Java Modules (JPMS) Cheat Sheet

Project Jigsaw, The Java Platform Module System (JPMS), Modularity, JSR 277, JSR 376... These are the names of one of the most significant features of Java 9. Modularity reduces the complexity of the system by providing:

Explicit dependencies
Clear interface
Strong encapsulation

Modularity is also arguably one of the terrifying Java features of all times. Despite these numerous benefits, proper modules are still rare species in the Java world.

The purpose of this cheat sheet is to show that modules in Java are friendly to us, developers, and easy to use. The Module Declaration (module-info.java) section explains how to define modules. The Module Path vs Class-Path section will be helpful for people migrating existing non-modular applications to JPMS.

FAQ

Here are some of important questions about support of old-style non-modular jar in Java 9+. It's better than what you think! 🌟

Will my plain old jar (without a module declaration) work?

...and...

Will my modularized jar work with dependencies which are not modularized yet?

YES! and YES! But... if...:ballot_box_with_check:it's Java 9 friendly, e.g., it doesn't use private API or split packages.

Read more:
📄 Java 9 Migration Guide: The Seven Most Common Challenges: https://blog.codefx.org/java/java-9-migration-guide/

In JPMS, you can convert existing non-modular jars into:

Unnamed modules, when passed on classpath;
Or, automatic modules, when passed on module path.
See Module Path vs Class Path.

Module Declaration (`module-info.java`)

A module declaration is a file which defines the dependency graph:

Dependencies: Which other modules this module depends on;
API: which parts of this module other modules can use: at compile time or at runtime.

To create a module declaration:
✔️ Create a file named module-info.java
✔️ By convention, it should be placed at the root source folder, e.g., next to the com or org folders.

Module Name and Minimal Module Declaration

A minimal module declaration defines its name:

module com.mycompany.myproject {
}

💣💥 The module name must be globally unique - the name conflicts are prohibited. A recommended practice is to use a reverse-domain-name pattern

Module's Dependencies

Dependencies are other modules this module needs for its work - it requires these modules.

module com.mycompany.myproject { // the module name should be unique

   // dependency on `com.example.foo.bar` which must be present both at compile time AND runtime
   requires com.example.foo.bar;
   
   // transitive dependency: any module which requires current module will also *implicitly* depend on this dependency
   requires transitive com.kitties.bar;
   
   // optional dependency is mandatory at compile time, but optional at runtime 
   requires static com.owls.foo;
}

Read more:
📄 Optional Dependencies: https://blog.codefx.org/java/module-system-optional-dependencies/
📄 Transitive Dependencies: https://blog.codefx.org/java/implied-readability/

Module's API

In pre-Java 9 world any public class was available everywhere and could be used by any other class. The "internal" packages were not in fact internal. With Java 9, module's contents are not available to the external classes by default; one must explicitly define what the module exposes to outside world so that other modules could use it either in compile time or via reflection. Module's API is now expressly specified in the module's declaration.

		Compile Access		Reflection Access
		What	By Whom	What	By Whom
Export Package	Unqualified	Public and protected types in the package Public and protected members of those types	All Code	Public and protected types in the package Public and protected members of those types	All Code
Export Package	Qualified		Modules from the "to" clause		Modules from the "to" clause
Open Package	Unqualified	NO access	All Code	All types in the package And all their members	All Code
Open Package	Qualified	NO access	Modules from the "to" clause	All types in the package And all their members	Modules from the "to" clause

How to export or open a package

A package can be exported or opened in

😃 Module declaration (module-info.java file). It's a recommended approach when you have control over this file.
😟 Or, as a command line option, if you have no other choice.

