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Head First Servlets and JSP, 2nd Edition by Bert Bates, Bryan Basham, Kathy Sierra

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Memorizing the JNDI <env-entry> DD tag

If you’re familiar with EJB and/or JNDI, this will make sense. If you’re not, it doesn’t really matter for the exam as long as you memorize the tag. (The details surrounding JNDI environment entries are covered in EJB/J2EE books like the lovely Head First EJB.)

Think of an environment entry as being something like a deploy-time constant that your app can use, much like servlet and context init parameters. In other words, a way for the deployer to pass values into the servlet (or in this case, an EJB as well if this is deployed as part of an enterprise application in a fully J2EE-compliant server).

At deploy time, the Container reads the DD and makes a JNDI entry (again, assuming this is a fully J2EE-compliant app, and not just a server with only a web Container), using the name and value you supply in this DD tag. At runtime, a component in the application can look up the value in JNDI, using the name listed in the DD. You probably won’t care about <env-entry> unless you’re also developing with EJBs, so the only reason you need to memorize this is for the exam.

Declaring an app’s JNDI environment entry

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Note

The <env-entry-type> must NOT be a primitive!

When you see an <env-entry-value> that’s an integer value (like the example above), you might think that the <env-entry-type> can be a primitive. But that would be... wrong.

You also might be tempted ...

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