I'm trying to learn java, and since I'm using 'javac' and 'java' all day, I'm using vim as my editor of choice. But I'm having a problem getting this system to work: I can't even get a minimal "Hello World" program to run! Within vim, I can use :!javac % to compile the current file just fine. However, with a file Test.java, any attempt to call :!java Test yields an UnsupportedClasVersionError, even though the same command works fine outside of vim. I'm completely stumped.
I'm using java7 on windows 7 (64bit) with cmd.exe as my shell.
The Java you invoke is an older version than the Javac you invoke.
You most likely have a Java 6 JRE installed (for browser plugins etc) at system level and a Java 7 JDK installed which you've added at the end of your path.
Either tell Javac to produce Java 6 compatible class files (with -target) or put the JDK in the front of your path.
Related
I have a jar file that runs fine from the Windows 10 command line, but is not working on my desktop Ubuntu 18.04 command line. The file was exported from Eclipse as a jar, and copied to Ubuntu. The project has about 30 classes over a half dozen packages. I've done some renaming in order to simplify things for this question.
This is the command used to run the jar:
java -jar myproject.jar
On Ubuntu, I get the error "Could not find or load main class com.a.b.LaunchThis"
Following are some of the things I've tried, based on suggestions from many similar posts. Sorry if this is a duplicate, but I couldn't find a workable answer.
I verified that the current Java is 1.8 using the commands:
$ update-alternatives --config java
and
$ java -version
openjdk version "1.8.0_252"
OpenJDK Runtime Environment (build 1.8.0_252-8u252-b09-1~18.04-b09)
OpenJDK 64-Bit Server VM (build 25.252-b09, mixed mode)
(I also verified that the program runs using java 8 on Windows.)
I ran the following check, which displays all the files, and confirmed that com/a/b/LaunchThis.class exists and is spelled exactly the same way.
java tj myproject.jar
The manifest seems correct.
Manifest-Version: 1.0
Main-Class: com.a.b.LaunchThis
I also tried unpacking the jar and running the program directly from the folder /myproject which contains both /com and META-INF. That should work, shouldn't it?
java com.a.b.LaunchThis
java com/a/b/LaunchThis
java com.a.b/LaunchThis
And from one level outside the myproject folder:
java myproject/com/a/b/LaunchThis
java myproject.com.a.b/LaunchThis
java myproject.com.a.b.LaunchThis
java myproject/com.a.b.LaunchThis
I am not totally clear on when the syntax requires . or /. The package command in "LaunchThis" which holds the main entry point is the following:
package com.a.b;
And in each of the above iterations I also tried including "-cp myproject.jar" as an option.
Always the exact same error message.
To be careful about not having a typo in the above, I first ran the ls command to make sure the file could be found (then edited this line to create the variants).
ls myproject.com.a.b.LaunchThis.class
Any suggestions as to what else to try?
For grins, running with Java11 gets this response:
Error: Could not find or load main class com.a.b.LaunchThis
Caused by: java.lang.NoClassDefFoundError: javafx/event/EventHandler
This makes some sense, because Java11 does not include JavaFX. But Java 8 does include EventHandler. It's been a part of Java since JavaFX 2.
UPDATE: I made two "Hello world" jars, one with and one without JavaFX on an Ubuntu installation of Eclipse running OpenJDK 8. To get the version with JavaFX to run, I downloaded the Oracle JDK 1.8, and linked to /lib/etc/jfxrt.jar as an "external jar" library. Both programs run in Eclipse but only the non-FX jar works. The FX jar gives exactly the same error message.
I think this pretty much establishes my problem to be one of not having JavaFX as part of OpenJDK 8.
My plan is to now try out two solutions: (1) copying the Oracle jfxrt.jar into the OpenJDK lib, (2) backloading OpenJFX 8.
Simply installing OpenJFX from repository loads OpenJFX 11, which is a little dated but will work with the OpenJDK 11.
The issue here is that the repository OpenJDK 8 on Ubuntu 18.04 does not include JavaFX. I'm guessing that the error message results from the fact that a JavaFX main() is located on a class that extends the JavaFX class Application.
How to fix this?
There are a number of things I tested.
First, one could use Oracle's JDK 1.8 instead. I ruled this out due to licensing issues.
