The public review draft of JSR 286 was recently released and the related files can be downloaded from this link. One of the major changes introduced in the new Portlet specification is how the caching of markup and resources can be done by the Portlet Container. I just want to highlight the differences between the caching part of JSR 168 and JSR 286 here.

 JSR 168:

• Only markup caching is supported. Resources are not accessed through a separate life cycle method.
  
• Only "Expiration" cache is supported.
  
• The cache is per user client and not shared across users.
  
• The cached content is expired after a pre-defined (in portlet.xml) period, or as set in the portlet (programatically).
  
• The expired cache is deleted and a fresh markup is fetched by calling the render method

JSR 286:

• Resources can be cached. A new life cycle method serveResource() has been introduced which allows access to the PortletContext object apart from providing more control for the portlet developers.
• "Validation" caching is supported along with "Expiration" caching. After the cache has expired the portlet has the option to check if the cache is still valid and indicate that the cache can be reused. To enable this, ETag has been introduced, which is similar to how the browsers, which conform to HTTP 1.1, perform caching. Once the cache has expired the Portlet Container calls the render/serveResource method with the ETag set in the RenderRequest/ResourceRequest, which the portlet can access and check if the cache is still valid. If the cache is still valid, then the portlet can set the USE_CACHED_CONTENT property in the response, and the new expiration time.
  
• Cache scope has been introduced, which when set to "public" indicates that the cache can be shared across users. The default value is "private" which behaves like the JSR 168 caching where the cache is per user.
  

The introduction of ETag ensures that the browser can be leveraged to cache resources and markup. With the new features of public sharing of caches, resource caching, validation caching and leveraging browser caching,  the performance of aggregation of portlets by a portal can be seen to increase significantly.