The Java 9 Module System

Beyond the Basics

Public Service Announcement

  • I assume you know the JPMS basics

  • there’s much to talk about
    ⇝ you pick and choose

  • slides at

Java Module System Basics

The very short version



  • have a unique name

  • express their dependencies
    (readabilityreliable configuration)

  • export specific packages
    and hide the rest
    (accessibilitystrong encapsulation)

Jigsaw Advent Calendar

A simple example

Find it on GitHub!


advent calendar structure


public static void main(String[] args) {
	List<SurpriseFactory> factories = asList(
		new ChocolateFactory(), new QuoteFactory());
	Calendar cal = Calendar.create(factories);

Module Structure


Module Structure

advent calendar module multi
module surprise {
	// requires no other modules
	exports org.codefx.advent.surprise;
module calendar {
	requires surprise;
	exports org.codefx.advent.calendar;
module factories {
	requires surprise;
	exports org.codefx.advent.factories;
module advent {
	requires calendar;
	requires factories;
	requires surprise;

Module Creation

Compilation, Packaging, Execution

# compile all modules at once
$ javac -d classes
    --module-source-path "src"
    --module advent
# package one by one, eventually advent
$ jar --create
    --file mods/advent.jar
    --main-class advent.Main
# launch the application
$ java --module-path mods --module advent

Advanced Java Module System

Going Deeper...

Transitive Dependencies

Not all dependencies are created equal:

  • most are used within a module

  • some are used on the boundary
    between modules

Transitive dependencies are about the latter.

Exposing dependencies

Example in calendar exposing surprise:

public static Calendar create(
		List<SurpriseFactory> factories) {
	// ...

⇝ Module calendar is unusable without surprise!

Try and Error?

How can users of exposing module
determine required dependencies?

Try and error?


Implied Readability

Exposing module can mark dependencies
on exposed modules with
requires transitive:

module A {
	requires transitive B;
  • A reads B as usual

  • modules reading A will read B
    without having to require it

A implies readability of B

Implied Readability

Applied to the advent calendar:

module calendar {
	requires transitive surprise;
	// ...

module factories {
	requires transitive surprise;
	// ...

Further Applications

Implied readability is surprisingly versatile

  • aggregator modules

  • splitting modules up

  • even merging modules

  • renaming modules

Aggregator Modules

Making it easier to consume
calendar, factories, surprise:

module adventcalendar {
	requires transitive calendar;
	requires transitive factories;
	requires transitive surprise;

Splitting Modules

If factories gets split into
api, chocolate, and quotes:

module factories {
	requires transitive factories.api;
	requires transitive factories.chocolate;
	requires transitive factories.quotes;

Merging Modules

If calendar, factories, surprise
are merged into adventcalendar:

module calendar {
	requires transitive adventcalendar;

module factories {
	requires transitive adventcalendar;

module surprise {
	requires transitive adventcalendar;

Careful: Users suddenly depend on a large module!

Renaming Modules

If factories becomes surprisefactories:

module factories {
	requires transitive surprisefactories;


With A requires transitive B:

  • A reads B

  • any module reading A reads B


  • make sure API is usable
    without further dependencies

  • aggregator modules

  • splitting, merging, renaming modules

Optional Dependencies

Not all dependencies are equally required:

  • some are needed for a module to function

  • some can be absent and code adapts

  • some are only needed to enhance
    another module’s features

Optional dependencies are about the latter two.

Use Cases

Adapting code:

  • a library may be absent from some deployments

  • code is aware and does not call absent modules

Enhancing a module:

  • a project may provide usability functions
    for other libraries

  • such code can not be called if library modules
    are absent


With what we know so far:

  • for code to compile against another module
    that module has to be required

  • a required module has to be present
    at launch time

⇝ If a module’s types are used
it has to be present at run time

(Reliable configuration!)

