displaying a list of entities in jsp file by using java

Question

I am trying to display list of entities in a .jsp file, but I this error:

Unable to compile class for JSP: 

An error occurred at line: 28 in the jsp file: /guestbook.jsp
Can only iterate over an array or an instance of java.lang.Iterable
25:     </tr>
26:     <tr>
27:     <%
28:         for (Entity drugtarget : "${drugtargets}") {
29:         %>
30:     <td> 
31:     <%

Here is my query from servlet class:

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Query query = new Query("DrugTarget").addSort("drug", Query.SortDirection.DESCENDING);
List<Entity> drugtargets = datastore.prepare(query).asList(FetchOptions.Builder.withLimit(10));

req.setAttribute("drugtargets", drugtargets);

And this is how I send drugtargets to jsp file:

String url = "/guestbook.jsp";
ServletContext sc = getServletContext();
RequestDispatcher rd = sc.getRequestDispatcher(url);
try {
    rd.forward(req, resp);
} catch (ServletException e) {
    e.printStackTrace();
}

Finally here is how I write jsp part:

<table border="1">
  <tr>
    <th>Drug Names</th>
    <th>Target Names</th>
  </tr>
  <tr>
    <%
      for (Entity drugtarget : "${drugtargets}") {
    %>
    <td> 
    <%
      pageContext.setAttribute("drug_content",
                                drugtarget.getProperty("drug"));
    %>
    ${fn:escapeXml(drug_content)}
    </td>
    <td>
    <%
      pageContext.setAttribute("target_content",
                                drugtarget.getProperty("target"));
    %>
    ${fn:escapeXml(target_content)}
    </td>   
    <%
      }
    %>
  </tr>
</table>

I think I'm making a syntax mistake, but I'm not sure. I checked some examples and the same way (to send list of entities from servlet to jsp and to reach entities in jsp file) was used. How can I resolve this error?

Answer 1

You shouldn't be using scriptlets (those oldschool <% %> things with Java code) at all when using servlets and EL. Use taglibs like JSTL instead. It offers the <c:forEach> tag to iterate over a collection.

For example,

<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>
...
<table border="1">
  <tr>
    <th>Drug Names</th>
    <th>Target Names</th>
  </tr>
  <c:forEach items="${drugtargets}" var="drugtarget">
    <tr>
      <td>${fn:escapeXml(drugtarget.drug)}</td>
      <td>${fn:escapeXml(drugtarget.target)}</td>
    </tr>
  </c:forEach>
</table>

(note that I also fixed the rendering of table rows by putting the <tr> inside the loop)

Much simpler, isn't it? You can by the way also just use <c:out value="${drugtarget.drug}"/> instead of those functions.

If you can, I suggest to add the following to your webapp's web.xml in order to disable scriptlets altogether so that you will be forced to do things the right way.

<jsp-config>
    <jsp-property-group>
        <url-pattern>*.jsp</url-pattern>
        <scripting-invalid>true</scripting-invalid>
    </jsp-property-group>
</jsp-config>

See also:

• How to avoid Java code in JSP files?

Answer 2

In this case "${drugtargets}" is a String, not the list you passed. I would recommend using the looping. It's way cleaner.

<c:forEach var="drugtarget " items="${drugtargets}"> 
.... Your code here ...
</c:forEach>