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You only need to compile those classes you have changed.However would normally use the ant build system to compile and jar a library. For good measure, if you are worried about only partial updates, you may want to 'clean' first.The eclipse builder tends to just put classes onto a classpath. And does not do the extra packaging steps, so if you are after a jar then its simplest to just use ant. Of course there's nothing to stop you from building a Jar in eclipse, its just that that's not the way that alfresco's out of the box eclipse projects are set up.