java - 无法在 JSP 中显示 servlet 值

Question

长话短说，我无法让 servlet 变量显示在 JSP 页面中。我尝试过类似的方法

 <p><$= request.getAttribute("foo") %></p>

以及

  <p>"${foo}"</p>

并在顶部导入页面，有或没有。

我的servlet(注释了测试变量所在的位置，朝向底部):

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException
{
    //String that gets returned as HTML
    StringBuilder returnAsHTML = new StringBuilder();

    //Cycles through the parameter(s) sent from the serialized form
    Enumeration<String> parameterNames;

    //Holds all of the parameter(s) values
    LinkedList<String> valuesIn = new LinkedList<String>();

    //Gets all of the parameter names from the serialized form
    parameterNames = request.getParameterNames();

    //Iterate through the enumeration of the parameters
    while(parameterNames.hasMoreElements())
    {
        String currentParameter = parameterNames.nextElement();
        String currentValue = request.getParameter(currentParameter);

        //Prevent a blank value from being added from the text fields (which is what they default to
        //if the user didn't enter anything for "Section:" or "Teacher:").
        if(!currentValue.contentEquals(""))
        {
            valuesIn.add(currentValue);
        }//Else doNothing();
    }

    //Set the section to query, if the user wanted one to search for
    if(request.getParameter("section").isEmpty())
    {
        sectionToQuery = "";
    }
    else
    {
        sectionToQuery = request.getParameter("section").toUpperCase();
    }

    //Set the teacher to query, if the user wanted one to search for
    if(request.getParameter("teacher").isEmpty())
    {
        teacherToQuery = "";
    }
    else
    {
        teacherToQuery = request.getParameter("teacher").toUpperCase();
    }

    //THIS BEGINS THE QUERY - BE SURE TO BREAK THIS DOWN TO OTHER METHODS

    //STAGE 1 See what semesters they are needing, eliminate the ones they don't need.
    resultSet = determineSemesters(valuesIn);

    //STAGE 2 See what locations they are needing, eliminate the ones they don't need.
    determineLocations(resultSet, valuesIn);

    //STAGE 3 See what sections they are needing, eliminate the ones they don't need.
    determineSections(resultSet, valuesIn);

    //STAGE 4 See what instructors they are needing, eliminate the ones they don't need.
    determineInstructors(resultSet, valuesIn);

    //STAGE 5 See if they want to include closed classes or not, eliminate what they don't need.
    determineClosedOrNotClosedCourses(resultSet, valuesIn);

    //SEARCH IS DONE, the remaining elements in "resultSet" are the product of their search. Find
    //The enrollment and the credits that is in resultSet.

    //THIS IS WHERE I AM TESTING IT////////////////
    int foo = 20;
    request.setAttribute("foo", foo);

    //Check to see if the result set is empty
    if(resultSet.isEmpty())
    {
        returnAsHTML.setLength(0);
        returnAsHTML.append("<tr><td colspan='15'><h1>No Results...</h1></tr>");
    }
    else//It's not empty, so style the classes.
    {
        //Make sure results are sorted
        Collections.sort(resultSet);

        //Style all the classes
        for(ClassInfo classes : resultSet)
        {
           returnAsHTML.append(styleClass(classes));
        }//End styling of classes
    }

    //Send back the result
    response.getWriter().write(returnAsHTML.toString());

}

作为 JSP 新手，我有一些想法。在我看到的示例中，似乎人们使用 response.sendRedirect() 或类似的方法来重定向或转发页面。另一个可能是因为我没有将 int 转换为字符串。有什么想法吗？

Answer 1

首先，要将 Servlet 重定向到适当的 JSP 页面，您应该使用 RequestDispatcher ，像这样:

RequestDispatcher rd = request.getRequestDispatcher("/someJsp.jsp"); 
rd.forward(request, response);

要获取您应该使用的值:

<p><%= request.getAttribute("foo") %></p>

但这是旧的且不正确的方法。您应该避免在 JSP 页面中使用 scriplet。我们有Expression Language为了这个目的。例如:

<p>
   <c:out value="${requestScope['foo']}"/>
</p>

另请参阅:

• SO Servlets Wiki page
• JavaServer Pages Standard Tag Library