有关java.lang.NullPointerException错误的问题

代码如下：
public static boolean validate(String username,String password)
{
boolean bool=false;

try
{
Class.forName("com.mysql.jdbc.Driver");
}
catch(java.lang.ClassNotFoundException e)
{
System.out.print("Class not found error occur.Message is:");
System.out.print(e.getMessage());
}

try
{
Statement stmt=null;
ResultSet rs;
String url="jdbc:mysql://localhost:3306/student";
String user="root";
String passwordOfDB="summer";
Connection conn=null;
try
{
conn=DriverManager.getConnection(url,user,passwordOfDB);
}
catch(Exception ex){ex.printStackTrace();}
String sql="select * from userinfo where username='"+username+"'and password='"+password+"'";
stmt=(Statement)conn.createStatement();
rs=(ResultSet)stmt.executeQuery(sql);

if(rs.next())
bool=true;
else
bool=false;

}
catch(SQLException e)
{
System.out.print("SQL Exception occur.Message is:");
System.out.print(e.getMessage());
}
return bool;
}
产生的错误：
exception

java.lang.NullPointerException
mypack.SQLQuery.validate(SQLQuery.java:38)
mypack.CheckA.doPost(CheckA.java:43)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)

上面的代码是SQLQuery.java中的，我被这个问题困扰将近一天了，始终是不知道该怎样解决？