Is Sun's javac used in Eclipse?

NO. Eclipse has its own Java compiler.

http://www.eclipse.org/jdt/core/

JDT Core is the Java infrastructure of the Java IDE. It includes:
An incremental Java compiler. Implemented as an Eclipse builder, it is based on technology evolved from VisualAge for Java compiler. In particular, it allows to run and debug code which still contains unresolved errors.

Reference:
How to change to use SUN JAVAC compiler [message #112141]
sun vs eclipse autoboxing difference