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SanderMak · 2017-03-26 · Original thread
Author here. Java 9 adds a new module system where module descriptors are introduced to explicitly demarcate the public API of a module, and to express its dependencies on other modules. Example of a module descriptor:

  module mymodule {
    exports mymodule.pkga;
    exports mymodule.pkgb;

    requires someothermodule;
  }
What happens is that every package except the ones exported are accessible to other modules. Non-exported packages are encapsulated, not even reflection can break through that barrier. The requires statements are used by the Java compiler and runtime to verify the current configuration of modules resolves correctly.

Obviously there's lots of more detail to go into. Of course I recommend you check out my upcoming book (early release available) for that: http://shop.oreilly.com/product/0636920049494.do

In short, Java makes a great step forward wrt. modularity. When regular JARs transition to modular JARs (adding a module descriptor), many more checks and balances are in place than are currently possible with the classpath.

SanderMak · 2017-02-01 · Original thread
I'm biased (currently authoring http://shop.oreilly.com/product/0636920049494.do), but I believe categorising the new module system as just something organisational is a bit too dismissive.

The module system brings two essential features: reliable configuration and strong encapsulation. Both are a boon for modular development and I believe all but trivially small codebases can benefit.

Reliable configuration means every module explicitly describes its dependencies on other modules. This is consistently available throughout all phases: compilation, run-time and even a new phase, link-time (for more information about linking see http://openjdk.java.net/jeps/282). No more classpath hell, and the explicit dependency information allows for new use-cases like linking.

Strong encapsulation brings new tools to truly modularise your application. Combine it with the services mechanism for maximum benefit (see for an example http://branchandbound.net/blog/java/2015/10/osgi-to-jigsaw/). As a side-effect, the platform becomes more secure by virtue of strong encapsulation of 'dangerous' platform internal classes.

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