Optional Dependencies

Dependency can be marked requires static:

module A {
	requires static B;
  • at compile time: A requires B as usual

  • at run time:

    • if B is present, A reads B

    • otherwise, app can launch
      but access to B can fail

Adapting Code

Assuming each factory has its own module,
advent may not need both all the time:

module advent {
	requires calendar;
	requires surprise;
	requires static factories.chocolate;
	requires static factories.quote;

Adapting Code

Checking whether module is present:

Optional<SurpriseFactory> createChocolateFactory() {
	if (isModulePresent("factories.chocolate"))
		return Optional.of(new ChocolateFactory());
		return Optional.empty();

Enhancing A Module

Hypothetical library uber-lib provides
usability functions for various libraries.

module uber.lib {
	requires static;
	requires static org.apache.commons.lang;
	requires static;
	requires static io.vavr;
	requires static;


  • nobody calls into Guava part without using Guava

  • no runtime checks necessary


With A requires static B:

  • at compile time: A requires B as usual

  • at runtime: B may be absent

Use to prevent absence of optional dependencies
from preventing application launch:

  • modules with code adapting to absense

  • utility libraries that aren’t called
    without that dependency


Consumers and implementations
of an API should be decoupled.

Service locator pattern:

  • service registry as central arbiter

  • implementors inform registry

  • consumers call registry to get implementations

In Java:

  • ServiceLoader is the registry

  • modules can interact with it


Module Declarations:

// consumer
module A {
	uses some.Service;

// provider
module B {
	provides some.Service
		with some.Type;


  • A never "sees" providers like B

  • module system picks up all providers

  • A can get providers from ServiceLoader


Factory Services

module advent {
	requires calendar;
	uses surprise.SurpriseFactory;

module factory.chocolate {
	requires surprise;
	provides surprise.SurpriseFactory
		with factory.quote.ChocolateFactory;

module factory.quote {
	requires surprise;
	provides surprise.SurpriseFactory
		with factory.quote.QuoteFactory;

Factory Services

public static void main(String[] args) {
	List<SurpriseFactory> factories = ServiceLoader
	Calendar cal = Calendar.create(factories);


To decouple API consumers and providers:

  • consumer uses Service

  • provider provides Service with Impl

Module system is service locator;
request implementations from ServiceLoader:


Qualified Exports

So far, exported packages are visible
to every reading module.

What if a set of modules wants to share code?

Known Problem

Similar to utility classes:

  • if class should be visible outside of package,
    it has to be public ⇝ visible to everybody

  • if package should be visible outside of module,
    it has to be exported ⇝ visible to everybody

Module system fixes the former.
What about the latter?

Qualified Exports

module A {
	exports some.pack to B;
  • B can access some.pack
    as if regularly exported

  • other modules can not access it
    as if not exported at all

Factory Utilities

To ease implementation of SurpriseFactory:

  • create new module factory

  • add class AbstractSurpriseFactory

  • export that package only to
    factory implementation modules

module factory {
	requires transitive surprise;
	exports factory
		to factory.chocolate, factory.quote;


With A exports pack to B:

  • only B can access types in some.pack

  • other modules behave as if some.pack
    is not exported

Use to share sensible code between modules.

Reflective Access

Exporting a package makes it public API:

  • modules can compile code against it

  • clients expect it to be stable

What if a type is only meant
to be accessed via reflection?

(Think Spring, Hibernate, etc.)

Open Packages

module A {
	opens some.pack;
  • at compile time:
    types in some.pack are not accessible

  • at run time:
    all types and members in some.pack
    are accessible

A qualified variant (opens to) exists.

Open Modules

open module A {
	// no more `opens` clauses

The same as open packages
but for all of them!


With open modules or open packages:

  • code can be made accessible
    at compile time only

  • particularly valuable to open
    for reflection

Use to make types available for reflection
without making them public API.

Command Line Arguments

The module system is pretty strict but…​

  • …​ not all modules are well-designed

  • …​ not all use cases were intended

What to do then?

Command Line Arguments to the rescue!
(I often call them "escape hatches".)

Command Line Arguments

All command line flags can be applied
to javac and java!

When used during compilation,
they do not change the resulting
module descriptor!

Add Modules

If a module is not required,
it might not make it into the module graph.

Help it get there with --add-modules:

$ java --module-path mods
#   --add-modules <module>(,<module>)*
    --add-modules factories
    --module advent

Add JavaEE Modules

JavaEE modules are not resolved by default!

  • java.activation

  • java.annotations.common

  • java.corba

  • java.transaction

  • java.xml.bind


They need to be added with --add-modules!

Add Readability Edges

Maybe a module in the graph is not readable
by another but you need it to be.

Add readability edges with --add-reads:

$ java --module-path mods
#   --add-reads <module>=<target>(,<more>)*
    --add-reads advent=factories
    --module advent

Add Exports

A common case:

A module needs types that
the owning module doesn’t export.

