7.1.3 Module Dependencies Explained

Module dependencies are a fundamental aspect of Java's modular system introduced in Java 9. They define the relationships between modules, ensuring that each module can access the necessary resources and functionalities from other modules. Understanding module dependencies is crucial for building robust and maintainable Java SE 11 applications.

Key Concepts

1. Module Declaration

A module is declared using a module-info.java file. This file specifies the module's name, its dependencies on other modules, and the packages it exports for use by other modules.

Example

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

2. Requires Clause

The requires clause in the module-info.java file specifies that the current module depends on another module. This ensures that the required module is available at runtime and compile time.

Example

        module com.example.mymodule {
            requires com.example.anothermodule;
        }
    

3. Transitive Dependencies

A transitive dependency means that if module A requires module B, and module B requires module C, then module A also requires module C. This is specified using the requires transitive clause.

Example

        module com.example.anothermodule {
            requires transitive com.example.thirdmodule;
        }
        
        module com.example.mymodule {
            requires com.example.anothermodule;
        }
    

4. Static Dependencies

A static dependency is a compile-time dependency that does not need to be present at runtime. This is specified using the requires static clause.

Example

        module com.example.mymodule {
            requires static com.example.optionalmodule;
        }
    

5. Exports Clause

The exports clause specifies which packages within the module are accessible to other modules. Only the exported packages can be used by other modules.

Example

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

6. Opens Clause

The opens clause is used to allow reflection access to packages. This is particularly useful for frameworks that rely on runtime reflection.

Example

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

7. Uses and Provides Clauses

The uses clause specifies that the module uses a service, and the provides clause specifies that the module provides an implementation of a service.

Example

        module com.example.mymodule {
            uses com.example.service.MyService;
        }
        
        module com.example.serviceprovider {
            provides com.example.service.MyService with com.example.service.MyServiceImpl;
        }
    

Examples and Analogies

Think of module dependencies as the blueprint of a building. Each module is a section of the building, and the dependencies define how these sections connect. For example, if you are building a house, the kitchen module might require the plumbing module, and the plumbing module might require the pipes module. By defining these dependencies, you ensure that all necessary parts are available when building the house.

By mastering module dependencies, you can create well-structured and modular Java SE 11 applications, ensuring that each module can access the resources it needs while maintaining encapsulation and separation of concerns.