Where to declare JDO Singleton PersistenceManagerFactory with multiple services

I am working with GWT and have 4 Service Implementations that need PersistenceManagerFactory. I followed Google's guidelines for creating a singleton class, however I'm not sure where this class should be instantiated and referenced in server-side code.

The class looks like this:

import javax.jdo.JDOHelper;
import javax.jdo.PersistenceManagerFactory;

public final class PMF {
    private static final PersistenceManagerFactory pmfInstance =
        JDOHelper.getPersistenceManagerFactory("transactions-optional");

    private PMF() {}

    public static PersistenceManagerFactory get() {
        return pmfInstance;
    }
}

      

        
        
        
      

    

But doing something like this in each service implementation seems to result in a unique copy of the singleton based on the error message I get: "Application code tried to create a PersistenceManagerFactory named transaction-optional, but one with that name already exists!"

@Override
 public void addCategory(Category category) {
  PersistenceManager pm = PMF.get().getPersistenceManager();
  try {
        pm.makePersistent(category);
  } finally {
       pm.close();
     }
 }

      

        
        
        
      

    

So basically the first ServiceImpl that calls addCategory is fine, everyone else fails with the error above. I am missing something vital here, I thought the whole point of the singleton class is to create a static PersistenceManagerFactory. Any insight into what I am missing is greatly appreciated.