Java Module System (Project Jigsaw)

Java Module System, introduced in Java 9 as part of Project Jigsaw, is a significant enhancement to the Java platform. It provides a way to modularize Java applications and the JDK itself, improving scalability, maintainability, and security. Below is a comprehensive guide to understanding and using the Java Module System.

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1. What is the Java Module System?

• Definition: A system for creating modular applications by encapsulating code into modules.
• Purpose:
• Strong Encapsulation: Hide internal implementation details.
• Reliable Configuration: Explicitly declare dependencies between modules.
• Scalability: Improve the scalability of large applications and the JDK.

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2. Key Concepts

a. Module

• Definition: A self-contained unit of code and data with a well-defined interface.
• Module Descriptor: Defined in a module-info.java file.

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

b. Module Path

• Definition: A path where the module system looks for modules.
• Usage: Replace the classpath with the module path (--module-path).

c. Module Types

1. Named Modules: Explicitly defined modules with a module-info.java file.
2. Automatic Modules: JARs without a module-info.java file, automatically converted to modules.
3. Unnamed Module: The default module for code not part of any named or automatic module.

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3. Creating a Modular Application

Step 1: Define Modules

• Create a module-info.java file for each module.

  // module-info.java for com.example.mymodule
  module com.example.mymodule {
      requires java.base;
      requires java.sql;
      exports com.example.mymodule.api;
  }

Step 2: Compile Modules

• Use the javac command with the --module-source-path option.

  javac --module-source-path src -d out $(find src -name "*.java")

Step 3: Package Modules

• Use the jar command to package modules.

  jar --create --file=lib/mymodule.jar -C out/com.example.mymodule .

Step 4: Run Modules

• Use the java command with the --module-path and --module options.

  java --module-path lib -m com.example.mymodule/com.example.mymodule.Main

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4. Module Descriptor Directives

a. requires

• Specifies a dependency on another module.

  requires java.sql;

b. exports

• Exports a package to other modules.

  exports com.example.mymodule.api;

c. opens

• Opens a package for reflection (e.g., for frameworks like Hibernate).

  opens com.example.mymodule.internal;

d. uses and provides

• uses: Specifies a service interface.

  uses com.example.mymodule.spi.MyService;

• provides: Specifies a service implementation.

  provides com.example.mymodule.spi.MyService with com.example.mymodule.MyServiceImpl;

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5. Modular JDK

• JDK Modules: The JDK itself is modularized into several modules (e.g., java.base, java.sql).
• Viewing JDK Modules:

  java --list-modules

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6. Migrating to Modules

a. Bottom-Up Migration

• Start by modularizing the lowest-level libraries and work upwards.

b. Top-Down Migration

• Start by modularizing the application and work downwards.

c. Using Automatic Modules

• Convert existing JARs to automatic modules by placing them on the module path.

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7. Best Practices

1. Start Small: Begin with a few modules and gradually modularize the application.
2. Use Strong Encapsulation: Hide internal implementation details by not exporting unnecessary packages.
3. Leverage Services: Use the uses and provides directives for loose coupling.
4. Test Thoroughly: Ensure compatibility and functionality after modularization.

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8. Common Issues and Troubleshooting

• Module Not Found: Ensure the module is on the module path and the module name is correct.
• Package Not Exported: Ensure the package is exported in the module descriptor.
• Reflection Issues: Use opens to allow reflection on internal packages.

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Resources

• Official Documentation: Java Module System
• Books: “Java 9 Modularity” by Sander Mak and Paul Bakker.
• Tutorials: Java Module System Tutorial

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