JSP中查询不稳定怎么解决？
在用JSP做网站的时候，为什么出现查询不稳定的情况！！就是说：输入同一个查询条件，有时候得到查询结果，有时候得不要查询结果，而数据库中有这样一条记录！有的时候甚至还出现空指针异常！！请问这是怎么回事？？急！
以下为执行查询的控制器servlet!
                                    String   condition=request.getParameter( "condition ").trim();
String   bookinfo=request.getParameter( "bookinfo ").trim();
BookMgr   book=new   BookMgr();

//Collection   c;
//Book   booklist;

if(condition.equals( "bookname "))
{
Collection   c=book.searchByName(bookinfo);
if(c.isEmpty())
{
//没有找到指定的书
request.setAttribute( "error ",   "没有找到符合条件的书，请重新选择查询条件！！！ ");
request.getRequestDispatcher( "error.jsp ").forward(request,   response);
}
else
{

request.setAttribute( "booklist ",   c);
request.getRequestDispatcher( "list.jsp ").forward(request,   response);
}
/*
while(bookinterator.hasNext())
{
booklist=(Book)bookinterator.next();

}
*/
}
if(condition.equals( "author "))
{
Collection   c=book.searchByAuthor(bookinfo);
if(c.isEmpty())
{
//没有找到指定的书
request.setAttribute( "error ",   "没有找到符合条件的书，请重新选择查询条件！！！ ");
request.getRequestDispatcher( "error.jsp ").forward(request,   response);
}
else
{

request.setAttribute( "booklist ",   c);
request.getRequestDispatcher( "list.jsp ").forward(request,   response);
}
}
if(condition.equals( "publish "))
{
Collection   c=book.searchByPublish(bookinfo);
if(c.isEmpty())
{
//没有找到指定的书
        request.setAttribute( "error ",   "没有找到符合条件的书，请重新选择查询条件！！！ ");
request.getRequestDispatcher( "error.jsp ").forward(request,   response);
}
else
{

request.setAttribute( "booklist ",c);
request.getRequestDispatcher( "list.jsp ").forward(request,   response);
}
}
if(condition.equals( "ISDN "))
{
//暂不支持这个查询方法，如果以ISDN来进行查询会抛出异常
throw   new   ServletException();
}

以下是BookMgr类是一个按书名执行查询的方法
/*
  *   task:   按书名进行模糊查询
  *   @param   BookName要查询的书名
  *   @return返回Collection对象;
  */
public   Collection   searchByName(String   BookName)
{
conn=DB.getConnection();
Collection   c=new   ArrayList();
String   sql= "select   *   from   book   where   BookName   like   '% "+BookName+ "% ' ";
try   {
Statement   stm=conn.createStatement();
ResultSet   rs=stm.executeQuery(sql);
Book   book;
while(rs.next())
{
book=new   Book();
book.setID(rs.getInt( "id "));
book.setBookName(rs.getString( "BookName "));
book.setBookClass(rs.getInt( "BookClass "));
book.setAuthor(rs.getString( "Author "