超链接不能传递参数，怎么解决啊

Question

book定义过没有
String itemIsbn=request.getParameter("isdn");中是isbn吧？

Answer 1

有啊
if(!"BY_ISBN".equals(request.getAttribute("type")))
   {
   Collection<Book> books =(Collection<Book>)request.getAttribute("book");
      if(books==null){
      return;
      }
      Iterator<Book> ibooks=books.iterator();
   while(ibooks.hasNext()){
   Book book=ibooks.next();
我在那个JSP页面中有这些代码啊

Answer 2

应该book的作用范围有问题，你这里的book是局部变量，未必能覆盖下面的引用
另外我觉得你可能有控件叫"isbn"而不是"isdn"，可能这里有手误

Answer 3

java.lang.NullPointerException把完整的错误信息发上来

Answer 4

at shiyin.bookShop.servlet.ShoppingCartServlet.doAdd(ShoppingCartServlet.java:94)
at shiyin.bookShop.servlet.ShoppingCartServlet.service(ShoppingCartServlet.java:49)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at shiyin.bookShop.util.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:18)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:215)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151)
at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:834)
at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:640)
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1286)
at java.lang.Thread.run(Thread.java:619)

Answer 5

  <table border="1">
<%
   if(!"BY_ISBN".equals(request.getAttribute("type")))
   {
   Collection<Book> books =(Collection<Book>)request.getAttribute("book");
      if(books==null){
      return;
      }
      Iterator<Book> ibooks=books.iterator();
   while(ibooks.hasNext()){
   Book book=ibooks.next();
  
%>
<tr>
<td><%=book.getIsbn() %></td>
<td><%=book.getTitle() %></td>
<td><%=book.getAuthor() %></td>
<td><%=book.getPrice() %></td>
<td><a href="shoppingCart?operation=addItem&isbn=<%=book.getIsbn() %>">加入购物车</a></td>
</tr>
<%
}
 %>
  </table>
以上代码运行是可以查询到BOOK的　只是一点“加入购物车”后就不能传递给后台的Servlet

Answer 6

而且浏览器的地址栏上也显示http://localhost:8080/bookShop/shoppingCart?operation=addItem&isbn=978-7-121-03655-2

Answer 7

之后就显示这样的代码java.lang.NullPointerException
shiyin.bookShop.servlet.ShoppingCartServlet.doAdd(ShoppingCartServlet.java:94)
shiyin.bookShop.servlet.ShoppingCartServlet.service(ShoppingCartServlet.java:49)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
shiyin.bookShop.util.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:18)

Answer 8

你的页面能找的到Servlet吗？
在你Servlet打印一句话看看

Answer 9

我知道什么原因了我把String operation=request.getParameter("operation");写成String operation=request.getParameter("opereation");String itemIsbn=request.getParameter("isbn");又写成String itemIsbn=request.getParameter("isdn");一字这差啊！！！谢谢各位的帮助。太感谢了。以前书看得很多，就是比较少写代码，经验不足。呵呵。

Answer 10

大哥看清楚！！！
<a href="shoppingCart?operation=addItem&isbn=<%=book.getIsbn() %>">加入购物车</a>
在后台Servlet中有接收代码
Java code
String operation=request.getParameter("opereation");
String itemIsbn=request.getParameter("isdn");标红的地不一样！！！