Question

我想将值servlet传递给jsp。在servlet中设置值并在jsp页面中使用

我在下面得到NullPointerException是我的代码

JSP

<body>
<%
  String message = (String) request.getAttribute("message");
  out.println("Servlet communicated message to JSP: "+ message);

  Vector vecObj = (Vector) request.getAttribute("vecBean");
  out.println("Servlet to JSP communication of an object: "+vecObj.get(0));
%>
</body>

的java

public class ServletToJSP extends HttpServlet {
      public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        //communicating a simple String message.
        String message = "Example source code of Servlet to JSP communication.";
        request.setAttribute("message", message);

        //communicating a Vector object
        Vector vecObj = new Vector();
        vecObj.add("Servlet to JSP communicating an object");
        request.setAttribute("vecBean",vecObj);

        //Servlet JSP communication
        RequestDispatcher reqDispatcher = getServletConfig().getServletContext().getRequestDispatcher("javaPapers.jsp");
        reqDispatcher.forward(request,response);
      }
    }

错误日志我没有在web.xml中执行任何操作我没有在web.xml中执行任何操作

org.apache.jasper.JasperException: An exception occurred processing JSP page /javaPapers.jsp at line 21

18:   out.println("Servlet communicated message to JSP: "+ message);
19:  
20:   Vector vecObj = (Vector) request.getAttribute("vecBean");
21:   out.println("Servlet to JSP communication of an object: "+vecObj.get(0));
22: %>
23: </body>
24: </html>


Stacktrace:
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:553)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:457)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:391)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)


root cause 

java.lang.NullPointerException
    org.apache.jsp.javaPapers_jsp._jspService(javaPapers_jsp.java:83)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:419)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:391)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)


note The full stack trace of the root cause is available in the Apache Tomcat/7.0.12 logs.

Answer 1

您可能会收到NullPointerException，因为“reqDispatcher.forward（request，response）;”完全创建一个新请求，vecObj变为null，因为它在请求对象中不存在。而不是使用reqDispatcher.forward使用response.redirect。

将值servlet传递给jsp

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