Modular Programming in Java 9 - Build large scale applications using Java modularity and Project Jigsaw

Last updated May 5, 2024 Published Jun 10, 2023

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Discover the comprehensive guide to constructing expansive applications with Java modularity and Project Jigsaw and learn the steps in migrating to Java 9 modules by initiating code segmentation.

Chapter 1 - Introduction to Modules

Objectives

  • pain points to maintain large code bases
  • discuss modularity before Java 9
  • The project jigsaw
  • JPMS - Java Platform Module System

Content

  • modular programming is a generally favored software design approach
    • divide and conquer approach
  • Achieving encapsulation and well-defined interfaces
  • modules give the ability to hide internal implementation
  • Java never had native support for modules
  • Java 9 introduces a new concept of modules
  • Java apps could work with the default package without creating another one
    • Before Java 9 apps could use encapsulation at the package level and class/method level
  • The problem until Java 9 is that one developer can create an internal library but the consumer could reference the internal implementation without any blocker
  • there is no way for Java apps to know if a dependency is loaded until it is required to be used
  • traditional apps are bundled in a single jar
Java module was accepted in JSR 376

Chapter 2 - The first module

  • Creating the first module from scratch
  • IntelliJ creates extra things that the book example does not need
  • The behavior of the application changes once a module is introduced - no more classes can be created without a package

Chapter 3 - Module dependencies

Objectives

  • split code into two or more modules
  • modules interdependencies
  • differences between classpath and initialization in Java 9 + modules

Content

  • creating a module with Intelij is accomplished via the file and then a new menu.
    • some gotchas need to be taken into account
      • first, IntelliJ when creating modules already creates packages inside it with tests and resources
      • the module is created from the root of the project. If the module needs to be inside src there is no option in the menu to do so.
  • compiling two modules will give an error of cannot find symbol, but the compilation works independently.
  • having a class in a module as public does not imply another module can access it
  • modules work with dependencies
    • each module needs to declare which dependencies can be accessed externally
    • and each module needs to declare what module it depends on
  • requires and exports are keywords
  • module version is not possible in JPMS
  • classpath has been a concept for many decades and now is not needed
    • given the module path the compiler and the runtime can now know where all classes are
    • when executing the code the check is not in run time anymore the check happens at the VM initialization

Chapter 4 - JDK with modules

Objectives

  • modular jdk
  • entire Java codebase has been modularized
  • built-in modules

Content

  • JRE has two folders
    • bin
      • executables
    • lib
      • key jar files for java
      • rt.jar contains almost all the classes needed for the runtime
  • having a big rt.jar file means shipping big files
    • sometimes the application does not use all of it
    • java 8 used to use compact
      • compact uses 11bm
  • internal vs public APIs
    • unsafe from SUN was never meant to be used publicly
    • Java is a backwards-compatible platform and it comes with a cost
    • attempt by SUN was made to bring the developer’s attention to not write programs that import sun.* packages
  • Project jigsaw was made to bring modularity to Java
    • internal classes were grouped into different modules
    • java.scripting
    • java.desktop
    • java.transaction
    • there is no need to import classes from java.lang java platform does that automatically
    • java –list-modules to see the available modules in java
      • observable module

Chapter 5 - using APIs from the Java platform

Objectives

  • hands-on creating other modules for:
    • adding logging (logging from java java.logging)
    • reading contacts from XML
      • Dependencies should be a directed acyclic graph - no circular dependencies
    • adding UI support with JavaFx
  • without requiring the modules the app won’t work

Chapter 6 - Module resolution

Objectives

  • dive into nuances of readability of modules, resolution and accessibility

Content

  • readability
    • describes how each module work with each other
    • cyclic dependencies are not allowed
    • module dependency graphs describe the relationships between the modules and the order of loading
  • accessibility

Chapter 10 - migrating to Java 9 modules

Objectives

  • path of migration for legacy code bases

Content

  • The first step is to get Java 9 or greater and compile the code

  • The second step is to start splitting the code to use modules

  • surprisingly enough the code that was shared works with Java 9 even without using modules
    • java has this behavior to support legacy code bases
    • this is also known as unnamed module
  • the unnamed module
    • it gives access to all the classes and packages by default
  • jdeps - Java dependency analysis tool
    • statically examines code to identify usage of internal API classes
  • tricks with module exports for apps without module
    • java provides overrides to support migration
      • if a given code does not have access to a module parameters can be given to the compiler to add this possibility
        • –add-exports
        • –add-opens
        • –permit-illegal-access -> disable all modularity features
    • the unnamed module is referenced by ALL-UNNAMED

Chapter 11 - migration to java 9

Objectives

  • migrate strategy for legacy code
  • Multi release jars
  • module bit by bit an application

Content

  • application in the real world are complex
  • gradually migration is preferred
  • three layers
    • application
    • frameworks
    • java platform

References

Made with 💖 by @MatheusMarabesi