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JAPICC, sample usage:
japi-compliance-checker OLD.jar NEW.jarSample reports for log4j: http://abi-laboratory.pro/java/tracker/timeline/log4j/

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PkgDiff, sample usage:
pkgdiff OLD.jar NEW.jarSee sample report for args4j.

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Clirr, sample usage:
java -jar clirr-core-0.6-uber.jar -o OLD.jar -n NEW.jar