I finished writing a java program and am ready to export it. I made a runnable jar from Eclipse. Running the jar works just fine on my computer, but throws the "Could not find main class" error on any other computer (including my other computer that I write Java on).
Whenever I search around for other people having the problem, it's always the same answer: Check the manifest file. I'm not seeing any problem with mine (Plus, can't imagine why it'd work on my computer but not someone else's)
Manifest:
Manifest-Version: 1.0
Main-Class: my.quick.monster.QuickMonster
And I've also tried:
Manifest-Version: 1.0
Class-Path: .
Main-Class: my.quick.monster.QuickMonster
Both work for me, but not other computers.
Thinking about the things that might be going wrong, here are a few other things to check:
make sure that there are no spurious characters (tabs, spaces) at the end of the lines.
check that the main class is actually in the JAR file with the right name.
on the machine that works, try changing your current directory and seeing if it still works.
check that you are using the same version of Java on each machine. Run java -version to check.
make sure that you are running it as an executable JAR; i.e. as java -jar foo.jar not as java -cp foo.jar.
(One theory is that the JAR is working on the one machine in spite of the manifest; e.g. that it is finding the class via the classpath in your CLASSPATH environment variable or something.)
To summarize for other folks, the OP's problem turned out to be that he had compiled his code with / for Java 7, and was trying to run the JAR on older Java installations. That wasn't working because of the classfile version numbers.
You can compile your code so that it will run on an older version of Java, but you need to use the -target option when compiling, and you ought to use the -bootclasspath option to compile against an rt.jar from the oldest Java version. A typical IDE will simplify this by allowing you to specify the target build platform, but it is worthwhile understanding the technical details, for cases where you are not using an IDE.
(I'm surprised that the java command didn't mention the classfile version number in the error message ...)
Make sure the MANIFEST.MF file contains a blank line at the end. If the Main-Class definition is on the very last line of the file, some class loaders ignore it.
Do not ever use 'eclipse-jar-worked-fine-on-my-computer'. I use maven shade jar plugin which excellently build a ready to run jar with all the dependencies, specified main class, etc.
EDIT:
What is the wrong with eclipse-builded-jar is that you won't been able to build it w/o elcipse. Maven is the common tool widely used to build packages of any kind. It's automated, and means that it can be used in CI environmet, etc. And the goal of a good developer is to write code so that it can be easily moved to CI.
However, if it's not a regular task, assuming to make just once/twice, theen, maybe, 'eclipse' solutio has also some benefits. But, I answered keeping in mind some cases of my past when people build packages in GUI just because they didn't manage to do it in maven.
So, I hope there is enough arguments for maven vs eclipse, so please stop downvote :D
Related
after a week I am no longer able to call main-classes of jar-files without adding the main-class to the manifest.mf. I don't know what I've broken so far.
I am trying the following:
compiling a jar (for hadoop for example) and calling it with
yarn -jar PathToMyJarFile ClassWithAMain Param1 Param2..
also I could start it with java instead of yarn, that doesnt matter.
My problem is:
The jars I compiled last week now throw an error
no main manifest found in PathToMyJarFile (or whatever the translation from the German Hauptmanifestattribut would be)
I explicit called the Class with main function and setting up a certain class-file is not a solution for my problem, because I also use jar files with more then one class containing a main.
Even a quite simple helloWorld program with just a System.out.println and only one class, I cannot call with
java -jar PathToJarFile ClassName.
Any suggestions?
I tried to do this again with deactivated antivir and firewall with the same result. I also reinstalled my jdk and jre (same version as before).
JavaVersion: 1.8.0_51 and didn't changed since quite a long time
OS: Windows 7U64
Im terribly sorry for that question, I just had to use
yarn jar jarfile class
instead of
yarn -jar jarfile class
incredible stupid of mine, but I mixed calling a jar with java for calling it with hadoop...
Can be closed.
So basically I made a terminal-dependent Java program and want to start handing it out, but I can't seem to find a way to do that without giving them my code. When making a .jar file it only works on Windows.
When trying to run on Linux (CentOS) I get
Exception in thread "main" java.lang.UnsupportedClassVersionError:
and
Could not find the main class: MyClass. Program will exit.
I'm using the command java -jar MyJar.jar to run them on both. I am using JavaMail in this program so perhaps that could be an issue. There is only one class and it has a main in it.
If anyone even has any alternatives to using a jar file to deliver the program where the user can't read the code that'd be great too. Thanks!
The JDK that you've actually used to compile and build your Jar is more recent than compared to the one installed on your client's Linux machine. That's when JVM throws an
Exception in thread "main" java.lang.UnsupportedClassVersionError:
Ask your client to report his Java version by running the following on console
java -version
Then if you're using Eclipse set the target runtime of your project accordingly. If you're compiling your Java source files on your own use
javac -target 1.4 source/MyClass.java
To me it sounds like the problem is that you compiled the code on an incompatible version that you're running the code on. For example, you compiled it with JDK 1.7 but the CentOS OS has only the java 1.5 version installed. Here is some more detailed information on the matter.
As for the title of your question, no, you don't have to give them the java source for them to run it. So something else is going wrong. Most likely the answer I gave.
Regarding question title:
Obfuscate your code. Dont print your name directly, use a generator method.
http://en.wikipedia.org/wiki/Obfuscation_(software)
http://www.yworks.com/en/products_yguard_about.htm
http://www.allatori.com/
http://www.zelix.com/
http://www.sable.mcgill.ca/JBCO/
In addition to the version issue others have described, and the fact that you don't need to distribute the .java files, you'll need to set up a Class-Path entry in the manifest file of your application jar file to reference the JavaMail jar file, and you'll have to deliver your application jar file and the JavaMail jar file to users. That can be tricky to get right. You might want to look at One-JAR to solve that problem.
first off let me start by saying I am completely new to Java, but to give you an idea of how new; I started reading lots of books, examples and so forth and began programming Java using Eclipse about 2 months ago. However, I found a really cool bit of advise about using notepad and the terminal to program instead. Kinda crazy for a newbie to go the hard route, but I love a challenge and I'm serious about learning.
