What are Automatic Modules in Java and How Do They Work?

Understanding Automatic Modules

What is an Automatic Module?

In Java 9 and onwards, automatic modules are created for JAR files that lack module descriptors. These modules offer a way to incorporate non-modular dependencies into modular applications.

How are Automatic Modules Named?

The module name is derived from the Automatic-Module-Name header in the manifest or, if absent, from the JAR file name.

Automatic Module Readability

Unlike explicit modules, automatic modules read all other modules, including the unnamed module containing class path artifacts. Automatic modules also grant readability to all other automatic modules.

Exports and Opens in Automatic Modules

In the absence of package declarations, automatic modules export and open all packages for API access and deep reflection.

Additional Features of Automatic Modules

• Provide services declared in META-INF/services.
• Allow Main-Class launch using the java command.

Purpose of Automatic Modules

Automatic modules bridge the gap between modular JARs and non-modular dependencies. By requiring automatic modules, modular JARs can depend on plain JARs that are placed on the module path.

Implicitly readable by the unnamed module, automatic modules serve as a bridge between the module graph and the class path. This allows indirect dependencies of modular JARs to remain on the class path, while direct dependencies are drawn onto the bridge as automatic modules.

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