JDK/JRE/JVM/Java SDK | What do they all mean? Sometimes you can develop with JRE and sometimes you need JDK?

Question

To tell the truth, I am quite confused on all these terms (JDK/JRE/Java SDK). I am not sure what each one does.

When I first started doing simple java examples in eclipse, I am pretty sure I only had the JRE, which I believed was the default java installer regular users use to be able to run java programs/applets on their system.

However, now in class we are using Google Appengine, and this requires the JDK which I am guessing is the same as Java SDK. After wasting some time finding out that installing the JDK meant I also had to add java/bin to the environment variables to get javac -version to work in the command prompt I find that only the JDK has javac...

How were my early java programs working without having installed the JDK and therefore not having javac? And really the main question... What is the difference between the JRE and JDK, and when do you use each one?

Thank you :)

Answer 1

JRE = Java Runtime Environment - what you need to run programs/software that require Java or use libraries written in Java. For example, OpenOffice requires the Java Runtime Environment

JDK/Java SDK = Java Development Kit/Java Software Development Kit - what you need to write programs that require Java or use libraries written in Java. For example, if you were to write your own word-processing tool in Java.

java comes with the JRE because it launches the VM (virtual machine). It can take in class files which are files that have been compiled using the JDK.

The JDK comes with javac because that's what you need to compile your .java files into .class files that can then run on the JRE.

Answer 2

Eclipse has its own built-in compiler (called ecj), which is probably the reason you could get away with not having the JDK installed to use it. It does not use javac.

Google App Engine uses the javac that comes with the JDK.