Accessibility is the other side of the Java modularity coin. If the readability relationship specifies what modules can read a given module, accessibility indicates what they actually do get when they read it. Not everything in a module is accessible to the other modules that read it. Only the public types in packages that are marked with an exports declaration are.

Thus, for a type in module B to be accessible in module A, the following needs to happen:

• Module A needs to read module B
• Module B needs to export the package that contains the type
• The type itself should be public

Let's look at an example and examine the readability and accessibility relationships. Consider two modules, app and lib. Module app has a requires qualifier for module lib. Module lib exports its package lib.external:

    module app { 
      requires lib; 
    } 
    module lib { 
      exports lib.external; 
    } 

Let's say the lib module has the following structure:

It has two packages--lib.external and lib.internal. Both packages contain one public interface and one package-private implementation.

Let's try to answer the following questions:

1. Does module app read module lib?

This one should be simple. The answer is yes, because of the requires qualifier.

2. Does module lib read module app?

The answer is no. Hopefully equally simple!

3. Is the type LibApi in module lib accessible to module app?

Let's verify the two requirements for accessibility. Is the type public? Yes. Is the type in a package that's exported by the module? Yes. So, the answer is LibApi is accessible to module app.

4. Is the type InternalService in module lib accessible to module app?

No. Because even though the type is public, it belongs to a package that is not exported in the module definition of the lib module. The type InternalImpl is also not accessible to the app module.

5. Is the type LibApiImpl in module lib accessible to module app?

The answer is no, because it fails the requirement--is the type public? Since LibApiImpl is package-private, it is not accessible outside the module, since it's not public. This is true even though the type belongs to an exported package. This scenario is, however, more interesting with a couple of important lessons we can learn from it. Let's look at them in detail.