RTFM on the ANT site for the jar task, specifically under the manifest element. "manifest The manifest nested element allows the manifest for the Jar file to be provided inline in the build file rather than in an external file. This element is identical to the manifest task, but the file and mode attributes must be omitted."
"Write beautiful code; then profile that beautiful code and make little bits of it uglier but faster." --The JavaPerformanceTuning.com team, Newsletter 039.