		Module Descriptor	Java Command Line Options
Export Package	Unqualified	`exports packageName`	n/a
Export Package	Qualified	`exports packageName to targetModule` ~~ALL-UNNAMED~~ is NOT allowed in the descriptor	`--add-exports sourceModule/packageName=targetModule(,targetModule)` The target-module can be ALL-UNNAMED* to export to all unnamed modules
Open Package	Unqualified	`opens packageName`	n/a
Open Package	Qualified	`opens packageName to targetModule`	`--add-opens sourceModule/packageName=targetModule(,targetModule)*`

Read more:
📄 Java Platform, Standard Edition Tools Reference > Main Tools to Create and Build Applications > java: https://docs.oracle.com/javase/9/tools/java.htm#GUID-3B1CE181-CD30-4178-9602-230B800D4FAE__GUID-AC22D7B8-E3C4-4554-AE49-A62C9421AA7C
📄 Five Command Line Options To Hack The Java 9 Module System: https://blog.codefx.org/java/five-command-line-options-to-hack-the-java-9-module-system/

Exported Packages

The exports directive defines what this module exposes to other modules at runtime and compile time.

module com.mycompany.myproject { // the module name should be unique

   // unqualified export => grants access to ALL other modules
   exports com.mycompany.myproject.foo;
   
   exports com.mycompany.myproject.foo.internal
       // having a "to" clause means that this is a **qualified export** => exports the package only to the friends of the current module - the modules specified in the `to` clause
       to com.example.x;
   exports com.mycompany.myproject.bar.internal to com.example.y, com.example.z;
          
}

Packages Open for Reflection

The opens directive opens the module to the outside modules for the reflection. It's usually useful when using reflection-based frameworks such as Spring or Guice;

✏️ As for the code from inside of the current module, it can still reflectively access all types and members from the module regardless whether they were opened or not

module com.mycompany.myproject { // the module name should be unique
   
   // `opens` the package for reflection to ALL other modules
   // * compile time: no access at all 
   // * runtime: to all types, including private types, and all their members, including private ones
   opens com.mycompany.myproject.ollie;
   
   // a qualified `opens` directive - only modules from the `to` cause can access this package at runtime
   opens com.mycompany.myproject.ollie.internal to org.anothercompany.coolframework;
   
   // Also see a section about the open modules - a module which opens ALL its packages
}

Shortcut: an open module

An open module opens all its packages. To define it, add the open modifier before the module:

open module com.mycompany.myproject {
}

Advanced: Services for DI

Services implemented with use of ServiceLoader existed in Java well before Java 9. Their goal is to do decouple the interface and its implementation(s). Now, in Java 9, we can declare services and their usages in the module-info.java: Service consumer:

module com.mycompany.serviceconsumer {
   // Use an existing service. The implementation can be accessed in code by using the ServiceLoader
   uses com.example.foo.ServiceInterface;
}

Service provider:

module com.mycompany.service {
   // Register a new service implementation for com.example.foo.ServiceInterface
   provides com.example.foo.ServiceInterface 
     with com.example.foo.ServiceImpl;
}

✏️ Service consumer and service provider can also be defined in the same module.

Read more: :page_facing_up: https://dzone.com/articles/java-9-kickoff-project-jigsaw-and-serviceloader-part-2

Advanced: Annotations in Module Declarations

Module declarations can also have annotations, e.g., the @Deprecated annotations:

/**
 * This module was replaced by a new module performing the same job much better!
 * @deprecated use {@link com.mycompany.newcoolmodule} instead.  
 */
@Deprecated
module com.mycompany.oldmodule {}

Advanced: The formal grammar of the module declaration file

OK, open modules, annotations... What else? ☕ A structure of a module declaration may seem too complicated. But it's not! Just see the grammar for module files (taken from section 7.7 of Java 9 Language Specification):

ModuleDeclaration:
  {Annotation} [open] module Identifier{.Identifier}
  { {ModuleDirective} }

ModuleDirective:
  requires {RequiresModifier} ModuleName ;
  exports PackageName [to ModuleName {, ModuleName}] ; 
  opens PackageName [to ModuleName {, ModuleName}] ; 
  uses TypeName ;
  provides TypeName with TypeName {, TypeName} ;

RequiresModifier:
  (one of)
  transitive static
  
ModuleName:
  Identifier{.Identifier}

Module Path vs Class Path

	Command line	Modular JAR (JAR with a module-info.class)	Plain JAR
Module Path	`--module-path modulepath` `-p modulepath`	Explicitly defined module	Automatic module
Classpath	`--class-path classpath` `-classpath classpath` `-cp classpath`	Unnamed module	Unnamed module

You can mix module path and classpath!

