The Java 9 Module System

Nicolai Parlog

codefx.org / @nipafx

Riga DevDays

#RigaDevDays

Impedance Mismatch

Where the JVM disagrees with us

How do you think about Software?

What is it made of?

How I think about Software

interacting parts
parts have
- names
- dependencies
- capabilities
creates a graph

How the JVM thinks about it

parts are packaged as JARs
to the JVM JARs
- have no names
- dependencies are unexpressed
- have no coherent surface
JVM rolls them into one big ball of mud

Consequences

JAR hell
- unexpressed, transitive dependencies
- shadowing, version conflicts
- complex class loading
slow
unsecure
maintenance nightmare

Project Jigsaw

Teaching the JVM about that graph

Quick History

2005: first discussions about modularity in JDK
2008: Project Jigsaw is created
2011-2014: exploratory phase; ends with JSR 376
2015: prototype is released
2017: Java 9 gets released with module system

Goals

Reliable Configuration
Strong Encapsulation
Scalable Systems (esp. the JDK)
Security, Performance, Maintainability

Non-Goals

Version Selection
Multiple Versions
Package Isolation

Means

Introducing modules, which

have a name
express dependencies
encapsulate internals

Everything else follows from here!

Concepts & Features

Modules, Readability, Accessibility
Implied Readability, Qualified Exports
Modular JARs, Module Path, Module Graph
Services
Unnamed Modules, Automatic Modules
Reflection, Layers
Run-time Images

Java Module System Basics

Getting started...

Modules

Pieces of a puzzle

These are the nodes in our graph.

Definition

Modules

have a unique name
express their dependencies
export specific packages
(and hide the rest)

Implementation

Modules are JARs with a module-info.class
(aka Modular JAR)

gets generated from module-info.java:

module $module_name {
	requires $other_module;
	exports $api_package;
}

this is called a Module Declaration or a
Module Descriptor

Readability

Putting the pieces together

Readability brings edges into our graph.

It is the basis for Reliable Configuration.

Definition

For two modules A and B with

module A {
	requires B;
}

we say

A requires B
A depends on B
A reads B
B is readable by A

Reliable Configuration

Java will only compile/launch when

every dependency is fulfilled
there are no cycles
there is no ambiguity

Consequences

boost for reliability
module system is strict and rigid
(no way to easily alter dependencies)
module name changes are not supported
accidental long cycles will cause problems

Accessibility

Hiding internals

Accessibility governs which types a module can see.

It builds on top of Readability.

It is the basis for Strong Encapsulation.

Definition

A type in one module is only accessible
by code in another module if

the type is public
the package is exported
the second module reads the first

Strong Encapsulation

public is no longer public
even reflection doesn’t work
more fine-grained mechanisms exist:
- for module authors in module declaration
- for module users as command line arguments

Consequences

great boost for maintainability
major reason for community unrest
critical APIs survive until Java 10
(e.g. sun.misc.Unsafe — see JEP 260)
life gets tougher for reflection-based
libraries and frameworks

Jigsaw Advent Calendar

A simple example

Find it on GitHub!

Structure

Code

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

No Module

modularization is not required
JARs continue to work as today!

(Unless you do forbidden things, more on that later.)

we can just put the application
on the class path as before

(Boring...)

No Module

Compilation, Packaging, Execution

# compile
javac -d classes/advent ${*.java}
# package with manifest
jar --create --file jars/advent.jar
	--manifest ${manifest}
	${*.class}
# run
java -jar jars/advent.jar

A single module

Modularization

module advent {
	// java.base is implicitly required
	// requires no other modules
	// exports no API
}

(Still Boring...)

A single module

Compilation, Packaging, Execution

# compile with module-info.java
javac -d classes/advent ${*.java}
# package with module-info.class
# and specify main class
jar --create --file mods/advent.jar
	--main-class advent.Main
	${*.class}
# run by specifying a module path
# and a module to run (by name)
java --module-path mods --module advent

A single module

Readability & Accessibility

Multiple Modules

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;
}

Multiple Modules

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
	${*.class}
# 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?

No!

