are in unnamed module of loader 'app'

The Concept of Modules in Java

When developers write Java applications, they often divide their code into smaller, reusable components known as modules. These modules can be thought of as self-contained packages that contain code and data related to a particular functionality of the application. Java 9 introduced the concept of modularization, which allows applications to be built around a collection of modules that can be managed, maintained, and reused independently.

Understanding Unnamed Modules

In Java 9, all code that is not explicitly part of a named module is considered to be part of an unnamed module. This is a default module that is automatically created by the Java platform when code from the classpath is loaded. Unnamed modules essentially act as catch-all modules that can load any code that is not part of a named module.

Loader App and Unnamed Modules

In Java, modules are loaded by a class loader, which is responsible for loading the classes and resources necessary to run a module. When a class is loaded, it is associated with a specific module and class loader. In the case of an unnamed module, the code is loaded by the application class loader, which is often referred to as the 'app' loader. This loader is responsible for loading all classes on the classpath, including those in the unnamed module.

Benefits of Using Unnamed Modules

While unnamed modules might seem like an unnecessary complication to developers, they actually offer a number of benefits. For one, they make it easier to migrate existing applications to Java 9 and beyond, since developers can continue to use the classpath to load code without having to make major changes to their codebase. Additionally, unnamed modules provide a convenient way to load code that is not compatible with the new modular structure of Java, such as legacy code that relies heavily on classpath scanning.

Drawbacks of Unnamed Modules

There are, however, some drawbacks to using unnamed modules. For one, they can make it more difficult to track down classpath issues since the loader app is responsible for loading all classes on the classpath, including those in the unnamed module. Additionally, using unnamed modules can make it more difficult to identify dependencies between modules, since classes in the unnamed module are not explicitly tracked by the modular system.

Conclusion

Overall, while unnamed modules might seem like an unnecessary complication to some developers, they play an important role in the Java modularization process. By providing a convenient way to load code from the classpath, unnamed modules make it easier to migrate existing applications to newer versions of Java. At the same time, however, they also pose some challenges, such as making it more difficult to track down classpath issues and identify module dependencies. As Java continues to evolve, it will be interesting to see how the use of unnamed modules evolves and changes over time.