Package is not visible error

Question

I was trying to use joptsimple package in my project but I get following error:

Error:(4, 20) java: package jdk.internal.joptsimple is not visible
  (package jdk         


        

Answer 1

Well, the message clearly reads (in fact notice the beautiful naming convention by JDK devs "internal" meaning they don't want to expose them) that the API you're trying to access is in a package that is not exported to any other module except these -

exports jdk.internal.joptsimple to jdk.jlink, jdk.jshell

even if you add requires jdk.internal.opt module in your declaration.

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To make use of classes from these packages should be avoided.

Also since even the Open JDK itself integrates JOpt Simple for internal usage by JDK tools. I would suggest you make use of the library jopt-simple for a long-term solution by adding the dependency on your modulepath(java9) or classpath(java8) using -

Maven -

<dependency>
    <groupId>net.sf.jopt-simple</groupId>
    <artifactId>jopt-simple</artifactId>
    <version>6.0-alpha-2</version>
</dependency>

Gradle:-

compile 'net.sf.jopt-simple:jopt-simple:6.0-alpha-2'

After the inclusion of which the package name in use in your classes shall simply be joptsimple.* instead of jdk.internal.joptsimple.*.

Answer 2

You are apparently using an internal API, which you shouldn't. Such APIs are not standardized - they can differ from JVM to JVM and may change at any time without notice.

That said, if you still want to use it, you can do so by adding the following command line flag at compile and run time (i.e. to javac and java):

--add-exports jdk.internal.opt/jdk.internal.joptsimple=ALL-UNNAMED

If you are creating a module you can replace ALL-UNNAMED by your module name.