struts2-tiles-plugin-json

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hello
i m working on struts2 and json in my application.
i m tring to get the json on my page but it gives me error

SEVERE: There was an exception while instantiating the result of type org.apache
.struts2.json.JSONResult
java.lang.NullPointerException
        at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:768
)
        at com.opensymphony.xwork2.ognl.OgnlUtil.compile(OgnlUtil.java:223)
        at com.opensymphony.xwork2.ognl.OgnlUtil.setValue(OgnlUtil.java:209)
        at com.opensymphony.xwork2.ognl.OgnlUtil.internalSetProperty(OgnlUtil.ja
va:405)
        at com.opensymphony.xwork2.ognl.OgnlUtil.setProperty(OgnlUtil.java:160)
        at com.opensymphony.xwork2.ognl.OgnlReflectionProvider.setProperty(OgnlR
eflectionProvider.java:91)
        at com.opensymphony.xwork2.ObjectFactory.buildResult(ObjectFactory.java:
232)
        at com.opensymphony.xwork2.DefaultActionInvocation.createResult(DefaultA
ctionInvocation.java:220)
        at com.opensymphony.xwork2.DefaultActionInvocation.executeResult(Default
ActionInvocation.java:367)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:277)
        at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doInte
rcept(DefaultWorkflowInterceptor.java:176)
        at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept
(MethodFilterInterceptor.java:98)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(V
alidationInterceptor.java:263)
        at org.apache.struts2.interceptor.validation.AnnotationValidationInterce
ptor.doIntercept(AnnotationValidationInterceptor.java:68)
        at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept
(MethodFilterInterceptor.java:98)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.interc
ept(ConversionErrorInterceptor.java:133)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept
(ParametersInterceptor.java:207)
        at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept
(MethodFilterInterceptor.java:98)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept
(ParametersInterceptor.java:207)
        at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept
(MethodFilterInterceptor.java:98)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.inter
cept(StaticParametersInterceptor.java:190)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(Multi
selectInterceptor.java:75)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(Checkbox
Interceptor.java:94)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUp
loadInterceptor.java:243)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(
ModelDrivenInterceptor.java:100)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.inte
rcept(ScopedModelDrivenInterceptor.java:141)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.interce
pt(DebuggingInterceptor.java:267)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(Cha
iningInterceptor.java:142)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(Pr
epareInterceptor.java:166)
        at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept
(MethodFilterInterceptor.java:98)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInt
erceptor.java:176)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(Ser
vletConfigInterceptor.java:164)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasI
nterceptor.java:190)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.inter
cept(ExceptionMappingInterceptor.java:187)
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionI
nvocation.java:248)
        at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.j
ava:52)
        at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.jav
a:485)
        at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(Exec
uteOperations.java:77)
        at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
.doFilter(StrutsPrepareAndExecuteFilter.java:91)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(Servlet
Handler.java:1254)
        at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java
:481)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.j
ava:119)
        at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.jav
a:516)
        at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandl
er.java:230)
        at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandl
er.java:956)
        at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:
411)
        at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandle
r.java:188)
        at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandle
r.java:891)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.j
ava:117)
        at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(Cont
extHandlerCollection.java:247)
        at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerColl
ection.java:151)
        at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper
.java:114)
        at org.eclipse.jetty.server.Server.handle(Server.java:353)
        at org.eclipse.jetty.server.HttpConnection.handleRequest(HttpConnection.
java:598)
        at org.eclipse.jetty.server.HttpConnection$RequestHandler.headerComplete
(HttpConnection.java:1059)
        at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:590)
        at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:212)

ex evaluated to null string.
The problematic instruction:
----------
==> ${ex} [on line 129, column 13 in org/apache/struts2/dispatcher/error.ftl]
----------

Java backtrace for programmers:
----------
freemarker.template.TemplateException: ex evaluated to null string.
        at freemarker.core.EvaluationUtil.getString(EvaluationUtil.java:81)
        at freemarker.core.Expression.getStringValue(Expression.java:108)
        at freemarker.core.Expression.getStringValue(Expression.java:93)
        at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
        at freemarker.core.Environment.visit(Environment.java:210)
        at freemarker.core.MixedContent.accept(MixedContent.java:92)
        at freemarker.core.Environment.visit(Environment.java:210)
        at freemarker.core.IteratorBlock$Context.runLoop(IteratorBlock.java:167)

