This is probably old news to veteran Java developers using Eclipse, but to me it’s new. Eclipse is an IDE (mainly) for Java development, written in Java. Thus you need Java runtime to run Eclipse itself plus the Java compiler to compile your code. Usually you’d just install the JDK and be done with it.

Or at least that’s what I thought… Even if you install just the runtime (JRE), Eclipse will compile your projects! This is somewhat strange because writing compilers is hard and IDE developers do not need yet another problem to solve. On the other hand, most modern IDEs offer code completion and refactoring plus a lot of helpers that warn you about code issues on the fly, while you are typing. To do that, the IDE needs a compiler, but not the “regular” one because this compiler needs to work in the presence of “errors”, that is unfinished code (you are still typing it).

Thus, it is not too strange that Eclipse has internal compiler, but it’s great that it is not just used for advanced editing but also for producing a final executable.