Making the Case for Modules – Java Module System

19.1 Making the Case for Modules

We begin by briefly examining the issues that modules address and the benefits they provide.

Stronger Encapsulation

Modules in Java provide a new level of encapsulation above packages. Whereas a package encapsulates classes and interfaces, a module encapsulates a set of packages and any other resources required by the module. An application is thus composed of reusable and related modules, where each module groups a set of packages and each package groups a set of classes and interfaces.

Fine-Grained Visibility of Public Types

The need for modules in Java arose from many concerns. For programming in the large, the concept of packages has many shortcomings, particularly in providing more fine-grained visibility of public types which are accessible in all packages, with no possibility of curtailing the accessibility of such a public type. A module can provide stronger encapsulation with a well-defined public API that other modules can use, and packages that are meant to be internal to a module are now guaranteed to be inaccessible to other modules.

Reusability, Maintainability, and Optimization

Applying the principles of encapsulation at each encapsulation level increases the reusability and aids the maintainability of the code, resulting in modules that can be optimized independently.

Reliable Configuration

Prior to modules, the Java language did not support any notion of dependencies between artifacts that comprise an application. Ad hoc use of JARs (Java Archives) proved inadequate for such relationships, as the compiler and the JVM saw the JAR files merely as containers of types and resources.

Now there is better encapsulation of modules in JARs, with one module in each JAR. The Java Module System explicitly utilizes the dependencies between modules to reason and maintain these dependencies both at compile time and at runtime, resulting in reliable application configurations. Before an application is launched, all modules needed by the application are resolved in order to avoid problems relating to missing modules or types during execution.