The main goal of this project is to explore basic features of modularity introduced in Java 9:

• exports
• exports X to Y
• requires
• requires transitive
• open module

Reference: Java 9 Modularity

• moduleA, moduleB, moduleC - exploring exports, exports X to Y, requires, requires transitive.

• openModule, ordinaryModule, reflection - exporing open module and correlate modularity with reflection.

Assume that we analyze module-info.java under module X

• exports Y - exporting package Y.
• exports Y to Z (Qualified Export) - export package Y to module Z.
  In general, we should avoid using qualified exports between modules in an application.
• requires Y - indicates a dependency on the module Y.
  Remember that the module Y has to be also added to dependencies of X (directly to module-path or to pom.xml).
• requires transitive Y - requiring automatically all modules required by Y.
• open module - all its types are available for deep reflection (break into private parts of types at run-time) by other modules, while strong encapsulation is still upheld at compile-time. This property holds regardless of whether any packages are exported.
  Deep reflection is used by many libraries. For example, Spring or Hibernate inject values into nonpublic fields of classes.

module moduleA {
    exports moduleA.export;
    
    exports moduleA.exportOnlyToB to moduleB;
}

so package:

• moduleA.export will be exported to other modules
• moduleA.exportOnlyToB will be exported only to moduleB
• moduleA.internal is encapsulated & other modules don't have access to it. For example if you uncomment the line in moduleB.TestB:

  //        InternalA internalA; // no access
  

  project will not compile.

module moduleB {
    requires transitive moduleA;

    exports moduleB.export;
}

<dependency>
    <groupId>basic</groupId>
    <artifactId>moduleA</artifactId>
    <version>1.0-SNAPSHOT</version>
</dependency>

so (we don't repeat keyword defined in previous step):

• moduleB is reporting dependency on moduleA, what is more - every further module that will be requiring moduleB will get access to moduleA also.
• Note that we have to put moduleA also in pom.xml (without pom.xml we should add moduleA to module-path of moduleB otherwise project will not compile).

module moduleC {
    requires moduleB;
}

<dependency>
    <groupId>basic</groupId>
    <artifactId>moduleA</artifactId>
    <version>1.0-SNAPSHOT</version>
    <scope>compile</scope>
</dependency>
<dependency>
    <groupId>basic</groupId>
    <artifactId>moduleB</artifactId>
    <version>1.0-SNAPSHOT</version>
    <scope>compile</scope>
</dependency>

so:

• if we want to use transitive dependency on moduleA (through moduleB) we have to also add it to pom.xml - if not, nothing more should be done.
• note that if you uncomment the line in moduleC.TestC:

  //        ExportAOnlyToB exportAOnlyToB; // no access
  

project will not compile, so package ExportAOnlyToB was correctly exported only to moduleB (moduleB.TestB)

• in moduleC.TestC we use transitive dependency on moduleA:

  ExportA.export();
  

Module with only one simple class Invoker to invoker static methods:

public static void invokeStatic(Object obj, String methodName) throws IllegalAccessException {
    try {
        obj.getClass().getMethod(methodName).invoke(null);
    } catch (InvocationTargetException | NoSuchMethodException e) {
        throw new RuntimeException(e);
    }
}

open module openModule {
    requires reflection;
}

Running openModule.Test class will cause printing to console: "Hello from openModule!" - because of that the modul is marked as open in the module-info.java file.

module ordinaryModule {
    requires reflection;
}

Running ordinaryModule.Test class will cause exception:

java.lang.IllegalAccessException: class Invoker (in module reflection) cannot access class 
Test (in module ordinaryModule) because module ordinaryModule does not export ordinaryModule 
to module reflection

• If we are using maven the file module-info.java should be added to the Sources Root (src/main/java)
• Note that if we put file moduleA.TestA on the same level as module-info.java compilation will succedd, but running main method will result in error:

  Error occurred during initialization of boot layer
  java.lang.module.FindException: Error reading module: path\moduleB\target\classes
  Caused by: java.lang.module.InvalidModuleDescriptorException: TestB.class found in top-level directory (unnamed package not allowed in module)