Comparing two .jar files
JAPICC, sample usage:
japi-compliance-checker OLD.jar NEW.jarSample reports for log4j: http://abi-laboratory.pro/java/tracker/timeline/log4j/
PkgDiff, sample usage:
pkgdiff OLD.jar NEW.jarSee sample report for args4j.
Clirr, sample usage:
java -jar clirr-core-0.6-uber.jar -o OLD.jar -n NEW.jar
If you select two files in IntellijIdea and press Ctrl + Dthen it will show you the diff. I use Ultimate and don't know if it will work with Community edition.