为什么我在jsp文件上收到NullPointerException？

一旦我点击我的jsp页面上的锚标签，它完全有效，但是在控制台中会抛出以下exception，根据这个答案 ，我从我的依赖项文件夹中删除了jap-api*.jar文件，但应用程序仍然抛出例外。

  WARNING: Servlet.service() for servlet jsp threw exception java.lang.NullPointerException at org.apache.jsp.products.ProductShow_jsp._jspx_meth_c_if_1(ProductShow_jsp.java:211) at org.apache.jsp.products.ProductShow_jsp._jspService(ProductShow_jsp.java:119) at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:411) at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:473) at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:377) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682) at org.apache.catalina.core.ApplicationDispatcher.doInvoke(ApplicationDispatcher.java:875) at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:739) at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:695) at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:626) at org.apache.jasper.runtime.JspRuntimeLibrary.include(JspRuntimeLibrary.java:977) at org.apache.jasper.runtime.PageContextImpl.include(PageContextImpl.java:701) at org.apache.tiles.request.jsp.JspRequest.doInclude(JspRequest.java:123) at org.apache.tiles.request.AbstractViewRequest.dispatch(AbstractViewRequest.java:47) at org.apache.tiles.request.render.DispatchRenderer.render(DispatchRenderer.java:45) at org.apache.tiles.request.render.ChainedDelegateRenderer.render(ChainedDelegateRenderer.java:68) at org.apache.tiles.impl.BasicTilesContainer.render(BasicTilesContainer.java:259) at org.apache.tiles.TilesContainerWrapper.render(TilesContainerWrapper.java:108) at org.apache.tiles.template.InsertAttributeModel.renderAttribute(InsertAttributeModel.java:188) at org.apache.tiles.template.InsertAttributeModel.execute(InsertAttributeModel.java:132) at org.apache.tiles.jsp.taglib.InsertAttributeTag.doTag(InsertAttributeTag.java:299) at org.apache.jsp.baseLayout_jsp._jspx_meth_tiles_insertAttribute_3(baseLayout_jsp.java:166) at org.apache.jsp.baseLayout_jsp._jspService(baseLayout_jsp.java:93) at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:411) at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:473) at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:377) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682) at org.apache.catalina.core.ApplicationDispatcher.doInvoke(ApplicationDispatcher.java:875) at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:739) at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:575) at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:546) at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:428) at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:378) at org.apache.tiles.request.servlet.ServletRequest.forward(ServletRequest.java:265) at org.apache.tiles.request.servlet.ServletRequest.doForward(ServletRequest.java:228) at org.apache.tiles.request.AbstractClientRequest.dispatch(AbstractClientRequest.java:57) at org.apache.tiles.request.render.DispatchRenderer.render(DispatchRenderer.java:45) at org.apache.tiles.impl.BasicTilesContainer.render(BasicTilesContainer.java:259) at org.apache.tiles.impl.BasicTilesContainer.render(BasicTilesContainer.java:397) at org.apache.tiles.impl.BasicTilesContainer.render(BasicTilesContainer.java:238) at org.apache.tiles.TilesContainerWrapper.render(TilesContainerWrapper.java:103) at org.apache.tiles.impl.mgmt.CachingTilesContainer.render(CachingTilesContainer.java:126) ..... WARNING: Servlet.service() for servlet jsp threw exception java.io.IOException: JSPException including path '/products/Product.jsp'. at org.apache.tiles.request.servlet.ServletUtil.wrapServletException(ServletUtil.java:61) at org.apache.tiles.request.jsp.JspRequest.doInclude(JspRequest.java:125) at org.apache.tiles.request.AbstractViewRequest.dispatch(AbstractViewRequest.java:47) at org.apache.tiles.request.render.DispatchRenderer.render(DispatchRenderer.java:45) 

码

 
  ..... 
 
 
ID:${product.id}
 
    1}"> Remove    Add   index      

删除和添加与他们自己的javascript函数相关联，两个函数完美地工作但exception将被抛出，并且从javascript函数返回后页面显示正确的结果。

Answer 1

你可以通过一点点工作自己解决这个问题。 JSP被编译成servlet，源代码可以在web服务器的工作文件夹下找到。 一般来说，找到’work’文件夹，然后通过软件包向下钻取，这对于Tomcat通常是org.apache ….（抱歉不能记住确切的，这台机器没有安装Tomcat）。 只需向下钻取，您应该找到ProductShow_jsp.java源代码。 转到指定的行号，您通常可以找出exception的原因。

Answer 2

请检查以下建议是否消除了错误：

a）检查是否定义了product.id或product.ID

b）检查product.available的属性名称是否正确

c）为添加结束标记