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Getting Geronimo
Apache Geronimo v2.2 is a Java EE 5 Certified server. Installation packages are available for both Windows and Unix (including Linux, Mac OS X) operating systems. In addition to the platform specific downloads, Geronimo is available in three main distributions which are the Java EE 5 Certified (Big-G), Little-G and Geronimo Framework. As suggested by the name, only the first one has full support of all the features and functions in the Java EE 5 specification and has passed SUN's Java EE 5 Certification Test Suite (CTS).
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You can find the source code and binary files of Geronimo v2.2 at http://geronimo.apache.org/downloads.html.
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You can also build a server from source code using either maven or eclipse. Look into Building Geronimo from source for the details.