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?
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.
There are a number of tools that come with the JDK that don't come with the JRE - JConsole (http://java.sun.com/developer/technicalArticles/J2SE/jconsole.html) springs to mind. This for example can help you understand & monitor the memory usage of your application and so on. Either way if you head into unfamiliar territory, I highly recommend you follow the Eclipse suggestion and use the JDK!
JDK is equipped with different helpful tools, as DotMasta mentioned. Eclipse's "version" of JDK is called JDT. Apart from range of shipped tools, there are also differences between javac and Eclipse built-in compiler ecj, so check here to see the comparison. One of the most important differences is that javac is strict, i.e. with ecj you can create a class file even in case of errors in code, which is perfectly suitable for testing phase, but not for launch :)
JDK contains software development tools which are used to compile and run the Java program.
Plenty of classes and methods in standard jdk library
Javac java compiler
Diffrences and why you will need this?
JDK includes the JVM, standard class libraries, and several other tools that a developer needs in order to create a Java program.
JRE includes the JVM, as the JRE provides some standard libraries and the JVM which can be used to execute a Java program.
You can even look there: http://en.wikipedia.org/wiki/Java_Development_Kit
Related
I always get the above mentioned error when i restart eclipse.I have gone through other posts in stackoverflow and understood how to resolve the problem( i.e by changing configurations in windows>prefernces and editing eclipse.ini file).
But the basic question is what is the meaning of that error?Someone please explain and also provide link to resource on how eclipse works internally with those preferences set.
Thanks in advance..
The JRE runs Java applications. The JDK can also compile Java source code, which is what your Eclipse is trying to do.
Basically Apache Maven requires a Java compiler (javac) which is not part of a Java runtime environment. (JRE) So it asks to use Java Development Kit (JDK) instead because JDKs are including a Java compiler. Nearest explanation for this can be found in below link.
http://wiki.eclipse.org/M2E_FAQ for question "Unable to locate the Javac Compiler Error". How to force Apache Maven to use the Eclipse Java Development Tools (JDT) as its default compiler can be found here.
In Eclipse when i create new Java Project, i see options like:
Now i want to know, what is the safest option to use considering when i export jar file any user will be able to use it. If selecting the shown option, does that mean if someone has JRE 1.6, he will not be able to run that jar file, or is it backward compatible??
As such my project has no specific 1.7 dependencies.
This is very broad question, which JRE to use depends on your requirement. If you have multiple projects, each specific to a JRE, then you can use project specific JRE. If you are happy with eclipse default JRE you can continue. It is purely your decision.
Someone with JRE 1.6 should be able to run on 1.6 as long as compilation level is set to 1.6 (This may flag if your code has any 1.7 features which are not part of 1.6).
If you use features which are specific to JDK 7 (such as the new NIO.2/The path API ) then they most likely won't work on java 6. Furthermore Java 6 has been tested and withstood the test of time as a result most (if not all) bugs have been ironed out. Java 7 is still fairly new and even after it was released there was a pesky bug that showed up couple of days after release which had to the with the loop optimisation. So unless you specifically need JDK7 features I'd suggest you stick with JDK6, on the other hand if your project is not mission-critical then you might experiment with the new JDK7 features given that your user base has JRE 7. Just my 2 cents...
I see JRE all over the place, but i can not be sure which JDK is compiling my code. How do i find out?
You can still check/set which JDK version you want to compile to. If you go to Window->Preferences, then click "Java" from the menu on the left and then "Compiler". The first setting allows you to see which JDK version is being complied with.
I know this doesn't directly answer your question since JDK isn't actually compiling it, but hopefully it helps!
Actually Eclipse has its own compiler. You can develop java apps using only the JRE and Eclipse
You don't find JDK explicitly mentioned anywhere because way to point to a JDK is under the name of 'Installed JRE'. That is Window > Preferences > Java > Installed JREs. This is useful when using m2e plugin.
secondly apart from having it's own built in compiler, it is recommended to you install one of your own to support development of Java programs. Except below from eclipse wiki page :
If you intend to use Eclipse for Java development, then you should install a JDK
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 :)
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.
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.
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.
You may write Java programs with whatever text editor, you don't need anything special to do this.
You need the JRE to run Java programs. The JRE includes the Java Virtual Machine, needed to run already compiled Java programs.
You need the JDK to compile Java programs. So if you are a Java developer, you may want to only install the JDK since it comes with the tools needed to compile, in addition to the Java Virtual Machine.
'Sometimes you can develop with jre'
No. Never.
You develop with the Java Development Kit. You run with the Java Runtime Environment or Engine or whatever it's called.
I've installed Eclipse on a Windows machine some time ago. A couple of days back I was doing some Java coding and I noticed I don't have a JDK, but still Eclipse could compile & run the Java classes. Does it have a compiler included?
Yes Eclipse contains it's own incremental compiler.
An incremental Java compiler.
Implemented as an Eclipse builder, it
is based on technology evolved from
VisualAge for Java compiler. In
particular, it allows to run and debug
code which still contains unresolved
errors.
For some history take a look here:
http://onjava.com/pub/a/onjava/2006/06/28/whats-new-in-eclipse-3-2-java-development-tools.html
Yes. And here's some more characters.