Quote from chapter 6.1 of JSF 2.0 specification:

FacesContext

JSF defines the
javax.faces.context.FacesContext
abstract base class for representing
all of the contextual information
associated with processing an incoming
request, and creating the
corresponding response.

A FacesContext
instance is created by the JSF
implementation, prior to beginning the
request processing lifecycle, by a
call to the getFacesContext method of
FacesContextFactory, as described in
Section 6.6 “FacesContextFactory”.

When the request processing lifecycle
has been completed, the JSF
implementation will call the release
method, which gives JSF
implementations the opportunity to
release any acquired resources, as
well as to pool and recycle
FacesContext instances rather than
creating new ones for each request.

It is specified in class javax.faces.context.FacesContext and in Mojarra implemented in class com.sun.faces.context.FacesContextImpl and in MyFaces implemented in class org.apache.myfaces.context.servlet.FacesContextImpl.

See also:

• What is the difference between FacesContext and ExternalContext