NoClassDefFoundError: javax.naming.directory.InitialDirContext is a restricted class. Using CCS (GCM) in Google App Engine

Question

Im trying to implement google's Cloud Connection Server with Google App Engine following this tutorial - Implementing an XMPP-based App Server. I copied latest smack jars from http://www.igniterealtime.org/projects/smack/ (smack.jar and smackx.jar), put them in WEB-INF/lib and added them to the classpath (im using eclipse).

In the code sample in the first link i posted, the XMPPConnection is initiated in a 'main' method. Since this is not really suitable to GAE i created a ServletContextListener and added it to web.xml.

public class GCMContextListener implements ServletContextListener {

   private static final String GCM_SENDER_ID = "*GCM_SENDER_ID*";
   private static final String API_KEY = "*API_KEY*";

   private SmackCcsClient ccsClient;

   public GCMContextListener() {
   }

   @Override
   public void contextInitialized(ServletContextEvent arg0) {
       final String userName = GCM_SENDER_ID + "@gcm.googleapis.com";
       final String password = API_KEY;

       ccsClient = new SmackCcsClient();

       try {
           ccsClient.connect(userName, password);
       } catch (XMPPException e) {
           e.printStackTrace();
       }
   }

   @Override
   public void contextDestroyed(ServletContextEvent arg0) {
       try {
           ccsClient.disconnect();
       } catch (XMPPException e) {
        e.printStackTrace();
       }
   }
}

web.xml

<web-app> 
   <listener>
       <listener-class>com.myserver.bootstrap.GCMContextListener</listener-class>
   </listener>
</web-app>

Now, when i start the GAE server i get the following exception :

java.lang.NoClassDefFoundError: javax.naming.directory.InitialDirContext is a restricted class. Please see the Google App Engine developer's guide for more details.

i searched the "Google App Engine developer's guide for more details" but couldnt find anything about this. can you please help me ?

Answer 1

Google App Engine restricts access to certain JRE classes. In fact they published a whitelist that shows you which classes are useable. It seems to me that the Smack library might require some reference to a directory context (maybe to create the XMPP messages?) and that is why your servlet causes this exception. The javax.naming.directory is not in the whitelist.

I'm currently working on setting up a GCM Server as well. It seems to me that you need to read through the example and see what that main method is doing. What I see is a connection to the GCM server:

try {
  ccsClient.connect(userName, password);
} catch (XMPPException e) {
  e.printStackTrace();
}

Then a downstream message being sent to a device:

// Send a sample hello downstream message to a device.
String toRegId = "RegistrationIdOfTheTargetDevice";
String messageId = ccsClient.getRandomMessageId();
Map<String, String> payload = new HashMap<String, String>();
payload.put("Hello", "World");
payload.put("CCS", "Dummy Message");
payload.put("EmbeddedMessageId", messageId);
String collapseKey = "sample";
Long timeToLive = 10000L;
Boolean delayWhileIdle = true;
ccsClient.send(createJsonMessage(toRegId, messageId, payload, collapseKey,
    timeToLive, delayWhileIdle));

}

These operations would be completed at some point during your application's lifecycle, so your servlet should support them by providing the methods the example is implementing, such as the connect method that appears in the first piece of code that I pasted here. It's implementation is in the example at line 235 if I'm not mistaken.

As the documentation says, the 3rd party application server, which is what you're trying to implement using GAE, should be:

Able to communicate with your client. Able to fire off properly formatted requests to the GCM server. Able to handle requests and resend them as needed, using exponential back-off. Able to store the API key and client registration IDs. The API key is included in the header of POST requests that send messages. Able to store the API key and client registration IDs. Able to generate message IDs to uniquely identify each message it sends.