Provides low-level JSP integration funtionality for OpenCms, also implements the FlexCache, a declarative parameter-based cache.

# How the JSP integration is done:

In OpenCms all resources are residing in the OpenCms Database, the so called "Virtual File System" or VFS. However, all JSP engines available read the JSP code from a file in the local file system. The option to re-write a JSP engine so that it can read from the VFS would limit OpenCms to running with this JSP engine only, which was not a choice. So the approach that was taken is to dump the contents of the OpenCms JSP page from the VFS to the "real" file system and then dispatch to the standard JSP engine.

Here is a top-level description of how this works:

1. OpenCms gets the request through the {@link org.opencms.main.OpenCmsServlet}
2. The requested resource is identified as a JSP page (resource type {@link org.opencms.file.types.CmsResourceTypeJsp})
3. The JSP loader is started up with the resource manager {@link org.opencms.loader.CmsResourceManager}
4. The JSP loader implements an additional interface: {@link org.opencms.loader.I_CmsResourceLoader}
5. A resource loader is a new interface that was introduced with the FLEX release. The main difference is that the original request / response is wrapped using a Servlet standard {@link javax.servlet.http.HttpServletRequestWrapper} / {@link javax.servlet.http.HttpServletResponseWrapper}.
6. On the first call to a page, the file contents of the JSP page are read from the OpenCms VFS using the {@link org.opencms.file.CmsObject}
7. The contents of the page are written to the "real" file system of the server, in the default configuration to the directories `{WEBAPP-PATH}/opencms/WEB-INF/jsp/online` or `{WEBAPP-PATH}/opencms/WEB-INF/jsp/offline` (depending if the request was for the online or the offline version of the page). The directory used can be configured in `opencms.properties`.
8. After the page is written to the "real" FS, the loader dispatches to the standard JSP mechanism with a simple `include()` call.
9. The standard JSP mechanim will handle the JSP and deliver the output through the wrapped request / response back to OpenCms.
10. OpenCms will then deliver the contents back to the user.

@version $Revision: 1.12 $ @since 6.0.0 @see org.opencms.flex.CmsFlexCache @see org.opencms.flex.CmsFlexCacheEntry @see org.opencms.flex.CmsFlexCacheKey