What is the purpose of Java JDK to Eclipse?

Question

I've just installed Eclipse, after i installed the java JDK. The Getting Started guide (in Eclipse) says i should reference my JDK installation in Window>Preferences>Java>Installed JREs, but that a JRE would also work.

Select the Java > Installed JREs preference page to display the installed Java Runtime Environments. Confirm that a JRE has been detected. By default, the JRE used to run the workbench will be used to build and run Java programs. It should appear with a checkmark in the list of installed JREs. We recommend that you use a Java SDK instead of a JRE. An SDK is designed for development and contains the source code for the Java library, easing debugging.

There was already a JRE set up there (not a JDK), so i did nothing and tried to compile a Hello World (just to see what would happen). To my surprise, it compiled!

I searched a little bit and it looks like this works because Eclipse has a built-in Java Compiler. I tried debugging using the same eclipse set up, and it was also successful.

So, what is the difference between setting a JDK and JRE there? Why is it important to download the JDK, since in my default configuration Eclipse doesn't seem to use it?

Answer 1

Probably the main difference is you get the source to all of the Java runtime libraries (with the JDK) which can be a big help. I always use the JDKs for that reason.

Also if you are debugging, this will allow you to meaningfully step into Java runtime libraries.