Eclipse has implemented its own compiler called as Eclipse Compiler for Java (ECJ)
Javac is the primary Java compiler, included in the Java Development Kit (JDK) from Sun Microsystems (now Oracle), and also in versions from other vendors.
"Eclipse compiler for java jdt ecj is much smarter tool than actual javac"
from question "VerifyError when build with ant, OK when build from Eclipse"
"To recap i want to know why a javac compilation utilizes so much more cpu than ecj"
from question "Javac goes crazy on CPU utilization ECJ is able to maintain a predictable CPU utilization for a multi-threaded Producer -Consumer"
"However i think that ecj is even better than javac my opinion ."
from question "How to configure Eclipse to compile using Oracle javac 1.7.0_09?"