at freemarker.core.Environment.visit(Environment.java:417)
        at freemarker.core.IteratorBlock.accept(IteratorBlock.java:102)
        at freemarker.core.Environment.visit(Environment.java:210)
        at freemarker.core.MixedContent.accept(MixedContent.java:92)
        at freemarker.core.Environment.visit(Environment.java:210)
        at freemarker.core.Environment.process(Environment.java:190)
        at freemarker.template.Template.process(Template.java:237)
        at org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:74
9)
        at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.jav
a:506)
        at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(Exec
uteOperations.java:77)
        at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
.doFilter(StrutsPrepareAndExecuteFilter.java:91)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(Servlet
Handler.java:1254)
        at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java
:481)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.j
ava:119)
        at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.jav
a:516)
        at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandl
er.java:230)
        at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandl
er.java:956)
        at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:
411)
        at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandle
r.java:188)
        at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandle
r.java:891)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.j
ava:117)
        at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(Cont
extHandlerCollection.java:247)
        at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerColl
ection.java:151)
        at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper
.java:114)
        at org.eclipse.jetty.server.Server.handle(Server.java:353)
        at org.eclipse.jetty.server.HttpConnection.handleRequest(HttpConnection.
java:598)
        at org.eclipse.jetty.server.HttpConnection$RequestHandler.headerComplete
(HttpConnection.java:1059)
        at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:590)
        at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:212)

at org.eclipse.jetty.server.HttpConnection.handle(HttpConnection.java:42
7)
        at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEn
dPoint.java:510)
        at org.eclipse.jetty.io.nio.SelectChannelEndPoint.access$000(SelectChann
elEndPoint.java:34)
        at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEnd
Point.java:40)
        at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool
.java:450)
        at java.lang.Thread.run(Thread.java:662)
2011-06-30 12:08:20.562:WARN::Committed before 500 Unable to show problem report
: freemarker.template.TemplateException: ex evaluated to null string.
2011-06-30 12:08:20.578:WARN::/index
java.lang.IllegalStateException: Committed
        at org.eclipse.jetty.server.Response.resetBuffer(Response.java:1056)
        at org.eclipse.jetty.server.Response.sendError(Response.java:260)
        at org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:75
3)
        at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.jav
a:506)
        at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(Exec
uteOperations.java:77)
        at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
.doFilter(StrutsPrepareAndExecuteFilter.java:91)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(Servlet
Handler.java:1254)
        at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java
:481)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.j
ava:119)
        at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.jav
a:516)
        at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandl
er.java:230)
        at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandl
er.java:956)
        at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:
411)
        at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandle
r.java:188)
        at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandle
r.java:891)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.j
ava:117)
        at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(Cont
extHandlerCollection.java:247)
        at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerColl
ection.java:151)
        at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper
.java:114)
        at org.eclipse.jetty.server.Server.handle(Server.java:353)
        at org.eclipse.jetty.server.HttpConnection.handleRequest(HttpConnection.
java:598)
        at org.eclipse.jetty.server.HttpConnection$RequestHandler.headerComplete
(HttpConnection.java:1059)
        at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:590)
        at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:212)

my action class is

package com.action;

import java.util.List;

import org.apache.struts2.components.Param;
import org.apache.struts2.convention.annotation.Action;
import org.apache.struts2.convention.annotation.Namespace;
import org.apache.struts2.convention.annotation.ParentPackage;
import org.apache.struts2.convention.annotation.Result;
import org.apache.struts2.convention.annotation.ResultPath;
import org.apache.struts2.json.annotations.JSON;

import com.gradeguru.api.domain.universities.UniversityList;
import com.opensymphony.xwork2.ActionSupport;
import com.sun.jersey.api.client.Client;
import com.sun.jersey.api.client.WebResource;

@ParentPackage(value = "default")
@Namespace("/")
@ResultPath(value = "/")
public class indexpage extends ActionSupport {

/**
	 * 
	 */

private static final long serialVersionUID = 1L;
	private UniversityList universitylist=null;

@Action(value = "index", results = { @Result(name = "success", type = "json", location="/json.jsp") })
	public String execute() {
		System.out.println("its universityIndex");
		Client client=Client.create();
		WebResource wr=client.resource("api call");
//above line code gives me the resource in json format.

universitylist =wr.get(UniversityList.class);
		System.out.println(universitylist);
		System.out.println("its home");
		return SUCCESS;
	}
	
	 @JSON(name="UniversityList")
		public UniversityList getUniversitylist() {
		 System.out.println(universitylist);
			return universitylist;
		}

public void setUniversitylist(UniversityList universitylist) {
			this.universitylist = universitylist;
		}

}

when i m not giving the location in @action annotation then it gives me a download file which contains the json data.

but when i m getting it on my jsp with property tag.

please help me to sort it out