I was trying to compile a Grails application referencing third party JARs on Mac OSX. Although my system's JRE and JDK is set to Java 1.6 I always got a Compilation error: java.lang.UnsupportedClassVersionError: Bad version number in .class file
when accessing classes in the JAR. Also when testing an existing Grails app the default stats of the app where showing that it's running with Java 1.6. So I really didn't have more ideas than to try to change the Java settings in Eclipse - I thought that's stupid as I'm not using Eclipse for the devlopment of this application - but voila - now the compilation of my app on the command line works just fine!
Can anyone explain me what Eclipse is doing here behind the scenes?
I've had set the JAVA_HOME manually before with no effect.
The JDK (JAVA_HOME) used to launched eclipse is not necessarily the one used to compiled your project.
To see what JRE YOU y can select for your project, check the preferences
General > Java Installed JRE
By default, if you have not added any JRE, the only one declared will be the one used to launched eclipse (which can be defined in your eclipse.ini).
You can add any other JRE you want, including one compatible with your project.
After that, you will need to check in your project properties (or in the general preferences) what JRE is used, with what compliance level:
Related
In my installation of Eclipse IDE (Mars 4.5) it has not automatically linked rt.jar and jfxrt.jar to src.zip and javafx-src.zip. I know how to do this manually by browsing to the .zip files within the JDK installation.
What I want to know is if there is a way to ensure this happens automatically when installing Eclipse / Java. I'm pretty certain I know someone who didn't link to the source manually and it is linked on their installation.
For example, if you install the JDK before Eclipse, or vice-versa, does this make any difference?
Also, when you make the link, where abouts is the preference information stored on the file system - is it somewhere within the same folder that eclipse.exe exists?
To make sure that Eclipse is running in your JDK instead of a JRE (The JDK installation typically puts a JRE on the system executable path), you should always specify it via eclipse.ini. The -vm option described there tells Eclipse exactly what JVM you want it to run in.
Another option is to add the JDK to the Installed JREs section of your Eclipse preferences and set that as the default for all projects. The default JVM used for projects is the same as the one Eclipse itself runs in, so you don't necessarily need to do both.
Once you have the JDK set as the default JVM, you can open a project's Properties, select Java Build Path section, then look at the Libraries tab. You should see your JDK as the JRE System Library, including its rt.jar and corresponding source attachment. Like this:
Is it possible to use Eclipse variables like ${workspace_loc} or ${eclipse_home} for adding a JRE to Installed JREs in the Eclipse preferences?
No, you cannot use variables for that, neither in the JRE preferences nor the execution environment specifications. And don't look at the eclipse.ini as suggested in comments, the JVM running your IDE has nothing to do with the JRE used to compile your project.
That said, there is still another easy solution (which we use in our company): Just install a second copy of the end user JVM at a known fixed location for your developers, so you can use that fixed path in the JRE preference page.
If you fear that the two JRE copies (in your project and at the fixed location) get out of sync over time, then you could use an Ant based builder to your Eclipse project, which conditionally copies the JRE from the workspace to the outside known fixed location during the build. But honestly, it does not really matter if those 2 copies get out of sync. If your code was compiled using a 32 bit JRE, it will run on another 32 bit JRE deployed at your customer anyway, as long as they are byte code compatible. And you can set that explicitly in the Java compiler settings of your Java project.
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
I'm not sure if this question has been answered in full or if my title is descriptive enough given my situation, but I've been asked to convert a project from being built with Ant to Maven. That part isn't too bad, but I've been told that this application was designed specifically for the JRE version 1.5 rather than the JRE 6 everything else I've been dealing with uses. Now, I'm incredibly new to Eclipse and Java themselves, so I was a bit confused when I was asked to tell either Eclipse or Maven to build this particular project using the JRE 1.5 instead. I have it installed, I believe, and I've tried to follow the steps outlined here: ( Eclipse: Build and conform to different JRE versions ), but I've run into a snag.
I've managed to change the Run Configuration to use the Alternate JRE jre1.5.0_11. When I then tried to build it, the console spat out the following:
[ERROR] Unable to locate the Javac Compiler in:
[ERROR] C:\Program Files (x86)\Java\jre1.5.0_11\..\lib\tools.jar
[ERROR] Please ensure you are using JDK 1.4 or above and
[ERROR] not a JRE (the com.sun.tools.javac.Main class is required).
[ERROR] In most cases you can change the location of your Java
[ERROR] installation by setting the JAVA_HOME environment variable.
I assume this means that I need to be using the JDK 1.5 in order to use the JRE 1.5, since I'm currently working with JDK 1.6. Now, is there a way to install the JDK 1.5 so that this project can use it? My JAVA_HOME variable currently points to the JDK 1.6, and I would like it to stay that way... Is there a way to be able to use both and simply tell the project (or Eclipse or Maven... however it works) to use a specific version of the JDK?
Yes, this is possible. You can install as many different versions of Java — JREs and JDKs — as you like, and tell Eclipse which one to use for each project.
First step is to install the JDK 1.5 on your machine (JDK is short for 'Java [SE] Development Kit', look for that on the Oracle download site). The JDK includes a JRE, so to make life easier you can usually just install the JDK and not worry about the JRE/JDK distinction.
Second step is to tell Eclipse about the new JDK. Go to Preferences... then Java > Installed JREs. Click the Search... button and Eclipse should find your newly installed JDK 1.5 and add it to the list.
Final step is to allocate that JDK 1.5 to your project. Bring up the Properties menu for your project (right-click it). Then set these values:
Java Compiler tab
Tick Enable project specific settings
Set Compiler Compliance level to 1.5
Java Build Path tab, Libraries subtab
Find the JRE System Library item, click Edit...
Set Alternate JRE to your newly installed JDK 1.5
It looks like you are using JRE 1.5 rather than JDK 1.5
Make sure you download the JDK 1.5 and your issue should go away. You can set the default JDK in the preferences, search for 'jre'.
Projects can also define their specific jre
Get JDK 1.5 from Here http://www.oracle.com/technetwork/java/javasebusiness/downloads/java-archive-downloads-javase5-419410.html
I have two JRE in my system. One is 32-bit and the other one 64-bit. In Eclipse I can configure both and choose which one to use when running my application.
I'm wondering if I can do the same thing in Netbeans. I've tried to go to Project Properties, Libraries and then tried to configure a new JRE there, through Manage Platforms, but it doesn't accept the directory of my JRE. How should I proceed?
Be aware that I'm talking about JRE, and not JDK ;)
EDIT: I managed to make it work by downloading a new 32-bit JDK and selecting its directory. I still couldn't make it accept a JRE directory.
Whether or not you can run Netbeans with just the JRE depends on what bundle you have downloaded. In Netbeans 7 only C/C++ and PHP bundles can be run with the JRE.
Older versions of Netbeans include more languages under this JRE umbrella, but the principle is the same.
The JDK contains the JRE plus tools to debug and compile code, so if you're doing anything Java based apart from just running the IDE, netbeans depends on the JDK.