can't forward info from servlet to jsp -- HF example p.87

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posted 15 years ago

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I got the servlet below to work when the servlet wrote out some html using out.println(). The html displayed correctly in my browser. Then I commented out the out.println() lines, and I tried to attach the info to the request object and then forward the request object to a jsp. However, now I get no output in my browser--just a blank page.

package com.example.web;
 
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import com.example.model.BeerExpert;
import java.util.*;
 
public class BeerSelect extends HttpServlet
{
  public void doPost(
      HttpServletRequest request,
      HttpServletResponse response) throws IOException, ServletException
  {
    String color = request.getParameter("color"); //retrieves beer color from a form
 
    BeerExpert expert = new BeerExpert(); 
    List<String> recommendations = expert.getBrands(color);  //getBrands() returns List<String>
 
    /*
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    
 
    out.println("<h2>Beer Selection Advice</h2>");
    out.println("<p>For beer color: " + color + "</p>");
    out.print("<div>Here are our recommendations: ");
 
    int count = 1;
    for(String beer: recommendations)
    {
      if(count == 1)
      {
        out.print(beer);
        count += 1;
      }
      else
      {
        out.print(", " + beer);
      }
    }
 
    out.print("</div>");
    out.println();
    */
 
    request.setAttribute("recommendations", recommendations);
    RequestDispatcher view = request.getRequestDispatcher("result.jsp");
    view.forward(request, response);
  }
}

webapps/Beer-v1/result.jsp:
I tried various casts on the value returned by request.getAttribute(), but nothing worked.

BeerExpert.java:

Hmmm...after some more testing, I think the problem is because I'm using List<String> instead of List. When I use List, my browser displays the output correctly, but I get some "Notes" when I compile the code:

Note: src/com/example/model/BeerExpert.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Are servlets unable to handle generics?

$ java -version
java version "1.5.0_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-113)
Java HotSpot(TM) Client VM (build 1.5.0_06-68, mixed mode, sharing)

apache-tomcat-6.0.13

Now both versions of my servlet are working. This is the cast that works:

[ May 29, 2008: Message edited by: sven studde ]

[ May 29, 2008: Message edited by: sven studde ]
[ May 29, 2008: Message edited by: sven studde ]

Ben Souther

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posted 15 years ago

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Servlets can handle generics.

Depending on which container (and which version) there may be some Java 1.5 features that don't work in JSP.
Which are you using?

Java API J2EE API Servlet Spec JSP Spec How to ask a question... Simple Servlet Examples jsonf

sven studde

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posted 15 years ago

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It's in my previous post:

$ java -version
java version "1.5.0_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-113)
Java HotSpot(TM) Client VM (build 1.5.0_06-68, mixed mode, sharing)

apache-tomcat-6.0.13

[ May 29, 2008: Message edited by: sven studde ]

Ben Souther

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posted 15 years ago

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Missed that in your previous post.

I know that Tomcat 5.5 was the first version to ship with the embedded compiler (JDT from the eclipse group) and that the compiler, at that time, was not fully Java 1.5 compliant. This meant that some of the 1.5 features would not work in scriptlet code. I'm not sure about Tomcat 6.

Java API J2EE API Servlet Spec JSP Spec How to ask a question... Simple Servlet Examples jsonf