JavaServer Faces (JSF) is a Java-based Web application framework intended to simplify development integration of web-based user interfaces.
JSF is a request-driven MVC web framework based on component driven UI design model, using XML files called view templates or Facelets views. Requests are processed by the FacesServlet, which loads the appropriate view template, builds a component tree, processes events, and renders the response (typically HTML) to the client. The state of UI components (and some other objects) is saved at the end of each request (called stateSaving (note: transient true)), and restored upon next creation of that view. Several types of state-saving are available, including Client-side and Server-side state saving. Out of the box, JSF 1.x uses JavaServer Pages (JSP) for its display technology, but can also accommodate other technologies (such as XUL and Facelets). JSF 2 uses Facelets by default for this purpose. Facelets is a more efficient, simple, and yet more powerful view description language (VDL).
Because JSF supports multiple output formats, Ajax-enabled components can easily be added to enrich JSF-based user interfaces. The JSF 2.0 specification provides built in support for Ajax by standardizing the Ajax request lifecycle, and providing simple development interfaces to Ajax events, allowing any event triggered by the client to go through proper validation, conversion, and finally method invocation, before returning the result to the browser via an XML DOM update.
The following companies and projects offer Ajax-based JSF frameworks or component libraries:
Facelets (which was designed specifically for JavaServer Faces) was adopted as the official view technology for JSF 2.0. This eliminates the well-known life-cycle conflicts that existed with JSP, forcing workarounds by Java developers. Facelets allows easy component/tag creation using XML markup instead of Java code, the chief complaint against JSF 1.x.
The new JSF developments also provide wide accessibility to Java 5 annotations such as @ManagedBean, @ManagedProperty and @FacesComponent which removes the need for faces-config.xml in all cases save framework extension. Navigation has been simplified, removing the need for faces-config.xml navigation cases. Page transitions can be invoked simply by passing the name of the desired View/Facelet.
Addition of Partial State Saving and DOM updates are part of the built in standardized Ajax support.
JSF 2.0 also includes a number of other changes like adding support for events, RAILS_ENV style development stages and significantly expanding the standard set of components.