Export them with --add-exports:

$ java --module-path mods
#   --add-exports <module>/<package>=<target>
    --add-exports factories/factories.quotes=advent
    --module advent

Use target ALL-UNNAMED to export to all modules.

Open Packages

Another common case:

A module reflects on types from a package that
the owning module doesn’t open.

Open packages with add-opens:

$ java --module-path mods
#   --add-opens <module>/<package>=<target>
    --add-opens factories/factories.quotes=advent
    --module advent

Use target ALL-UNNAMED to open to all modules.

(It is not possible to open an entire module.)


Edit module graph with:

  • --add-modules to add modules

  • --add-reads to add readability edges

  • --add-exports to export packages to modules

  • --add-opens to open packages to modules

The latter two accept ALL-UNNAMED as target.

Migration Challenges

What to look out for
when running on JDK 9

Break Stuff

Some internal changes break existing code!

Just by running on JDK 9
(even without modularizing the application).

Internal APIs

  • internal APIs are:

    • all in sun.*

    • most in com.sun.*
      (unless marked @jdk.Exported)

  • encapsulated at compile time

  • accessible at run time
    for some time

  • critical APIs may survive longer
    (e.g. sun.misc.Unsafe)

What to look for?

JDeps can report internal dependencies:

$ jdeps --jdk-internals
	-recursive --class-path 'libs/*'

> batik-codec.jar -> JDK removed internal API
>     JPEGImageWriter -> JPEGCodec
> guava-18.0.jar -> jdk.unsupported
>     Striped64 -> Unsafe
> scaffold-hunter-2.6.3.jar -> java.desktop
>     SteppedComboBox -> WindowsComboBoxUI

What else to look for?

  • look for reflection, especially

    • Class::forName

    • AccessibleObject::setAccessible

  • recursively check your dependencies!

What to do?

  1. fix your code

  2. contact library developers

  3. look for alternatives
    (in the JDK or other libraries)

  4. consider command line flags
    --add-exports, --add-opens, or

Split Packages

  • packages should have a unique origin

  • no module must read the same package
    from two modules

The implementation is even stricter:

  • no two modules must contain
    the same package (exported or not)

  • split packages on class path
    are inaccessible


  • some libraries split java.xml.*, e.g. xml-apis

  • some JBoss modules split, e.g.,

  • jsr305 splits javax.annotation

What to look for?

JDeps reports split packages:

$ jdeps -summary
	-recursive --class-path 'libs/*'

> split package: javax.annotation
>     [jrt:/,
>         libs/jsr305-3.0.2.jar]

What to do?

Your artifacts:

  1. rename one of the packages

  2. merge package into the same artifact

  3. merge the artifacts

  4. place both artifacts on the class path


  1. upgrade the JDK module with the artifact

  2. --patch-module with the artifact’s content

Java EE Modules

  • java.activation (javax.activation)

  • java.corba (CORBA packages)

  • java.transaction (javax.transaction)

  • java.xml.bind (javax.xml.bind.*)

  • (JAX-WS packages)

  • (javax.annotation)

These are

  • deprecated for removal

  • not resolved by default

What to look for?

JDeps shows dependencies on platform modules:

$ jdeps -summary sh-2.6.3.jar

> sh-2.6.3.jar -> java.base
> sh-2.6.3.jar -> java.datatransfer
> sh-2.6.3.jar -> java.desktop
> sh-2.6.3.jar -> java.logging
> sh-2.6.3.jar -> java.prefs
> sh-2.6.3.jar -> java.sql
> sh-2.6.3.jar -> java.xml

What to do?

  • pick a third-party implementation

  • add Java EE platform modules
    with --add-modules

Run-Time Images

  • new JDK/JRE layout

  • internal JARs are gone (e.g. rt.jar, tools.jar)

  • JARs are now JMODs

  • application class loader is no URLClassLoader
    (no way to append to its class path)

  • new URL schema for run-time image content

What to look for?

  • does the code rummage around
    in the JDK / JRE folder?

  • are URLs to JDK classes / resources handcrafted?

  • search for casts to URLClassLoader


  • Compact Profiles

  • Extension Mechanism

  • Endorsed Standards Override Mechanism

  • Boot Class Path Override

  • JRE version selection with -version:N

But wait, there’s more!

