This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The "mappedName" is used to identify the resource in JNDI, but it is container specific. The way you write it depends on the container, so it's not portable. To come over this problem, the "lookup" attribute was added to J2EE6. You can use it to specify the complete JNDI path to your resource, and this will work with any containers.