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A geronimo installation typically can run multiple jvms for different purposes. For instance, there's the main JavaEE server, a command line deploy tool, and a JavaEE application client container. Each of these requires configuration information from different files. The set of such information sufficient to configure such a jvm is collected together in a "server instance". In a geronimo server, these are represented by gbeans in the plugin module. When assembling a server using maven these are represented with xml in the maven pom. In any case, these have names, and for the plugin metadata specific to a server instance, you specify which instance you intend using the server attribute. This defaults to "default", the JavaEE server.
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Building,installing plugins and extracting a server from an exsiting server
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More details about this topic, please see Buidling,installing plugins and extracting a server from an exsiting server
Assembling a server using maven.
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