Testing and optimizing Java code without test automation for handling concurrent activities is rather difficult.

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1. Concurrency Testing in Java Applications
   Testing and optimizing Java code without test automation for handling concurrent activities is rather difficult. Even with test automation, being able to correlate the test activity from the client side to observations of thread, memory, object and database connection use on the server side is difficult at best.
    
2. The Singleton Beans
   A new feature of Singleton Beans is added in EJB 3.1 that is used to store application-wide shared data. The JEE container maintains a single shared instance of an EJB 3.1 Singleton that can cache state across the application tier. Like all other EJBs, Singletons are simply annotated POJOs.

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http://www.javajazzup.com/issue9/page5.shtml

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