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How to check JSP variable and hide if value is null/empty or some specific string?

I am creating a project where i am getting set data in JSP page from database.
if any field data value is null the jsp page is showing null but i do not want to show it on jsp page. please help. i am getting data from bean.
<%=p.getOffer()%>
<% String s = p.getOffer() %>
<% if (<%=s ==null) { %>
show nothing

If you are coding java inside a jsp, you need to use scriptlet tags(<% and %>). So if you are checking for conditions you need to open a scriptlet tag.
<%
String s = p.getOffer();
if (s != null && !s.equals("")) {
out.print(s);
} else { %>
<!-- s is either null or empty. Show nothing -->
<% }%>

What exactly do you want to show when the value is null? Anyway, your approach looks good:
<%=p.getOffer()%> // Here you print the value "offer". If you don't want to show it when it is null, remove this line
<% String s = p.getOffer() %>
<% if (<%=s ==null) { %> // the <%= is unnecessary. if(s==null) is enough
show nothing // show nothing, why not inverse the if and show something
Here another approach:
<%
String s= p.getOffer();
if(s != null){ %>
Offer: <%= s%>
<% }%>
That way you only print the offer when the variable is not null.
By the way: naming a String variable "s" is not recommended, call it "offer" or something to facilitate the reading.

Session value is null servlet doGet at times

I have the following code
<%
String projectId = request.getParameter("projectId");
%>
<iframe width="100%" id="uploadFrame"
src="testframe.jsp?projectId=<%=projectId %>"></iframe></body>
</html>
and in testframe.jsp I am setting session value as
<%
String projectId = request.getParameter("projectId");
request.getSession(true).setAttribute("prj",projectId);
%>
and finally in servlet, I am getting session value in doGet method as
String prjId = request.getSession(false).getAttribute("prj").toString();
Problem I am facing is sometimes session value is null in doGet method, not all the time, although request.getParameter("projectId") is not null in testframe.jsp
What could be the reason for this?

Send data from servlet to jsp

I tried to send a list from my servlet to a jsp page. This is the servlet code:
Query q = new Query("post").addSort("time", SortDirection.DESCENDING);
PreparedQuery pq = datastore.prepare(q);
QueryResultList<Entity> results = pq.asQueryResultList(fetchOptions);
for (Entity entity : results) {
System.out.println(entity.getProperty ("content"));
System.out.println(entity.getProperty ("time"));
}
req.setAttribute("postList",results);
req.getRequestDispatcher("/tublr.jsp").forward(req, resp);
The jsp code:
<%
QueryResultList<Entity> result = request.getAttribute("postList");
for (Entity entity : results) {
<b> IT WORRRKKKK !!! </b> <br>
}
%>
But I get an error
EDIT : I added
<%#page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%# page import="java.util.List,com.google.appengine.api.datastore.Query.SortDirection,com.google.appengine.api.datastore.*" %>
And now i get a new error
An error occurred at line: 37 in the jsp file: /tublr.jsp Type
mismatch: cannot convert from Object to QueryResultList .....
Caused by:
org.apache.jasper.JasperException: Unable to compile class for JSP:
I m do it for the school and we have to di it like this now , we have to use java in the jsp page.

1) You need to add import statements at top of the JSP.
Example:
<%# page import="java.util.List" %>
2) It is NOT good practice to have Java code directly embedded in JSP
Read more here on SO Wiki

Don't do any coding on JSP page. There is a JSTL library for this kind of stuff, and to iterate and display stuff you should use forEach tag:
<%# taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%# taglib prefix="x" uri="http://java.sun.com/jsp/jstl/xml" %>
and for loop
<x:forEach select="${postList}" var="item">
... code
</x:forEach>

You forgot <% %> for the html code
<%
QueryResultList<Entity> result = request.getAttribute("postList");
for (Entity entity : results) {
%> <b> IT WORRRKKKK !!! </b> <br><%
}
%>

Have you imported QueryResultList in your jsp?

You need to cast list obtained from request.getAttribute("postList") to QueryResultList.
<%
QueryResultList<Entity> result =(QueryResultList)request.getAttribute("postList");
for (Entity entity : result) {
// Your code goes here You can use <%= %> to print values.
// <b> IT WORRRKKKK !!! </b> <br>
}
%>
For more about expression

using a scriplet inside a javascript function

I wanted to use a scriplet inside a java script function. I wanted to check for some attribute's vale and give an alert according to that. Following is the function in which the only scriplet statement gives an error.
function UploadMessage() {
<% if((String)request.getAttribute("SuccessMessage").compareTo("Uploaded successfully") == 0) { %>
alert("File Successfully uploaded !");
<%
} %>
}
Is there any way i can do this ? What is the problem here ?
NOTE : I have placed the above snippet in a jsp page

function UploadMessage() {
<% if(((String)request.getAttribute("SuccessMessage")).equals("Uploaded successfully")) { %>
alert("File Successfully uploaded !");
<%
} %>
}
Problem was -
The method compareTo(String) is undefined for the type
Object
Incompatible operand types String and int

Java Facelets and session?

How can I use session for facelets ?
What's the syntax...?
I would put a code like this
<% String loginSession = (String)session.getAttribute("login"); %>
<% if(loginSession != null){ %>
Welcome <%= session.getAttribute("firstName") %> !
<% }else{ %>
Guest
<% } %>
Thanks

#{sessionScope.login}
You can't have if-s in JSF (you can with JSTL, but it has complications). Instead you can choose to render or not a component:
<h:outputText value="Guest" rendered="#{sessionScope.login != null}" />