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 HttpServlet
s 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.
Related questions
It's present as a request attribute with the name as specified by the RequestDispatcher.ERROR_EXCEPTION
constant.
#{requestScope['javax.servlet.error.exception']}
This gives you the whole Exception
object. Getting its stacktrace requires a bit more work. You basically need to create a custom EL function which does basically something like this:
public static String printStackTrace(Throwable exception) {
StringWriter stringWriter = new StringWriter();
exception.printStackTrace(new PrintWriter(stringWriter, true));
return stringWriter.toString();
}
so that you can use it as follows:
<pre>#{my:printStackTrace(requestScope['javax.servlet.error.exception'])}</pre>
The JSF utility library OmniFaces offers this as well. See also the FullAjaxExceptionHandler
showcase page.
Best Answer
Faces messages are request scoped. A redirect basically instructs the webbrowser to send a brand new HTTP request (that's also why you see the URL being changed in browser address bar). The faces messages which are set in the previous request are of course not available anymore in the new request.
There are several ways to get it to work:
Don't send a redirect. Send a forward instead. You can do it by
ExternalContext#dispatch()
or by just navigating the usual way if you're already inside an action method
Create a common error page master template and use separate template clients for every individual type of error and put the message in the view instead.
Then you can just redirect to this specific error page like so
redirect("error-login.xhtml")
.Pass some error identifier as a request parameter though the redirect URL like so
redirect("error.xhtml?type=login")
and let the view handle it.Keep the faces messages in the flash scope.
Mojarra has however a somewhat buggy flash scope implementation. With the current releases, this won't work when you need to redirect to a different folder, but it will work when the target page is in the same folder.