code: public boolean invokeOnComponent(FacesContext context, String clientId, ContextCallback callback)
              throws FacesException
      {
          // java.lang.NullPointerException - if any of the arguments are null
          if (context == null || clientId == null || callback == null)
          {
              throw new NullPointerException();
          }
  
          pushComponentToEL(context, this);
          try
          {
              // searching for this component?
              boolean found = clientId.equals(this.getClientId(context));
              if (found)
              {
                  try
                  {
                      callback.invokeContextCallback(context, this);
                  }
                  catch (Exception e)
                  {
                      throw new FacesException(e);
                  }
                  return found;
              }
              // Searching for this component's children/facets
              // [perf] Use getFacetsAndChildren() is nicer but this one prevents
              // create 1 iterator per component class that does not have
              // facets attached, which is a common case. 
              if (this.getFacetCount() > 0)
              {
                  for (Iterator<UIComponent> it = this.getFacets().values().iterator(); !found && it.hasNext(); )
                  {
                      found = it.next().invokeOnComponent(context, clientId, callback);
                  }                
              }
              if (this.getChildCount() > 0)
              {
                  for (int i = 0, childCount = getChildCount(); !found && (i < childCount); i++)
                  {
                      UIComponent child = getChildren().get(i);
                      found = child.invokeOnComponent(context, clientId, callback);
                  }
              }
              return found;
          }
          finally
          {
              //all components must call popComponentFromEl after visiting is finished
              popComponentFromEL(context);
          }
      }
docs: Invokes the <code>invokeContextCallback</code> method with the component, specified by <code>clientId</code>.
  
  @param context
  <code>FacesContext</code> for the current request
  @param clientId
  the id of the desired <code>UIComponent</code> clazz
  @param callback
  Implementation of the <code>ContextCallback</code> to be called
  @return has component been found ?
  @throws javax.faces.FacesException