Valuechangelistener Doubt in JSF -

hi,

please see following code:

                <h:selectonemenu id="countries" value="#{countrybean.selectedcountry}" onchange="submit()                                     valuechangelistener="#{countrybean.changecountry}">                     <f:selectitems value="#{countrybean.countries }" />                 </h:selectonemenu>   

backing bean

public void changecountry(valuechangeevent event){           string newvalue = (string)event.getnewvalue();     string oldvalue = (string)event.getoldvalue();      system.out.println("new value : " + newvalue);     system.out.println("old value : " + oldvalue);      if ("1".equals(newvalue)){         this.countries = new arraylist<selectitem>();         this.cities.add(new selectitem("1","delhi"));         this.cities.add(new selectitem("2","mumbai"));     }     if ("2".equals(newvalue)){         this.cities = new arraylist<selectitem>();         this.cities.add(new selectitem("1","mossco"));     }        } 

please let me know if implementation correct. working fine. questions are:

• what advantage of adding f:valuechangelistener tag inside h:selectonemenu tag. have used normal attribute valuechangelistener="#{countrybean.changecountry}".
• is necessary use onchange="submit() code change values.
• what difference between writing custom listeners implementing actionlistener interface , using attribute in uicomponent tags (action="methodname"). please explain me.

the valuechangelistener called when form submitted, not when value of input changed. thus, if want run listener when value modified, have 2 solutions:

1. submit form when onchange event fired (this did in code);
2. use ajax call instead, using dedicated components (already integrated in jsf2, <f:ajax>, or third-parties libraries such richfaces, primefaces...).

here example richfaces:

<h:selectonemenu id="countries" value="#{countrybean.selectedcountry}" valuechangelistener="#{countrybean.changecountry}">     <a4j:support event="onchange" .../>     <f:selectitems value="#{countrybean.countries }" /> </h:selectonemenu> 

regarding code of listener, seems correct, why question why need valuechangelistener here? indeed, listener usefull when want track modification of value. that's why valuechangeevent provides both getoldvalue() , getnewvalue() methods.

in code, not care old value, basically, "simply" action instead of valuechangelistener (ex. richfaces):

<h:selectonemenu id="countries" value="#{countrybean.selectedcountry}">     <a4j:support event="onchange" actionlistener="#{countrybean.changecountry}"/>     <f:selectitems value="#{countrybean.countries }" /> </h:selectonemenu> 

finally, regarding difference between valuechangelistener attribute , <f:valuechangelistener> first binds java method (#{mybean.mymethod}), while second binds java class (type="com.foo.mylistenerclass") implements valuechangelistener interface. second 1 more generic first one...