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"nb-javac" consists of two libraries that provide a NetBeans-specific fork of the Java JDK Java compiler, and is explicitly excluded by Oracle from the NetBeans code donation to Apache.
Where is nb-javac found in the NetBeans sources?
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What does nb-javac do that's different to vanilla javac?
A few years back, the NetBeans team wrote a page describing what nb-javac does in addition to/differently to vanilla javac:
http://wiki.netbeans.org/JavacDiff
Looking at the page now, some of the things listed above are not part of nb-javac anymore (e.g., cancelling and some part of the error recovery paragraph) but it still gives a reasonable overview and is good to get an idea what kinds of things nb-javac does.