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 "));
book.setPublish(rs.getString( "Publish "));
book.setBookNo(rs.getString( "BookNo "));
book.setContent(rs.getString( "Content "));
book.setPrice(rs.getFloat( "Price "));