Yes, yes, there’s more:

Java 9 Migration Guide


And there are new version strings:

  • goodbye 1.9.0_31, hello 9.1.4
    (soon 18.3?)

General Advice I

The most relevant for most applications:

  • internal APIs

  • split packages

  • Java EE modules

General Advice II

  • get your code in shape
    (and prevent relapses)

  • check your dependencies and tools

  • if any are suspicious
    (automatically true for IDEs, build tools):

    • make sure they’re alive

    • get them up to date!

    • or look for alternatives

  • download Java 9 and try it!

Incremental Modularization

Moving Into Module Land

Why Is It Even An Option?

  • most module systems are "in or out",

  • but modularized JDK and legacy JARs
    have to cooperate!

  • there is a boundary between
    legacy and modules

Incremental modularization means
moving that boundary.


Incremental modularization is enabled by two features:

  • Unnamed Module(s)

  • Automatic Modules

And the fact that module and class path coexist:

  • modular JARs can be put on either

  • "regular" JARs can be put on either

The Unnamed Module

Why The Class Path "Just Works"


The Unnamed Module
contains all JARs on the class path
(including modular JARs).

  • has no name (surprise!)

  • can read all modules

  • exports all packages

Inside the unnamed module
"the chaos of the class path" lives on.


Put all your JARs on the class path.

modularization unnamed

No Access

  • what if your code was modularized?
    and your dependencies were not?

  • proper modules can not depend on
    "the chaos on the class path"

  • this is not possible:

    module advent {
    	requires unnamed;

No Access

modularization unnamed dependency

Automatic Modules

From Modules To The Class Path


An Automatic Module
is created for each "regular" JAR
on the module path.

  • name defined by manifest entry
    derived from JAR name

  • can read all modules
    (including the Unnamed Module)

  • exports all packages


  • put guava-19.0.jar on the module path

  • then this works:

    module advent {
    	requires guava;


modularization automatic

What Goes Where?

Class PathModule Path

Regular JAR

Unnamed Module

Automatic Module

Modular JAR

Unnamed Module

Explicit Module

Unnamed or named module?
The user decides, not the maintainer!

Modularization Strategies

Three strategies emerge:

  • bottom-up

  • top-down

  • inside-out


Works best for projects without
unmodularized dependencies

  • turn project JARs into modules

  • they still work on the class path

  • clients can move them to the module path
    whenever they want


Required for projects with
unmodularized dependencies

  • turn project JARs into modules


  • modularized dependencies:

    • require direct ones

    • put all on the module path

  • unmodularized dependencies:

    • require direct ones with automatic name

    • put direct ones on the module path

    • put others on the class path


When dependencies get modularized:

  • hopefully the name didn’t change

  • if they are already on the module path,
    nothing changes

  • otherwise move them there

  • check their dependencies


What about published projects with
unmodularized dependencies

  • top-down mostly works

  • but there’s an important detail
    about automatic module names!


Automatic Module Names

  • automatic module name may
    be based on JAR name

  • file names can differ
    across build environments

  • module name can change
    when project gets modularized

⇝ Such automatic module names are unstable.


Impossible Module Requirements

  • dependencies might require the same
    module by different names

  • the module system does not support that

  • there is no way to launch that application!

Do not publish modules
that depend on automatic modules
whose names are based on file names!


Manifest Entry

  • thanks to manifest entry,
    projects can publish their module name

  • assumption is that it won’t change
    when project gets modularized

  • that makes these names stable

⇝ It is ok to publish modules
that depend on automatic modules
whose names are based on manifest entry.

About Nicolai Parlog

37% off with
code fccparlog

Want More?

⇜ Get my book!

You can hire me:

  • training (Java 8/9, JUnit 5)

  • consulting (Java 8/9)

What About OSGi?

Brief comparison
of Jigsaw and OSGi

Jigsaw vs. OSGi

OSGi Bundles:

  • are JARs with a descriptor (MANIFEST.MF)

  • have a name

  • import packages or bundles

  • define public API by exporting packages

Jigsaw vs. OSGi



not at all

packages and modules

Run-time Behavior

mostly static



declarative via ServiceLoader

declarative or programmatically;
more flexible

Class Loaders

operates below

one per bundle

Image Credits


Project Jigsaw

Java Module System

Incremental Modularization

Migration Challenges