java threadlocal singleton - what is it?

Question

In layman speak, what does it mean when somebody says an object is a threadlocal singleton in Java? I was at a lecture about Java Server Faces, and everytime the FacesContext was spoken of - the instructor always reminded us that it is a threadlocal singleton.

Answer 1

There is only one unique instance of the FacesContext per thread.

The FacesServlet creates a ThreadLocal<FacesContext> on the beginning of the HTTP servlet request and removes it on the end of the HTTP servlet response associated with the HTTP servlet request. Whenever you do a FacesContext#getCurrentInstance() in your JSF code, you'll always get the one and the same instance throughout the entire HTTP servlet request/response processing.

Since HTTP servlet requests are executed by distinct threads and the FacesContext instance is attached as thread local variable to a single thread, no two HTTP servlet requests share the same FacesContext instance.