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• When a portlet sets an ETag on any response with an expiration time the portal caches the content using the ETag, until the expiration time passes any subsequent request for the same ETag results in immediate replay from cache
• One the expiration time for an ETagged response has passed the portlet is consulted to see if the ETag is still valid for the request, if the portlet returns true the content is replayed and the cache expiration updated.
• On resource responses the ETag is passed to the browser as the ETag HTTP header and the expiration time is passed as the 

More Information

For a complete description of how portlet caching works it is recommended to read section PLT.22 of the JSR-286 specification.

Caching of static assets

uPortal and some portlets are configured to cache static assets such as *.js and *.css configuration in web.xml, a spring context file, and ehcache.xml, for example.  

<filter>
    <filter-name>pageCachingFilter</filter-name>
    <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
    <init-param>
        <param-name>targetFilterLifecycle</param-name>
        <param-value>true</param-value>
    </init-param>
</filter>
    
<filter-mapping>
    <filter-name>pageCachingFilter</filter-name>
    <url-pattern>*.js</url-pattern>
    <url-pattern>*.css</url-pattern>
</filter-mapping>

Other configuration is needed in applicationContext.xml and ehcache.xml, and you typically also use a cacheExpiresFilter  See the following for examples:

• https://github.com/Jasig/CalendarPortlet/blob/CalendarPortlet-2.1.3-M4/src/main/webapp/WEB-INF/web.xml#L86
• https://github.com/Jasig/CalendarPortlet/blob/CalendarPortlet-2.1.3-M4/src/main/webapp/WEB-INF/context/applicationContext.xml#L61
• https://github.com/Jasig/CalendarPortlet/blob/CalendarPortlet-2.1.3-M4/src/main/resources/ehcache.xml#L38