7.1.1 Module Declaration Explained

The module declaration in Java is a fundamental aspect of the Java Platform Module System (JPMS) introduced in Java 9. It allows developers to define the structure and dependencies of their applications in a modular way. Understanding module declarations is crucial for creating modular, maintainable, and scalable Java applications.

Key Concepts

1. Module Declaration Syntax

A module declaration is defined in a file named module-info.java located in the root of the module's source directory. The declaration starts with the keyword module followed by the module name and a block containing various module directives.

Example

        module com.example.mymodule {
            requires java.base;
            exports com.example.mymodule.api;
        }
    

2. Module Name

The module name is a unique identifier for the module. It follows the same naming conventions as package names, typically using reverse domain name notation. The module name is used to reference the module in other module declarations and in the module path.

Example

        module com.example.mymodule {
            // Module directives
        }
    

3. Requires Directive

The requires directive is used to declare dependencies on other modules. It specifies that the current module depends on the functionality provided by the named module. The requires directive can also include modifiers like transitive and static.

Example

        module com.example.mymodule {
            requires java.base;
            requires transitive com.example.anothermodule;
        }
    

4. Exports Directive

The exports directive is used to make packages within the module accessible to other modules. Only the exported packages are visible to other modules, ensuring encapsulation and controlled access to the module's API.

Example

        module com.example.mymodule {
            exports com.example.mymodule.api;
        }
    

5. Opens Directive

The opens directive is used to allow reflective access to packages within the module. This is particularly useful for frameworks and tools that rely on reflection, such as dependency injection libraries.

Example

        module com.example.mymodule {
            opens com.example.mymodule.internal;
        }
    

6. Uses and Provides Directives

The uses directive is used to declare a service dependency, while the provides directive is used to declare a service implementation. These directives are part of the ServiceLoader mechanism, which allows for loose coupling between service providers and consumers.

Example

        module com.example.mymodule {
            uses com.example.mymodule.api.MyService;
            provides com.example.mymodule.api.MyService with com.example.mymodule.impl.MyServiceImpl;
        }
    

Examples and Analogies

Think of a module declaration as a blueprint for a building. The module name is the address of the building, the requires directive is the list of materials needed to construct the building, the exports directive is the list of rooms open to the public, and the opens directive is the list of rooms that can be accessed by maintenance personnel. The uses and provides directives are like the building's services, such as electricity and plumbing, that are provided by external contractors but used by the building's occupants.

By mastering module declarations, you can create well-structured, modular Java applications that are easier to maintain, test, and scale. This modular approach is essential for modern Java development and is a key skill for any Java developer aiming for the Oracle Certified Professional Java SE 11 Developer certification.