stupid Q: setters are not called if properties are empty

["Kan Leung, MK" <lm...@digitalempires.net>]

Sorry if you find if this is a bit off topic. ;)

Hi all,

Hope this is not a FAQ. (I spent 2 hours to do net search but didn't
get a good answer)

8<
<jsp:useBean name="abc" scope="session" .... />
<jsp:setProperty name="abc" property="*"/>
8<

Introspection magic makes the property setters called if they match the
input parameters by GET or by POST.

e.g. URL?address1=a&address2=&address3=


There are exceptions: setters of some parameters which have no value
won't get called. In the above string, the later two are not called
because they have no values assigned. Suppose my bean has session scope.
Values kept unchanged.

My question is: how can the web users clear a text field if the field is
incorrectly filled?

I suppose the introspection magic is a life-saver. I just don't want to
check the return value of request.getParameter("address2) for each input
to determine if it's empty.

Can anybody give me a good alternative to clear unwanted fields easily?

--
Kan LEUNG, M K
email: lmk@digitalempires.net
Digital Empires Company Limited

Re: stupid Q: setters are not called if properties are empty

["Craig R. McClanahan" <Cr...@eng.sun.com>]

"Kan Leung, MK" wrote:

> Sorry if you find if this is a bit off topic. ;)
>
> Hi all,
>
> Hope this is not a FAQ. (I spent 2 hours to do net search but didn't
> get a good answer)
>

It is actually a pretty subtle point, and on-topic in my book :-)

>
> 8<
> <jsp:useBean name="abc" scope="session" .... />
> <jsp:setProperty name="abc" property="*"/>
> 8<
>
> Introspection magic makes the property setters called if they match the
> input parameters by GET or by POST.
>
> e.g. URL?address1=a&address2=&address3=
>
> There are exceptions: setters of some parameters which have no value
> won't get called. In the above string, the later two are not called
> because they have no values assigned. Suppose my bean has session scope.
> Values kept unchanged.
>

The exception you mention is specifically required in the JSP
specification.  For any property that would be set to a zero-length string,
and you are using <jsp:setProperty name="xxx" property="*"/>, the setter
method is *not* called.

A very similar problem occurs with checkboxes -- if the checkbox is checked,
the value is included and the corresponding boolean setter is called.  But,
if it is not checked, *nothing* is sent to the server, so your boolean
property will remain set :-(.

>
> My question is: how can the web users clear a text field if the field is
> incorrectly filled?
>

Struts deals with these issues via a combination of features:

* First, you should do your submits to an action defined in the
  controller servlet, rather than back to the JSP page.  This makes
  the behavior of <jsp:setProperty> irrelevant, because it will never
  be utilized.

* The Struts configuration file allows you to configure a form bean
  for each input form.  If you do so, the Struts controller will
automatically
  populate the bean's properties based on your request parameters,
  similar to what <jsp:setProperty> does.

* However, you would seem to still have a problem with boolean properties
  and with setting String properties to zero length.  This is resolved by
  requiring your bean to implement a reset() method, which will set
  all of the bean properties to default values (normally false for booleans,

  and null for Strings).

Now, if your user erases the contents of a String field, the following
things happen:
* The reset() method of your form bean is called, setting the string
  property to null.
* The setXxxxx() method is *not* called (to be consistent with
  <jsp:setProperty/> semantics).
* The end result is that your property has been set to a null string,
  which is what you normally want.  You can also reset to a zero
  length String constant if you prefer.

>
> I suppose the introspection magic is a life-saver. I just don't want to
> check the return value of request.getParameter("address2) for each input
> to determine if it's empty.
>
> Can anybody give me a good alternative to clear unwanted fields easily?
>
> --
> Kan LEUNG, M K
> email: lmk@digitalempires.net
> Digital Empires Company Limited

Craig McClanahan

Re: stupid Q: setters are not called if properties are empty

[ma...@tumbleweed.com]

It's not a stupid question. I'm not sure if it's off-topic or not. :-)

The JSP fragment you posted suggests that your JSP is being invoked 
directly. That is, the URL coming from the browser references the JSP, and 
not a Struts Action (e.g. doSomthing.do). If this is the case, then I don't 
think Struts can help you much, since it won't have a chance to touch the bean.

In a typical Struts application, the request will be handled by the 
controller servlet, which will parse the request and populate a form bean 
with the parameters. An Action will then process the request and forward to 
a JSP for displaying the results to the user. In this scenario, since 
Struts populated form bean for you, there is usually no need to use the 
useBean/setProperty mechanism you have.

Also - and this is the piece you need - the controller servlet will call 
the reset() method on your form bean immediately before populating it with 
the values from the new request. That gives you the chance to clear out the 
old values, so that they are not left in the form bean if there is no 
corresponding value in the request.

Hope this helps.

--
Martin Cooper
Tumbleweed Communications


At 10:42 PM 2/27/01 +0800, Kan Leung, MK wrote:
>Sorry if you find if this is a bit off topic. ;)
>
>Hi all,
>
>Hope this is not a FAQ. (I spent 2 hours to do net search but didn't
>get a good answer)
>
>8<
><jsp:useBean name="abc" scope="session" .... />
><jsp:setProperty name="abc" property="*"/>
>8<
>
>Introspection magic makes the property setters called if they match the
>input parameters by GET or by POST.
>
>e.g. URL?address1=a&address2=&address3=
>
>
>There are exceptions: setters of some parameters which have no value
>won't get called. In the above string, the later two are not called
>because they have no values assigned. Suppose my bean has session scope.
>Values kept unchanged.
>
>My question is: how can the web users clear a text field if the field is
>incorrectly filled?
>
>I suppose the introspection magic is a life-saver. I just don't want to
>check the return value of request.getParameter("address2) for each input
>to determine if it's empty.
>
>Can anybody give me a good alternative to clear unwanted fields easily?
>
>--
>Kan LEUNG, M K
>email: lmk@digitalempires.net
>Digital Empires Company Limited