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 :)
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
Google App Engine uses the
javac that comes with the JDK.
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.
What is the difference between the JRE and JDK and when do you use each one?
JRE: Java Runtime Environment. It is used to run Java programs only. As Chris Jester -Young mentioned, Eclipse had a built in compiler. That's why you just needed JRE ( to run eclipse )
If you ship a Java program, the only thing the client need to have is this runtime environment
JDK: Java Develpment Kit, this also includes a JRE inside, but additionally have other tools for program development such as the java compiler javac among many others.
If you want to create java program you should use this.
There's no way you used the JRE to compile Java programs. javac, the Java compiler, only comes with the JDK.
'Sometimes you can develop with jre'
You develop with the Java Development Kit. You run with the Java Runtime Environment or Engine or whatever it's called.