java - NullPointer Exception - JSF Event Handling ( ValueChangeListener ) -

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i learning jsf event handling , when try run sample code, getting null pointer exception.

this index.xhtml snippet,

<h:form>             <h2>implement valuechangelistener</h2>             <hr />             <h:panelgrid columns="2">                 selected locale:                 <h:selectonemenu value="#{userdata.selectedcountry}" onchange="submit()">                     <f:valuechangelistener type="com.cyb3rh4wk.test.localechangelistener" />                     <f:selectitems value="#{userdata.countries}" />                 </h:selectonemenu>                 country name:                 <h:outputtext id="countryinterface" value="#{userdata.selectedcountry}" />             </h:panelgrid>         </h:form>

userdata.java

@managedbean(name = "userdata", eager = true) @applicationscoped public class userdata implements serializable{     private static map<string, string> countrymap;     private string selectedcountry = "united kingdom";     static  {         countrymap = new linkedhashmap<string, string>();         countrymap.put("en", "united kingdon");         countrymap.put("fr", "french");         countrymap.put("de", "german");         countrymap.put("def", "default");     }       public string getselectedcountry() {         return selectedcountry;     }     public void setselectedcountry(string selectedcountry) {         this.selectedcountry = selectedcountry;         system.out.println("locale set");     }         public map<string, string> getcountries() {         return countrymap;     }             public void localechanged(valuechangeevent event) {         selectedcountry = event.getnewvalue().tostring();     }         }

localechangelistener.java

public class localechangelistener implements valuechangelistener    {     @override     public void processvaluechange(valuechangeevent event) throws abortprocessingexception {         userdata userdata = (userdata) facescontext.getcurrentinstance().getexternalcontext().getsessionmap().get("userdata");         string newlocale = event.getnewvalue().tostring();         if (newlocale != null)             userdata.setselectedcountry(newlocale);         else             userdata.setselectedcountry("default");        }  }

when run these on glassfish server, error, 

java.lang.nullpointerexception     @ com.cyb3rh4wk.test.localechangelistener.processvaluechange(localechangelistener.java:25)     @ com.sun.faces.facelets.tag.jsf.core.valuechangelistenerhandler$lazyvaluechangelistener.processvaluechange(valuechangelistenerhandler.java:128)     @ javax.faces.event.valuechangeevent.processlistener(valuechangeevent.java:134)

can me ?

  1. you getting nullpointerexception because userdata not found in session scope.

  2. the reason happening put userdata in application scope (@applicationscoped annotation on managed bean) , searching in session scope.

  3. eventhough verified userdata null still prints locale set because bean in application scope described in 2. above.

  4. so solution? either change @applicationscoped @sessionscoped or access userdata changing:

    userdata userdata = (userdata) facescontext.getcurrentinstance().getexternalcontext().getsessionmap().get("userdata");

to

    facescontext ctx = facescontext.getcurrentinstance();     userdata userdata = (userdata)ctx.getexternalcontext().getapplicationmap().get("userdata");

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