Some sites suggested copying the jfxrt.jar from either the Oracle JDK 1.8 or from a working Java program that uses JavaFX and uses a self-contained JRE. I found that if I linked to the Oracle jfxrt.jar (located in the /jre/lib/ext folder) as an external jar, I could run a simple, "Hello world" javafx program in Eclipse. But I could not run the program after it was exported to a jar.
One answer on a related stackoverflow thread suggested copying over several additional files along with jfxrt.jar, which I tried. This did not work, so I'm not going to pass that on.
The solution that has been most successful came from a link provided in the comment by #dave_thomson_085, How do I get Java FX running with OpenJDK 8 on Ubuntu 18.04.2 LTS?.
This answer involves purging and reinstalling openjfx with an older version, and marking it to NOT be subjected to updates.
The code provided by Wolfgang Fahl (which he credits to Druidefix) follows:
apt purge openjfx
apt install openjfx=8u161-b12-1ubuntu2 libopenjfx-jni=8u161-b12-1ubuntu2 libopenjfx-java=8u161-b12-1ubuntu2
apt-mark hold openjfx libopenjfx-jni libopenjfx-java
Now, I can run and test jars containing Java8 using JavaFX that were built on my Windows Eclipse, on my Ubuntu desktop.
I do get a warning message which I haven't dealt with yet:
Gtk-Message: 13:25:40.829: Failed to load module "canberra-gtk-module"
But this is not preventing my programs from running.
I have created an executable JAR file developed on Java version 8. The JAR file was opening on double click. But as the Oracle applications support only Java 6, I had to install JRE 6, but then after the JRE 6 installation, my executable JAR file is not opening.
I have set the JDK 8 bin path in Path environment variables. Is there a solution for this problem? Why is the JAR file not opening after two Java versions in the system?
JAR should open even if two versions 6 and 8 of Java are installed in the system.
You are facing a backward compatibility problem. Backwards compatibility means that you can run a Java 6 program on a Java 8 runtime, but not the other way around.
You can run a lower configuration on a higher configuration, not vice-versa
There are several reasons for that:
Bytecode is versioned and the JVM checks if it supports the version it finds in .class files.
Some language constructs cannot be expressed in previous versions of bytecode.
There are new classes and methods in newer JREs which won't work with older ones.
If you really, really want (tip: you don't), you can force the compiler to treat the source as one version of Java and emit bytecode for another, using something like this:
javac -source 1.8 -target 1.6 MyClass.java
You can compile your code to Java 1.6 bytecode using JDK 1.8. Just take care of the following:
-source=1.8 and -target=1.6 compiler options
If you use Maven, consider having two pom.xml files, with an optional parent file.
Source: Can program developed with Java 8 be run on Java 7?
I am not sure if this solution going to work or not.
Try to run command java -version and look if it returns java 6 or 8 path. Also try to give path of JDK 8 as JAVA_HOME variable and add that into path like this path=%JAVA_HOME%/bin and see if it works. If you get the java 6 as java version try to use above method and then install JRE 6
Hi All Thank you for your response. I kept java6 and reinstalled java8 and now forms and jar both are working!.
In the short term,
the answer is yes. Since both JDK files are downloaded as jar fils it will ok to download both jar files. The reason to not opening after two java versions is as #Elliott said: "in the system is Java 6 can't run Java 8 compiled code, you should be getting an error." That's exactly true but the problem is how to use multiple versions of JDK in a single machine.
Then we have to move on to long term,
The tricky thing is to manage these multiple JDKs and IDEs. It’s a piece of cake if I just use Eclipse for compiling my code because the IDE allows me to configure multiple versions of Java runtime. Unfortunately (or fortunately), I have to use the command line/shell to build my code. So, it is important that I have the right version of JDK present in the PATH and other related environment variables (such as JAVA_HOME).
Manually modifying the environment variables every time I want to switch between JDKs, isn’t a happy task. But, thanks to Windows Powershell, I’m able to write a script that can do the heavy lifting for me.
Basically, what you want to achieve is to set the PATH variable to add the Java bin folder and set the JAVA_HOME environment variable and then launch the correct Eclipse IDE. And, I want to do this with a single command. Let’s do it.
Open a Windows Powershell.
I prefer writing custom Windows scripts in my profile file so that it is available to run whenever I open the shell. To edit the profile, run this command: notepad.exe $profile - the $profile is a special variable that points to your profile file.
