code: public boolean containsKey(Object key) {
          if(key == null)
              throw new NullPointerException("Binding to a resource bundle does not accept a null key");
  
          BundleNodeMap map = lookupScriptableBundle(key.toString());
          return map != null;
      }
docs: Implementation of Map.containsKey for the bundle implicit object.
  
  This method is required by JSP 2.0 EL and performs the lookups of the
  various available bundles which have been registered either explicitly or
  implicitly.
  
  @param key The name of a bundle to lookup
  @return <code>true</code> if the bundle is available; <code>false</code> otherwise