Read more:
📄 Java Platform, Standard Edition Tools Reference > Main Tools to Create and Build Applications > java: https://docs.oracle.com/javase/9/tools/java.htm#GUID-3B1CE181-CD30-4178-9602-230B800D4FAE__BABDJJFI

Types of Modules: Named and Unnamed

As you can see from the table above:

Everything on the module path, regardless whether it's a plain jar or a jar with a module-info, becomes a named module.
Similarly, everything on the classpath, regardless whether it's a plain jar or a jar with a module-info, becomes an unnamed module.

In fact, there are three module types:

Named:
- Explicit
- Automatic
Unnamed

Explicit Modules

Explicit modules follow the rules for Dependencies and API defined in its Module Declaration (module-info.java)

Automatic Modules

Automatic modules are plain JARs (no module descriptor) on the module path.

✏️ reads vs requires
"Module1 reads Module2" means that Module1 can access types from Module2's exported packages.
A module that requires another module also reads the other module.

As they don't have a module-info.class, this information will be calculated using the following rules:

Name: Automatic-Module-Name from MANIFEST.MF if it's present. Derived from the JAR name otherwise.
- They can be referenced using this name;
- See http://branchandbound.net/blog/java/2017/12/automatic-module-name/ for details;
- For Eclipse plug-ins, see https://dev.eclipse.org/mhonarc/lists/cross-project-issues-dev/msg14888.html;
Dependencies:
- It requires transitive all other resolved modules;
- Reads the unnamed module;
API:
- Exports all its packages;
- The module can be referenced in other modules' module-info.java using its calculated name.

Unnamed Module

Everything on the classpath becomes an unnamed module.

~~Name~~: No name;
Dependencies:
- Reads all other modules
- Can read or open otherALL-UNNAMED in the command line
API:
- Because it does not have a name, it cannot be referenced in module-info.java.
- Readable only by automatic modules

Resources

📄 Java Language Specification. Chapter 7. Packages and Modules. https://docs.oracle.com/javase/specs/jls/se9/html/jls-7.html
📚💲 The Java Module System by Nicolai Parlog : https://www.manning.com/books/the-java-module-system
Disclaimer: I was a manuscript reviewer of this book and, thus, got a free copy of it from Manning.

📚💲 Java 9 Modularity by Sander Mak, Paul Bakker: https://javamodularity.com/
📄 Understanding Java 9 Modules: https://www.oracle.com/corporate/features/understanding-java-9-modules.html
📄 Programming with Modularity and Project Jigsaw. A Tutorial Using the Latest Early Access Build https://www.infoq.com/articles/Latest-Project-Jigsaw-Usage-Tutorial
🔈 Sander Mak on the Java Module System: https://www.infoq.com/podcasts/Sander-Mak-module-system-Java9
🎥 Designing for Modularity with Java 9: https://www.youtube.com/watch?v=tamVhtV18dY

Acknowledgments

Special thanks to Vera Zvereva whose feedback was invaluable.
ajaxorg/ace#3628

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Java Modules (JPMS) Cheat Sheet

Table of Contents

FAQ

Will my plain old jar (without a module declaration) work?

Will my modularized jar work with dependencies which are not modularized yet?

Module Declaration (module-info.java)

Module Name and Minimal Module Declaration

Module's Dependencies

Module's API

Compile Access

Reflection Access

What

By Whom

What

By Whom

Export Package

Unqualified

Qualified

Open Package

Unqualified

Qualified

How to export or open a package

Module Descriptor

Java Command Line Options

Export Package

Unqualified

Qualified

Open Package

Unqualified

Qualified

Exported Packages

Packages Open for Reflection

Shortcut: an open module

Advanced: Services for DI

Advanced: Annotations in Module Declarations

Advanced: The formal grammar of the module declaration file

Module Path vs Class Path

Command line

Modular JAR (JAR with a module-info.class)

Plain JAR

Module Path

Classpath

Types of Modules: Named and Unnamed

Explicit Modules

Automatic Modules

Unnamed Module

Resources

Acknowledgments

Module Declaration (`module-info.java`)