What is the difference between JVM and JRE and JDK?

Question

Hi , I want to know what actually is JVM,JRE and JDK ? and the difference between them?

Answer 1

They should at least try. When it comes to
terminology, if they can find this site, they should
be able to find google, wikipedia, etc. It's
reasonable to expect them to try there first, and
then, if they still have questions, post here,
indicating what they looked at and how it fell short,
what they're still confused about, etc.Perhaps this is a valid point.
I believe physical JVM hardware has also been
manufactured. I wonder what we'd call that? A JPM
(Java Pysical Machine)?No. It is simply a hardware implementation of the abstract Java Virtual Machine.
.class files are executed by software (in fact,the
software that executes the .class files is calledthe
JRE and is short for the Java
Runtime Environment). The JavaRuntime
Environment is a native program (i.e. it is run by
hardware) which provides the ability to executejava
bytecodes (.class files)That's the JVM.No it's not. The JVM is an abstract architecture.
The JRE is the JVM plus native libraries plus core
API Java classes.The JRE is an implementation of the JVM along with some additional facilities. The JVM may be used to describe concrete implementations in industry vernacular, but this is not a technically accurate usage of the term per the specification and is therefore incorrect.
From the preface to the Java Virtual Machine Specification, Second Edition, viewable at http://java.sun.com/docs/books/jvms/second_edition/html/Preface.doc.html:
"The Java virtual machine is an abstract machine."
From Wikipedia (http://en.wikipedia.org/wiki/JVM):
"A Java Virtual Machine (JVM) is a set of computer software programs and data structures which implements a specific virtual machine model."
Perhaps this misconception is the source of the confusion. This is one reason why I answered deepakorsun rather than refer him to somewhere else: anywhere that would be reliable and/or completely accurate would be too dense to readily understand, and anywhere understood easily enough might not be completely accurate.
>
The Java Development Kit (JDK) just provides the
facilities necessary to write programs that willrun
on the Java Virtual Machine. It contains acompiler
It also contains tools such as jar and javap, the API
docs for the core API classes, the source code for
the API classes, probably a few other things, and
generally the JRE (though I'm not sure if that's
officially a required part of the JDK).I never said it didn't contain jar, javap, javac, java, jdb, jconsole, or any of the other programs or resources shipped with jdk. I simply stated affirmatively something it did contain. As I implied before, technical comprehensiveness is exactly what makes resources such as the JVM specification almost unusable for answering questions like this. That is why I omitted such details in my description. And since we're being all technical here, the API documentation does not ship with the Java Development Kit (at least not as of JDK 5.0), it is a separate download. It is the core API source files that ship in a .zip file with the JDK.
Don't get me wrong, I do understand that you have probably seen a lot more people come and go from these forums than I may ever understand and that you have helped more people than you can remember, and I don't mean to disrespect that. I understand that dealing with so many people over such a long period of time and hearing the same questions over and over again can probably start to wear on your patience, but I don't think the right approach is to treat anyone that tries to help them with utter contempt and sarcasm. That's not constructive or useful to anyone. I understand why you would be upset if somebody was purposely trying to be lazy, I probably would be too, but you have no real way of determining whether or not they are being lazy or just don't know something and don't know how to figure it out for themselves so they are asking a question. And if they are honestly just asking a question in good faith and looking up the answer on Wikipedia just did not occur to them (or they couldn't understand what it said), yet they somehow ended up on this site, they have done nothing wrong and there is no reason to get angry at them.
Oh come on, that wasn't nice. Not everybody isable
to answer their own questions. WTF are you talking about?!? Why waste the time to
type something out when they can click on a link and
find 1000 times more information than you can fit
into one post? Either way - they're
STILL going to have to read the
information themselves...
Ridiculous.So I suppose you've either read the Java Virtual Machine Specification, the Java Language Specification, and the Java Native Interface Specification in their entireties, or absolutely nothing about programming in Java. This is because, by your reasoning, these three books contain 1000 times more than anyone can fit on a tutorial, Wikipedia entry, mailing list, forum, blog, or anything else.
Criticizing somebody who isn't doing something the exact way you think it should be done is often quite idiotic and non-constructive. I believe this is this case here. Please post something more useful, meaningful, and/or polite next time. And just for future reference, yelling at a beginner does not help them learn faster or better. It just makes them hate you.
Oh come on, that wasn't nice. Not everybody isable
to answer their own questions. Java terminologycan
be confusing and difficult to understand in part
because of all the acronyms used and all the name
changes undergone by the various development kitsand
platforms.Point taken. You have much more patience than
I, but with lots of intensive therapy and proper
adjustment of my medications, I may get better.Thank you for this post. A display of wisdom and patience. You single handedly caused me to reconsider participating in a flame war.