Java modules were introduced in Java 9 to enhance the modularity of the Java platform. A module in Java is a collection of related packages, classes, and resources that can be packaged and deployed as a single entity. Modules are designed to help developers create more maintainable and scalable applications by improving encapsulation and reducing dependencies.
Modules define a set of exported and non-exported packages, as well as a set of dependencies on other modules. Exported packages are visible to other modules, while non-exported packages are only accessible within the module itself. This enables developers to limit the visibility of their code and reduce the risk of conflicts with other modules.
Modules are declared using the module-info.java file, which is located in the root directory of a module. This file specifies the module name, the packages it exports, and the modules it depends on.
To use a module in Java, you need to add it to the module path, which is a list of directories that contain modules. The module path is separate from the classpath, which is used to locate classes and resources.
Table Of Contents
1. Java Modules Benefits
Using Java modules offers several advantages, including:
- Encapsulation: Modules allow developers to encapsulate their code and limit the visibility of their classes and packages. This can help to reduce the risk of conflicts and make it easier to maintain and update the application over time.
- Improved scalability: By breaking down the application into smaller, more manageable modules, developers can make it easier to scale the application as it grows. New modules can be added, and existing modules can be modified without affecting the rest of the application.
- Stronger security: Modules enable developers to control access to their code and prevent unauthorized access to sensitive information or functionality within the application.
- Simplified dependency management: Modules allow developers to declare their dependencies explicitly, which can help to avoid version conflicts and make it easier to manage the application’s dependencies.
- Improved performance: By reducing the size of the runtime environment, modules can help to improve the startup time and reduce the memory footprint of the application.
- Better readability and maintainability: By organizing the application into smaller, more cohesive modules, developers can make it easier to understand and maintain the codebase over time.
Overall, using Java modules can help developers to create more modular, scalable, and maintainable applications, while also improving security and performance.
2. Java Modules General Idea in 10 Questions
Q: What are Java modules?
A: Java modules are a way of organizing code and encapsulating functionality in a reusable and maintainable way. They were introduced in Java 9 as a new feature to modularize the Java platform.
Q: What are the benefits of using Java modules?
A: The benefits of using Java modules include better code organization and encapsulation, improved security and performance, easier maintainability, and more reliable dependency management.
Q: How do you create a Java module?
A: To create a Java module, you need to define a module descriptor file (module-info.java) in the root directory of your module. This file specifies the module’s name, dependencies, and exported packages.
Q: What is the module descriptor file?
A: The module descriptor file (module-info.java) is a file that defines a Java module’s metadata, including its name, dependencies, and exported packages.
Q: How do you specify dependencies in a module descriptor file?
A: You can specify dependencies in a module descriptor file using the requires keyword, followed by the name of the module that your module depends on.
Q: What is the difference between an automatic module and an explicit module?
A: An automatic module is a module that is created automatically from a JAR file that does not have a module descriptor file. An explicit module is a module that has a module descriptor file.
Q: How do you migrate an existing Java project to use modules?
A: To migrate an existing Java project to use modules, you need to create a module descriptor file for your project, specify your project’s dependencies, and refactor your code to be modular.
Q: What is the main class in a module, and how do you specify it?
A: The main class in a module is the class that contains the main method that is used to launch the application. You can specify the main class in the module descriptor file using the main-class attribute in the module directive.
Q: How do you run a modular application from the command line?
A: To run a modular application from the command line, you need to use the java command with the –module option, followed by the name of the module containing the main class, and the name of the main class.
Q: How do you package a modular application as a JAR file?
A: To package a modular application as a JAR file, you need to use the jar command with the –create –file options, followed by the name of the JAR file, the module descriptor file, and the compiled classes and resources of the module.
3. Java Module Sample Example
Having introduced the main idea of Java Modules let’s create an example of a Java modular application with two modules:
First, create a directory called myapp to hold the modules:
mkdir myapp cd myapp
Next, create a module descriptor file called module-info.java in the myapp directory for module moduleone:
module moduleone {
requires java.base;
exports com.example.moduleone;
}
This module requires the java.base module and exports the package com.example.moduleone.
Create a package called com.example.moduleone in the myapp directory, and create a class called ModuleOne in that package:
package com.example.moduleone;
public class ModuleOne {
public void hello() {
System.out.println("Hello from Module One!");
}
}
Create another module descriptor file called module-info.java in the myapp directory for module moduletwo:
module moduletwo {
requires moduleone;
exports com.example.moduletwo;
}
This module requires moduleone and exports the package com.example.moduletwo.
Create a package called com.example.moduletwo in the myapp directory, and create a class called ModuleTwo in that package:
package com.example.moduletwo;
import com.example.moduleone.ModuleOne;
public class ModuleTwo {
public void greet() {
ModuleOne moduleOne = new ModuleOne();
moduleOne.hello();
System.out.println("Hello from Module Two!");
}
}
This class uses a public class ModuleOne from moduleone and prints a greeting message.
Compile the modules:
javac -d out --module-source-path . $(find . -name "*.java")
This command compiles all .java files in the current directory and its subdirectories and places the compiled classes in the out directory.
Create a JAR file containing the moduleone and moduletwo modules and their dependencies:
jar --create --file myapp.jar --main-class=com.example.moduletwo.ModuleTwo --module-version=1.0 -C out/moduleone . -C out/moduletwo .
This command creates a JAR file called myapp.jar that contains the moduleone and moduletwo modules and their dependencies, and specifies the ModuleTwo class as the main class.
Run the moduletwo module using the java command:
java -p myapp.jar -m moduletwo
This command runs the moduletwo module using the myapp.jar JAR file as the module path, and specifies the moduletwo module as the module to run. The output should be:
Hello from Module One! Hello from Module Two!
4. Conlcusion
In conclusion, Java modules provide a way to encapsulate code into logical units, called modules, which can be easily reused and managed. Modules help to simplify the development process by reducing the complexity of large codebases, improving security, and making code more maintainable.
With Java modules, you can declare dependencies between modules, control which classes and packages are exposed to other modules, and enforce strong encapsulation. This makes it easier to create modular, scalable, and robust applications that are easier to maintain and extend over time.
