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ellen
Asked: 2018-04-19 20:53:15 +0800 CST

Java from terminal: "Unable to access jarfile Example.jar"

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I'm a newbie when it comes to using Java on linux. I cloned a repository I'm interested in working on and I can't seem to run the code from my machine. The command I'm trying is:

java -Djava.library.path=/home/myname/path/to/project -jar example.jar

The error I'm getting is:

Error: Unable to access jarfile example.jar

First I thought it was a permissions thing, so I tried running the command with sudo (got the same error), I also saw one example with the filepath in quotes so I tried that, but I also got the same error.

I looked through the project files and there actually isn't a file named "example.jar". Is there a command I'm supposed to run first to generate it?

Thanks in advance for your help!

command-line java

1 Answers

  1. Best Answer

    Yes, to run a jar, it has to exist.

    While creating it, you have to match the directory structure to the package structure.

    For instance, your directories might be:

    /home/ellen/proj/java/foo/example/src/example/Foo.java 
/home/ellen/proj/java/foo/example/src/example/util/MyList.java

    for sources, and for class-files:

    /home/ellen/proj/java/foo/example/classes/example/Foo.class
/home/ellen/proj/java/foo/example/classes/example/util/MyList.class

    (often you find classes in a ../bin/.. instead of ../classes/.. named folder or source and class files are mixed in the same directories)

    corresponding to a package structure:

    package example
package example.util

    then you should move to the base directory of the hierarchy: 

    cd /home/ellen/proj/java/foo/example/classes

    or set it with the parameter -C jar -C /home/ellen/proj/java/foo/example/classes ... or from the home jar -C proj/java/foo/example/classes ... as relative path.

    You create the jar file for example with: 

    jar -cf example.jar *.class

    and inspect it with:

    jar -tf example.jar

    Note that there is a manifest file, automatically generated and useful, as a template, where you can add the information of what your main class is, if you have such, but which is pretty strict about what the syntax is (upper/lower case, line breaks). 

    Manifest-Version: 1.0
Created-By: 9-internal (Oracle Corporation)    
Main-Class: example.Main

    Such a main class entry has to be added, if you want to run it without specifying the main class: 

    java -jar example.jar

    or else you have to use:

    java -cp example.jar example.Main

    In essence, a jar file is a zip archive and can be worked on with these tools.

    A clean separation of src/ and bin/ files is useful, when you have more than a handful of classes and probably a deep packet structure. 

     jar -cf example.jar -C classes .

    will then, starting in the directory classes/ include everything which is often what you want. (example/class, example/util/.class, example/net/*class ...)

     jar --help

    informs you about the syntax.

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