Can not get some service in JavaXPCOM

Hello, everybody
    I am working on JavaXPCOM now, but I have some trouble with
Extension Manager. I can't get the nsIExtensionManager interface in
following code:

    nsIExtensionManager em =
(nsIExtensionManager)serviceManager.getServiceByContractID("@mozilla.org/
extensions/manager;1", nsIExtensionManager.NS_IEXTENSIONMANAGER_IID);

I got such error:

"getServiceByContractID" returned an error condition  (0x80570016)

When I try to look into the problem, I discovered that I couldn't get
many service interfaces, such as nsIXULAppInfo of "@mozilla.org/xre/
app-info;1" component.

I would be appreciated if you could give me some advice, such as:point
out my fault and the reason.

Best wishes.