<c:if test="${companies.size() > 0}">
</c:if>
This syntax works only in EL 2.2 or newer (Servlet 3.0 / JSP 2.2 or newer). If you're facing a XML parsing error because you're using JSPX or Facelets instead of JSP, then use gt instead of >.
<c:if test="${companies.size() gt 0}">
</c:if>
If you're actually facing an EL parsing error, then you're probably using a too old EL version. You'll need JSTL fn:length() function then. From the documentation:
length( java.lang.Object) - Returns the number of items in a collection, or the number of characters in a string.
Put this at the top of JSP page to allow the fn namespace:
<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>
Or if you're using JSPX or Facelets:
<... xmlns:fn="http://java.sun.com/jsp/jstl/functions">
And use like this in your page:
<p>The length of the companies collection is: ${fn:length(companies)}</p>
So to test with length of a collection:
<c:if test="${fn:length(companies) gt 0}">
</c:if>
Alternatively, for this specific case you can also simply use the EL empty operator:
<c:if test="${not empty companies}">
</c:if>
JSP is a Java view technology running on the server machine which allows you to write template text in client side languages (like HTML, CSS, JavaScript, ect.). JSP supports taglibs, which are backed by pieces of Java code that let you control the page flow or output dynamically. A well-known taglib is JSTL. JSP also supports Expression Language, which can be used to access backend data (via attributes available in the page, request, session and application scopes), mostly in combination with taglibs.
When a JSP is requested for the first time or when the web app starts up, the servlet container will compile it into a class extending HttpServlet and use it during the web app's lifetime. You can find the generated source code in the server's work directory. In for example Tomcat, it's the /work directory. On a JSP request, the servlet container will execute the compiled JSP class and send the generated output (usually just HTML/CSS/JS) through the web server over a network to the client side, which in turn displays it in the web browser.
Servlet is a Java application programming interface (API) running on the server machine, which intercepts requests made by the client and generates/sends a response. A well-known example is the HttpServlet which provides methods to hook on HTTP requests using the popular HTTP methods such as GET and POST. You can configure HttpServlets to listen to a certain HTTP URL pattern, which is configurable in web.xml, or more recently with Java EE 6, with @WebServlet annotation.
When a Servlet is first requested or during web app startup, the servlet container will create an instance of it and keep it in memory during the web app's lifetime. The same instance will be reused for every incoming request whose URL matches the servlet's URL pattern. You can access the request data by HttpServletRequest and handle the response by HttpServletResponse. Both objects are available as method arguments inside any of the overridden methods of HttpServlet, such as doGet() and doPost().
JSF is a component based MVC framework which is built on top of the Servlet API and provides components via taglibs which can be used in JSP or any other Java based view technology such as Facelets. Facelets is much more suited to JSF than JSP. It namely provides great templating capabilities such as composite components, while JSP basically only offers the <jsp:include> for templating in JSF, so that you're forced to create custom components with raw Java code (which is a bit opaque and a lot of tedious work) when you want to replace a repeated group of components with a single component. Since JSF 2.0, JSP has been deprecated as view technology in favor of Facelets.
Note: JSP itself is NOT deprecated, just the combination of JSF with JSP is deprecated.
Note: JSP has great templating abilities by means of Taglibs, especially the (Tag File) variant. JSP templating in combination with JSF is what is lacking.
As being a MVC (Model-View-Controller) framework, JSF provides the FacesServlet as the sole request-response Controller. It takes all the standard and tedious HTTP request/response work from your hands, such as gathering user input, validating/converting them, putting them in model objects, invoking actions and rendering the response. This way you end up with basically a JSP or Facelets (XHTML) page for View and a JavaBean class as Model. The JSF components are used to bind the view with the model (such as your ASP.NET web control does) and the FacesServlet uses the JSF component tree to do all the work.
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You seem to concentrate on only the presentation and flow-control part of the scriptlets as in using
if,forandswitchstatements andout.print()things. You seem to compare scriptlets 1:1 with JSTL. This is wrong. I was not talking about the flow control part only (which is indeed to be replaced by JSTL), but about writing raw Java code in JSP files in general. I.e. gathering request parameters, validating and converting values, interacting with database and other Java classes/methods, etc. All things you normally (indirectly) do in a Servlet or Filter.