So, In Eclipse I had a really good grasp of how to import, add jars compile etc. When I started using pico and using the terminal (I'm running ubuntu) to compile all went really well, until I wanted to use packages. I've spent two days pulling my hair out because no matter what I do I can't figure it out.
I'm trying to use the acm.jar (which I have many times in Eclipse) however I'm completely lost on how to use it when compiling from the javac in terminal.
So what I'm asking for, is for someone to explain the process getting my jar file to work.
All I'm using to create my java programs is the pico (or notepad) and the javac in the terminal.
To compile and run a java class using external libraries, you have to add that library to the classpath. The classpath is the set of places where the java compiler and the JVM look to find any external libraries/classes that it needs during the process of compiling/executing.
Setting the classpath can be done in 2 ways:
Set an environment variable called CLASSPATH
Set it when your run javac/java
Setting the classpath when running javac/java is done like this:
javac -cp path/to/jar1:path/to/jar2:path/to/jar3:path/to/dirContainingClasses
yourMainClass.java
To run:
java -cp path/to/jar1:path/to/jar2:path/to/jar3:path/to/dirContainingClasses
yourMainClass
: is used as a separator on Linux, for windows use ;
Assuming your source files are in src. Assuming you want your compiled classes to be in classes. Assuming your source files reference classes that are in lib/acm.jar:
javac -cp classes:lib/acm.jar -d classes src/com/foo/bar/MyClass.java
will compile the class com.foo.bar.MyClass and put the generated MyClass.class file in classes/com/foo/bar.
You need the acm.jar file in the classpath. That's what the -cp lib/acm.jar option does. You also need classes in the classpath, because MyClass probably references other classes that you have already compiled and that are in your classes directory.
To run your class, it has to be in the classpath, and acm.jar as well:
java -cp classes:lib/acm.jar com.foo.bar.MyClass
As you see, the classpath contains jar files, and directories containing the folder hierarchy which matches the package hierarchy.
I wouldn't use javac from the command line directly, though. Try using a real build tool, that will build all your classes at once, like Gradle or Ant. Maven is also very popular, but I hate it with passion.
I can run my software without any problems in my IDE, but when I try to compile it to a jar, and try to run it, it says that the main class can't be found. I succeeded to make it work perfectly several times in the past, but I found this problem few time ago, so I can't really know when it went wrong.
The jar has a META-INF, all properly set up, like in my older versions, and the main class is there. Other peoples can't compile too, so it's not just me. The code is on Github, so that could be a tool to help me solve this.
But I want to know, just by reading this, is there some common mistakes that could cause this, or this is uncommon?
EDIT:
I'm using IntelliJ IDEA
and I will check if the versions are the same.
EDIT2:
They do have the same version, and it's exporting an executable jar with the correct main class defined, as I have beeing doing for all the previous versions.
Check that the MANIFEST.MF is present in the META-INF folder of the jar, and make sure that the line :
Main-Class: yourMainClass
is present inside the file, then make sure that you start the jar with this command line :
java -jar yourjar.jar
Have you tried to launch the jar file specifying the main class ? For example :
java yourjar.jar yourMainClass
Does it work in this way ?
I am new to Java (and Eclipse) but I have used .NET (and Visual Studio) a fair amount. I also know about compiling C/C++ code and things like that. I know that at the end I get either an EXE or a nice binary file that can be run from the command line.
I have been making a Java utility that uses some external libraries. I need to compile this into an executable that I can run from the command line on a unix machine, but I cannot find any way to do this.
I can build and run/debug in Eclipse, but that is no use to me as this code will be run on a webserver. I just need all the dependancies compiled in to one file but after hours of searching on Google, the best thing I could find was the Fat-JAR plugin for Eclipse and after using that I just get the following error when I try to run the file:
Exception in thread "main" java.lang.NoClassDefFoundError: Network/jar
This is really confusing me and as it is such an essential thing to be able to do I am sure I must be missing something blindingly obvious, but as I said, after hours of searching I have gotten nowhere.
I really appreciate any help you can give me. Thanks.
If you build your java app using Maven (which can be done with every major IDE), then you can use the maven Shade Plugin to build a shaded jar file; this is a jar file with all of its dependencies included.
A shaded jar can be run from the command line like this:
java -jar myjar.jar command line options
You're doing something standard and you're using eclipse. This means, in your case, Maven is your friend. Download and install the M2Eclipse plug-in. Maven is best at managing dependencies. So, creating a jar with dependencies included will be very, very straight forward. There are thousands of examples on the web and in StackOverflow. If you have problems setting it up, comment on this and I can point you in the right direction.
Sounds like your class path on the server needs to be modified to pick up the jar file containing the Network class. How are you executing your program? What path(s) are you putting in the -cp option?
If you are not sure how to find out the contents inside a jar file, run jar tf , this will list the packaged classes. Validate that one of the jars in your CLASSPATH has that class it says missing.
Give us more details and we can help solve it.
I think I should first explain some basics. A Java class can be run as an application if it has a public static void main(String[] args) method. If it has this method, you can run it from command line as:
java my.package.MyClass <attributes>
Before launching your app, you need to make sure that all required .jar files (and the root of your own class folders, if you did not make a jar from your classes) are in the CLASSPATH environment variable.
These are the absolute basics. When you are building a more complex app, or an app for distribution, you'll probably use maven or ant or some other tool that makes your life easier.