Homepage contains data fetch by database

As soon as my homepage(1st page)opens,it should contain some data fetch by database. How should i do it? Should i go for any listener? I am using hibernate for the db connection.

What is your home page? .jsp,.html,.xhtml,.jsf etc?

If jsp then apply a standard MVC pattern, have the home page invoke a servlet, load up the model
and forward to the view to render the data.

WP

Its a .jsp file.

To should should i forward the request?

Here is what i did.

Web.xml

HomePage.jsp

CommentsServlet.java

CommentsDao.java

Error---

Apr 12, 2012 9:56:17 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet jsp threw exception
org.apache.jasper.JasperException: /HomePage.jsp(26,2) "${All Comments}" contains invalid expression(s): javax.el.ELException: Error Parsing: ${All Comments}
at org.apache.jasper.compiler.DefaultErrorHandler.jspError(DefaultErrorHandler.java:40)
at org.apache.jasper.compiler.ErrorDispatcher.dispatch(ErrorDispatcher.java:407)
at org.apache.jasper.compiler.ErrorDispatcher.jspError(ErrorDispatcher.java:198)
at org.apache.jasper.compiler.Validator$ValidateVisitor.checkXmlAttributes(Validator.java:1149)
at org.apache.jasper.compiler.Validator$ValidateVisitor.visit(Validator.java:819)
at org.apache.jasper.compiler.Node$CustomTag.accept(Node.java:1507)
at org.apache.jasper.compiler.Node$Nodes.visit(Node.java:2336)
at org.apache.jasper.compiler.Node$Visitor.visitBody(Node.java:2386)
at org.apache.jasper.compiler.Node$Visitor.visit(Node.java:2392)
at org.apache.jasper.compiler.Node$Root.accept(Node.java:489)
at org.apache.jasper.compiler.Node$Nodes.visit(Node.java:2336)
at org.apache.jasper.compiler.Validator.validate(Validator.java:1737)
at org.apache.jasper.compiler.Compiler.generateJava(Compiler.java:178)
at org.apache.jasper.compiler.Compiler.compile(Compiler.java:306)
at org.apache.jasper.compiler.Compiler.compile(Compiler.java:286)
at org.apache.jasper.compiler.Compiler.compile(Compiler.java:273)
at org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:566)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:317)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)

Since you are not passing or require any parameters, jsp:useBean would solve your problem.

http://www.exampledepot.com/egs/javax.servlet.jsp/usebean.jsp.html

Create a bean say, CommentData.java and when it instantiates, load up the comments for rendering in the jsp.

Please let us know if this works for you ...

WP

Try after removing spaces in "All Comments".

Piyush, removing the spaces solve the exception, but comments are not fetched up in homepage

Hi, Did you also remove the space in your servlet code as well?

req.setAttribute("All Comments", com);

Please also make sure that has some data in it.

I can see you have started the transaction but where are you doing a commit?

William, that doesn't solves the problem. I need some code that runs even before the homepage runs. Something as similar to ServletContextListeners which runs before the servlet can service the client

Your welcome file should not be the JSP. It should be the URL of the servlet controller for the JSP.

I do not recommend following the deprecated Model 1 approach with useBean.

If you want to show comment on your HomePage.jsp, then you should call a servlet, get data from Model , set data in request attribute , dispatch the request to HomePage.jsp which will then be able to show comment.

Exactly, as I said. No JSP should be able to be directly addressed without going through its controller -- including any "home page".

In fact, I recommend placing all your JSPs in a folder such as WEB-INF/views so that they cannot be mistakably served up without a controller.

Please make sure to use doGet as this is method that gets called when you directly access an URL.

Kunal, if your homepage should contain the comments as soon as it loads, you should
change your web.xml to
<servlet>
<servlet-name>Comm</servlet-name>
<servlet-class>org.nit.servlet.CommentsServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>

and possibly place the call to your commentsList method in the Comm servlet's init method.

I applied the following changes

Web.xml

CommentServlet

Removed <from> tag from HomePage.jsp

But still not getting any Comment.

It seems control doesn't goes to CommentServlet, as the println() line doesn't runs

Vanessa Danin wrote:Kunal, if your homepage should contain the comments as soon as it loads, you should
change your web.xml to
<servlet>
<servlet-name>Comm</servlet-name>
<servlet-class>org.nit.servlet.CommentsServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>

and possibly place the call to your commentsList method in the Comm servlet's init method.

It will not work if you directly access HomePage.jsp. You will have to go through servlet to show comments on HomePage.jsp.

Hi Kunal,

Why are you using doPost? Please use doGet.

Piyush, its works. Great.
Vanessa Danin, load on startup doesn't works. Just wonder, why?
Thanks Bear Bibeault, Piyush, Vanessa Danin,William P O'Sullivan.

Kunal Lakhani wrote:Vanessa Danin, load on startup doesn't works. Just wonder, why?

That's because "load on startup" does just what it says... it loads the servlet when the application starts. It makes no difference to your code whether the servlet is loaded at the time of the request or whether it is loaded 30 minutes earlier, so configuring it to load on startup won't make any difference to your code.

The only reason to use the load-on-startup feature is if the servlet is doing something which causes it to take a long time to initialize, such as setting up large quantities of data in a cache. And servlets shouldn't be doing that sort of thing any more, it should be done by listeners instead. So not only is load-on-startup unhelpful for your problem, it has been obsolete since listeners were brought into the servlet spec. Which was a very long time ago.

Thanks Paul Clapham. Any listener helpful in this case?

Kunal Lakhani wrote:Thanks Paul Clapham. Any listener helpful in this case?

No. You've already been give then answer, why aren't you busy acting on it?

Because I can ... ;)

JSP:

Add, run this and you will see that the constructor is called each time.
This approach would allow your comments to change dynamically as well.

May not be the most efficient but a very easy and straightforward way to load startup pages etc..

WP

I posted earlier. The problem has been solved.

Thanks Bear Bibeault, William.