I hope that you're planning to get the user ID for casual information purposes only and not as part of a do-it-yourself web application security system, because anyone over the age of 5 years could stomp a security system like that flat without half thinking.
JSF does not "load" backing beans, it instantiates them. That means, specifically, that if a View calls for a bean property, the name of the bean in looked up in the compiled faces-config.xml file, the scope is searched for that attribute, and if the bean doesn't exist, it's instantiated using the class.newInstance() method. Then, if there are any managed properties, they are injected into the newly-instantiated bean, recursing on the process if any of the referenced managed propertiy beans didn't exist previously.
If you want to load/reload specific values into a managed bean, you have to explicitly do that yourself, and prior to JSF 2, and the @PostConstruct annotation, that could be an annoyance. @PostContruct at least gives you a point where the management framework has instantiated and initialized the bean's managed properties before anything actually start to use the bean.