Nullpointerexception in entitymanager

hy,

I'm having trouble in using entitymanager in a java web applicaton.

My code is:

public class HelloWorldResource extends ServerResource {
@PersistenceContext(unitName = "TestRestletPU")
EntityManagerFactory emf;

@Get  
public String represent() {
    EntityManager em = emf.createEntityManager();

    if(em.isOpen())
        return "good";
    else 
        return "bad";
} 

and my persistence.xml is

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence"  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns    /persistence/persistence_2_0.xsd">
  <persistence-unit name="TestRestletPU" transaction-type="JTA">
    <jta-data-source>Test</jta-data-source>
    <exclude-unlisted-classes>false</exclude-unlisted-classes>
    <properties/>
  </persistence-unit>
</persistence> 

All my entity classes and persistence.xml are auto generated, therefore i think there shouldn't be anything wrong.

I got everytime this error:

java.lang.NullPointerException
    at firstSteps.HelloWorldResource.represent(HelloWorldResource.java:23)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.restlet.resource.ServerResource.doHandle(ServerResource.java:449)
    at org.restlet.resource.ServerResource.get(ServerResource.java:645)
    at org.restlet.resource.ServerResource.doHandle(ServerResource.java:527)
    at org.restlet.resource.ServerResource.doNegotiatedHandle(ServerResource.java:587)
    at org.restlet.resource.ServerResource.doConditionalHandle(ServerResource.java:299)
    at org.restlet.resource.ServerResource.handle(ServerResource.java:846)
    at org.restlet.resource.Finder.handle(Finder.java:510)
    at org.restlet.routing.Filter.doHandle(Filter.java:156)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.routing.Router.doHandle(Router.java:497)
    at org.restlet.routing.Router.handle(Router.java:737)
    at org.restlet.routing.Filter.doHandle(Filter.java:156)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.routing.Filter.doHandle(Filter.java:156)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.routing.Filter.doHandle(Filter.java:156)
    at org.restlet.engine.application.StatusFilter.doHandle(StatusFilter.java:151)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.routing.Filter.doHan开发者_开发百科dle(Filter.java:156)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.engine.ChainHelper.handle(ChainHelper.java:111)
    at org.restlet.engine.application.ApplicationHelper.handle(ApplicationHelper.java:72)
    at org.restlet.Application.handle(Application.java:388)
    at org.restlet.routing.Filter.doHandle(Filter.java:156)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.routing.Router.doHandle(Router.java:497)
    at org.restlet.routing.Router.handle(Router.java:737)
    at org.restlet.routing.Filter.doHandle(Filter.java:156)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.routing.Router.doHandle(Router.java:497)
    at org.restlet.routing.Router.handle(Router.java:737)
    at org.restlet.routing.Filter.doHandle(Filter.java:156)
    at org.restlet.engine.application.StatusFilter.doHandle(StatusFilter.java:151)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.routing.Filter.doHandle(Filter.java:156)
    at org.restlet.routing.Filter.handle(Filter.java:203)
    at org.restlet.engine.ChainHelper.handle(ChainHelper.java:111)
    at org.restlet.Component.handle(Component.java:388)
    at org.restlet.Server.handle(Server.java:488)
    at org.restlet.engine.http.connector.BaseServerHelper.handle(BaseServerHelper.java:158)
    at org.restlet.engine.http.connector.BaseServerHelper.handleInbound(BaseServerHelper.java:167)
    at org.restlet.engine.http.connector.BaseHelper.handleNextInbound(BaseHelper.java:418)
    at org.restlet.engine.http.connector.Connection.readMessages(Connection.java:695)
    at org.restlet.engine.http.connector.Controller$2.run(Controller.java:95)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:662)

Can anyone help me out.

I think your class needs to be marked @Stateless or @Statefull to use @PersistenceContext

As far as I know, you can use either:

@PersistenceContext(unitName = "TestRestletPU") EntityManager em; (inside a stateless or statefull EJB session bean)

OR create your Entity Manager via the EntityManagerFactory, like you tried in your code with EntityManager em = emf.createEntityManager();

But you should not mix that code.