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;
}

Summary

With A requires transitive B:

A reads B
any module reading A reads B

Applications:

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
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 module absent

Conundrum

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 factories;
	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());
	else
		return Optional.empty();
}

Enhancing A Module

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

module uber.lib {
	requires static com.google.guava;
	requires static org.apache.commons.lang;
	requires static org.apache.commons.io;
	requires static io.vavr;
	requires static com.aol.cyclops;
}

Assumptions:

nobody calls into Guava part without using Guava
no runtime checks necessary

Summary

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

Services

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

Services

Module Declarations:

// consumer
module A {
	uses service.Interface;
}

// provider
module B {
	provides service.Interface
		with some.Type;
}

Services

A never "sees" providers like B
module system picks up all providers
A can get providers from ServiceLoader

ServiceLoader.load(service.Interface.class)

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 =
			new ArrayList<>();
	ServiceLoader
			.load(SurpriseFactory.class)
			.forEach(factories::add);
	Calendar cal = Calendar.create(factories);
	System.out.println(cal.asText());
}

Summary

To decouple API consumers and providers:

consumer uses Service
provider provides Service with Impl

Module system is service locator;
request implementations from ServiceLoader:

ServiceLoader.load(Service.class)

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;
}

Summary

With A exports pack to B:

only B can access types in pack
other modules behave as if pack
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!

Summary

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
java.xml.ws

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.)

Summary

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

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!
so migration has to be possible

Enablers

Migration 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"

Definition

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.

Example

Put all your JARs on the class path.

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

Automatic Modules

From Modules To The Class Path

Definition

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

gets a name based on the file name or a
manifest entry AUTOMATIC-MODULE-NAME
can read all modules
(including the Unnamed Module)
exports all packages

Example

put guava-19.0.jar on the module path
then this works:
```
module advent {
	requires guava;
}
```

Example

What Goes Where?

	Class Path	Module Path
Regular JAR	Unnamed Module	Automatic Module
Modular JAR	Unnamed Module	Named Module

Class Path

Module Path

Regular JAR

Unnamed Module

Automatic Module

Modular JAR

Unnamed Module

Named Module

Migration Strategies

Three strategies emerge:

bottom-up migration
top-down migration
inside-out migration

Bottom-Up Migration

Works best for projects without
unmodularized dependencies
(libraries).

turn project JARs into modules
they still work on the class path
clients can move them to the module path
whenever they want

Top-Down Migration

Required for projects with
unmodularized dependencies
(applications).

turn project JARs into modules

Top-Down Migration

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

Top-Down Migration

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

Inside-Out Migration

What about published projects with
unmodularized dependencies
(libraries)?

top-down mostly works
but there’s an important detail
about automatic module names!

Inside-Out Migration

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.

Inside-Out Migration

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!

Inside-Out Migration

Manifest Entry

thanks to manifest entry projects
can already 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.

Compatibility

What to look out for
when running on JDK 9

Break Stuff

Some internal changes can break existing code!

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

JEP 261 contains a list of risks.

Internal APIs

internal APIs are:
- all in sun.*
- most in com.sun.*
  (unless marked @jdk.Exported)
most if them are encapsulated
critical APIs will survive until Java 10
(e.g. sun.misc.Unsafe — see JEP 260)
six deprecated methods adding/removing
PropertyChangeListener got removed
from LogManager and Pack200

What to look for?

use jdeps (preferably from JDK 9)
- either manually:
  jdeps --jdk-internals {.class/.jar}
- or as part of your build (e.g. with JDeps Mvn)
look for reflection, especially
- Class::forName
- AccessibleObject::setAccessible
recursively check your dependencies!

What to do?

contact library developers
look for alternatives
(in the JDK or other libraries)
consider command line flags
--add-exports, --add-opens, or
--permit-illegal-access

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

Examples

some libraries split java.xml.*, e.g. xml-apis
some JBoss modules split, e.g.,
java.transaction, java.xml.ws
jsr305 splits javax.annotation

What to look for?

search your code and dependencies
for java(x) packages (jdeps can help)
no tool support

What to do?

is the split on purpose / necessary?
find other ways to solve the problem
upgradeable modules
to replace run-time modules
command line --patch-module
to add individual classes

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
new URL schema for run-time image content

What to look for?

does the code rummage around
in the JDK / JRE folder?
search for casts to URLClassLoader
are URLs to JDK classes / resources handcrafted?

Obsolete

Endorsed Standards Override Mechanism
Extension Mechanism
Boot Class Path Override

But wait, there’s more!

yes, yes, there’s more
you should really check JEP 261
and JEP 223 (new version strings)
- goodbye 1.9.0_31
- hello 9.1.4

General Advice I

The most relevant for most applications:

internal APIs
split packages

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
get an EA-build and try it!

About Nicolai Parlog

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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

Jigsaw OSGi

	Jigsaw	OSGi
Versioning	not at all	packages and modules
Run-time Behavior	mostly static	dynamic
Services	declarative via `ServiceLoader`	declarative or programmatically; more flexible
Class Loaders	operates below	one per bundle

Versioning

not at all

packages and modules

Run-time Behavior

mostly static

dynamic

Services

declarative via ServiceLoader

declarative or programmatically;
more flexible

Class Loaders

operates below

one per bundle