WebWork value stack & Interceptor

The value stack is central to the dynamic context-driven nature of webwork. A stack of objects against which expressions can be evaluated to find property values dynamically, by searching for the first object (from the top of the stack down) that has a property of that name. WebWork builds up the value stack during execution by pushing action onto stack (so action property is available to expressions in view pages). Note that not property itself but the contained object in the stack.

It provides a convenient way to expose common values such as security permission to the front-end. Creating a custom interceptor SecurityGateInterceptor, which push a secInfo object into the value stack, then all the properties available through getxxx() in SecInfo object will be exposed to all the view pages.

Relate methods are: context.getValueStack.push(obj); context.getValueStack().findValue("propertyName");

Interceptors are conceptually the same as servlet filters and JDK's Proxy class. They provides a way to supply pre-processing and post-processing around the action. With it, we can do dependency injection using a combination of set injection and interface injection. With the assistance of interfaces such as PrincipalAware, SessionAware, etc.