Write the below script in the profile file and save it.
function myIDE{ $env:Path = “C:vraajavajdk7bin;” $env:JAVA_HOME = “C:vraajavajdk7” C:vraaideeclipseeclipse set-location C:vraaworkspacemyproject play }
function officeIDE{
$env:Path = "C:vraajavajdk6bin;"
$env:JAVA_HOME = "C:vraajavajdk6"
C:officeeclipseeclipse
}
Close and restart the Powershell.
Now you can issue the command myIDE which will set the proper PATH and environment variables and then launch the Eclipse IDE.
As you can see, there are two functions with different configurations. Just call the function name that you want to launch from the Powershell command line (myIDE).
If any issue please put a comment below!
I have a C++ application that uses JNI to create a Java virtual machine. I have the latest version of Java installed - JDK 8-144. If I run "java -version" it comes up and tells me that Java Hotspot 8 is the JVM. I have JAVA_HOME defined in the .profile file, and it works -- JAVA_HOME points to the JDK8-144 folder.
My application is including jni.h from the latest JDK. Our code creates a JVM with the JNI_VERSION_1_8 version flag. On Windows, this compiles and works perfectly.
The problem is that when we compile this code on the Mac, and use the JVM Framework for linking, we end up with an executable that, when run, says that there's "No Java Runtime present" and a dialog comes up that says we need to install the Version 6 JVM. This happens if we run the application from either the Bash terminal or from XCode.
I've researched this and there are other people who have had this identical issue. Everyone seems to suggest that JAVA_HOME needs to be pointing to the correct place, and mine is. Again, java -version works correctly. The whole thing is quite puzzling.
Does anyone know the right way to solve this? I've tried two different Macs with identical results. They are both running the latest version of macOS Sierra.
Thanks!
Dan
I got tired of using Eclipse because it was too "plain" and "old style". So I moved on to a program called Sublime Text 3 and I used a package named "material theme" which made the program look flat and all nice and stuff.
But I wanted to code Java in that but I don't know how to compile stuff without using an IDE and it was not easy to find out but I found out that you could do it through the terminal/cmd of your operating system.
But what I don't understand is how I could actually use the javac command which means Java Compile and I searched and I didn't find out because I couldn't understand anything that was posted on either YouTube or Stack Overflow because I'm new to this whole terminal/cmd Java Compiling thing.
You're almost there buddy!
Just be sure you have the Java Development Kit (JDK) installed in your system. The JDK provides you the command javac -which you need to compile your .java program files.
The javac command is not that friendly as you think it is. You have to let it know where to find the java file you want to compile.
For example you saved Main.java inside C:\Projects\Java you must compile it this way javac C:\Projects\Java\Main.java.
Then you can run your program this way too java C:\Projects\Java\Main
As mentioned here... You can find a more detailed explanation to the official tutorial.
You need to install java JDK which you can find here
Then you need to right-click on my computer->properties->advanced system settings->Environment variables -> Click on path -> New
Then add the path to the bin folder inside the install folder for JDK.
Then in cmd you can compile with
javac *.java
inside the directory of your code
Install JDK
(Make sure your PATH variable is set up)
Use SublimeText's Build System, which allows you to compile open java documents with the Ctrl + B command.
You did not install JDK. You need to install JDK which has javac command.
Installing JDK on MAC instructions
Installing JDK on Windows instructions
After installing JDK(Java Development Kit), you can run the command javac -version on command line and let it show the version that is installed.
I recently noticed that my PATH setting in Windows had incorrect pointers to older versions of Java. What is odd is that the java -version command still works at the command prompt and it gives the version of Java that does correspond to the latest JRE I have installed. This is also the version represented in Control Panel and it is the VM version referenced by Eclipse. What I'm confused about is how does the command prompt know where to find the java executables if the path is set incorrectly?
There is no mystery. There are going to be at least two versions of Java installed on your computer: The one you installed manually, and the one Windows uses (most likely inside the \system32 folder).
when you type the java command at the command prompt, the operating system (windows) resolves this command by: 1) executing the command located on the current directory, 2) finding the command in one of the paths declared in the PATH.
I have multiple Java versions installed on my computer. The one I want to use to develop software is the one I made sure is declared first on my PATH variable.