnection();
    }
}
}



import java.sql.*;
public class Test {
    public static void main(String[]args) throws InstantiationException, IllegalAccessException{
        DataBaseManager db=new DataBaseManager();
        ResultSet rs;
        BorrowBook bb=new BorrowBook();
        bb.borrow("some", "ghost", "2011-12-5", "2012-3-12");
        rs=db.getResult("select* from bookbrowse");
        try {
            if(!rs.first()){
                System.out.println("学生没有借阅过此书");
            }
            else{
                System.out.println(rs.getString("studentName"));
                System.out.println(rs.getString("bookName"));
                System.out.println(rs.getString("borrowDate"));
                while(rs.next()){
                    System.out.println(rs.getString("studentName"));
                    System.out.println(rs.getString("bookName"));
                    System.out.println(rs.getString("borrowDate"));
                }
            }
            db.closeConnection();
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}
这是数据库编程中的出现的一个问题。
抛出异常：java.lang.NullPointerException
可能是由于关闭数据库的链接对象时，将所有方法共享的conn关掉，出现异常。以后方法使用时找不到conn，请问此处问题是什么？
conn是否是在所有DataBaseManager类中共享？？？
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有谁熟悉数据库编程，希望指点
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