Enterprise JavaBeans 3.1, 6th Edition

Example: The RSSCacheEJB

Because the singleton bean represents one instance to be used by all incoming requests, we may take advantage of the extremely small impact this session type will have upon our overall memory consumption. The factors to beware are state consistency (thread-safety) and the potential blocking that this might impose. Therefore, it’s best to apply this construct in high-read environments.

One such usage may be in implementing a simple high-performance cache. By letting parallel reads tear through the instance unblocked, we don’t have the overhead of a full pool that might be supplied by an SLSB. The internal state of a cache is intended to be shared by many distinct sessions, so we don’t need the semantics of an SFSB. Additionally, we may eagerly populate the cache when the application deploys, before any requests hit the system.

For this example, we’ll define an RSS Caching service to read in a Really Simple Syndication (RSS; http://en.wikipedia.org/wiki/RSS) feed and store its contents for quick access. Relatively infrequent updates to the cache will temporarily block incoming read requests, and then concurrent operation will resume as normal, as illustrated in Figure 7-8.

Figure 7-8. A concurrent singleton session bean servicing many requests in parallel

This example is covered in greater detail in Appendix D.

Value Objects

As we’ll be representing the RSS entries ...