Question

I just started to develop with Google AppEngine and now I have my first problem.

I take the Todo-Example-Project from the Google-IO 2011 and make some changes now I get the following StackTrace in my Application:

Caused by: java.lang.NullPointerException: null
    at com.google.web.bindery.requestfactory.shared.impl.AbstractRequestContext.doFire(AbstractRequestContext.java:1102)
    at com.google.web.bindery.requestfactory.shared.impl.AbstractRequestContext.fire(AbstractRequestContext.java:569)
    at com.google.web.bindery.requestfactory.shared.impl.AbstractRequest.fire(AbstractRequest.java:54)
    at com.google.web.bindery.requestfactory.shared.impl.AbstractRequest.fire(AbstractRequest.java:59)

I try to create a new Entity in the GWT-Client with this Code:

MyRequest request = requestFactory.myRequest();
request.createTime().fire();

Code in the Service-Class

static DataStore db = new DataStore();

public static Time createTime() {
    return db.update(new Time());
}

And this is the Code in the Datastore-Class

public Time update(Time item) {
    PersistenceManager pm = PMF.get().getPersistenceManager();
    try {
        pm.makePersistent(item);
        return item;
    } finally {
        pm.close();
    }
}

I hope you can help me, with this Problem.

Thank you

Edit:
I think I should add, that of course I have implemented a TimeLocator-Class and a ServiceLocator-Class.

Here is my web.xml, because I think there could be the problem:

<?xml version="1.0" encoding="utf-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/xmlschema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
    xsi:schemalocation="http://java.sun.com/xml/ns/javaee
                        http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
    version="2.5">

  <!-- Default page to serve -->
  <welcome-file-list>
    <welcome-file>Test.html</welcome-file>
  </welcome-file-list>

  <!-- RequestFactory Servlet -->

  <servlet>
    <servlet-name>requestFactoryServlet</servlet-name>
    <servlet-class>com.google.web.bindery.requestfactory.server.RequestFactoryServlet</servlet-class>
    <init-param>
      <param-name>symbolMapsDirectory</param-name>
      <!-- You'll need to compile with -extras and move the symbolMaps directory
           to this location if you want stack trace deobfuscation to work -->
      <param-value>WEB-INF/classes/symbolMaps/</param-value>
    </init-param>
  </servlet>

  <servlet-mapping>
    <servlet-name>requestFactoryServlet</servlet-name>
    <url-pattern>/gwtRequest</url-pattern>
  </servlet-mapping>

  <!-- Other servlets -->

  <servlet>
    <servlet-name>dataMessagingServlet</servlet-name>
    <servlet-class>
      com.google.android.c2dm.server.C2DMRetryServlet
    </servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>dataMessagingServlet</servlet-name>
      <url-pattern>/tasks/c2dm</url-pattern>
  </servlet-mapping>

  <!-- Security -->

  <!--  Require user login for the .html welcome page -->
  <security-constraint>
    <web-resource-collection>
      <url-pattern>/Test.html</url-pattern>
    </web-resource-collection>
    <auth-constraint>
      <role-name>*</role-name>
    </auth-constraint>
  </security-constraint>

  <!--  Require admin role for the tasks servlet -->
  <security-constraint>
    <web-resource-collection>
      <web-resource-name>tasks</web-resource-name>
      <url-pattern>/tasks/*</url-pattern>
    </web-resource-collection>
    <auth-constraint>
      <role-name>admin</role-name>
    </auth-constraint>
  </security-constraint>

</web-app>

Answer 1

This is how you start the RequestFactory to solve the problem above:

public class ClientFactory {

  private final ApplicationRequestFactory requestFactory = GWT.create(ApplicationRequestFactory.class);

  private EventBus eventBus;

  public ClientFactory() {
   eventBus = new SimpleEventBus();
   requestFactory.initialize(eventBus);
  }

  public ApplicationRequestFactory getRequestFactory() {
    return requestFactory;
  }

}