The Java 9 Module System In Action

Nicolai Parlog

codefx.org / @nipafx

DEVFEST KA

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 Jigsaw

Disclaimer / Call to arms

this is based on early access builds
some things can still change
time for community feedback
is running out

Goals

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

Non-Goals

Multiple Versions
Version Selection

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

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
command line provides escape hatches

Consequences

great boost for maintainability
also the major reason for community unrest
critical APIs might survive until Java 10
(e.g. sun.misc.Unsafe — see JEP 260)
life might get 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<SurpriseF_> fac = asList(
		new ChocolateF_(), new QuoteF_());
	Calendar cal = Calendar.create(fac);
	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...)

A single module

Modularization

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

(Still Boring...)

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

# First compile/package the other modules
#   ('surprise', 'calendar', 'factories')
#   into folder 'mods'.
# Compile/package 'advent':
javac -p mods -d classes/advent ${*.java}
jar -c --file=mods/advent.jar
	--main-class=org.codefx.advent.Main
	${*.class}
# Launch the application:
java -p mods -m advent

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

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
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

Two strategies emerge:

bottom-up migration
top-down 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

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 that will be 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 flag --add-exports
turn to the Jigsaw mailing lists

Split Packages

packages should have a unique origin
no module must read the same package
from two modules

The current implementation is even stricter:

no two modules must contain
the same package (exported or not)
split packages on the class path
will be 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 (yet?)

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

Find Me

codefx.org
sitepoint.com/java
@nipafx / +NicolaiParlog

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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

Image Credits

Introduction

puzzle-people: Kevin Dooley (CC-BY 2.0)
binary-code: Christiaan Colen (CC-BY-SA 2.0)
graph: chef_ele (CC-BY 2.0)
ball-of-mud-2: Andi Gentsch (CC-BY-SA 2.0)
jar-hell: Wellcome Library, London (CC-BY 4.0)

Project Jigsaw

flag-amsterdam: George Rex (CC-BY-SA 2.0)
puzzle-cubed: David Singleton (CC-BY 2.0)

Java Module System

puzzle-piece-green:
StockMonkeys.com (CC-BY 2.0)
puzzle-pieces-put-together:
Ken Teegardin (CC-BY-SA 2.0)
iceberg:
NOAA’s National Ocean Service (CC-BY 2.0)
advent-calendar: Tina D (CC-BY 2.0)
class and module diagrams:
Nicolai Parlog (CC-BY-NC 4.0)

Migration

keep-out: Brian Smithson (CC-BY 2.0)
garbage-only: Peter Kaminski (CC-BY 2.0)
golden-gate: Nicolas Raymond (CC-BY 2.0)
confusion: Procsilas Moscas (CC-BY 2.0)
module diagrams:
Nicolai Parlog (CC-BY-NC 4.0)

Compatibility

broken-glass:
Eric Schmuttenmaer (CC-BY-SA 2.0)
internals: David French (CC-BY 2.0)
cut: Jinx! (CC-BY-SA 2.0)
cells: Jonathan Lin (CC-BY-SA 2.0)
obsolete: Trevor King (CC-BY 2.0)
sign: Duncan Harris (CC-BY-SA 2.0)

Rest

question-mark: Milos Milosevic (CC-BY 2.0)
bundles: Danumurthi Mahendra (CC-BY 2.0)

The Java 9 Module System In Action

Impedance Mismatch

Where the JVM disagrees with us

How do you think about Software?

What is it made of?

How I think about Software

How the JVM thinks about it

Consequences

Project Jigsaw

Teaching the JVM about that graph

Quick History

Disclaimer / Call to arms

Goals

Non-Goals

Means

Concepts & Features

Java Module System Basics

Getting started...

Modules

Pieces of a puzzle

Definition

Implementation

Readability

Putting the pieces together

Definition

Reliable Configuration

Accessibility

Hiding internals

Definition

Strong Encapsulation

Consequences

Jigsaw Advent Calendar

A simple example

Structure

Code

No Module

A single module

Modularization

A single module

Readability & Accessibility

Multiple Modules

Multiple Modules

Multiple Modules

Compilation, Packaging, Execution

Migration

Moving Into Module Land

Why Is It Even An Option?

Enablers

The Unnamed Module

Why The Class Path "Just Works"

Definition

Example

No Access

No Access

Automatic Modules

From Modules To The Class Path

Definition

Example

Example

What Goes Where?

Migration Strategies

Bottom-Up Migration

Top-Down Migration

Top-Down Migration

Top-Down Migration

Compatibility

What to look out forwhen running on JDK 9

Break Stuff

Internal APIs

What to look for?

What to do?

Split Packages

Examples

What to look for?

What to do?

Run-Time Images

What to look for?

Obsolete

But wait, there’s more!

General Advice I

What to look out for
when running on JDK 9

Brief comparison
of Jigsaw and OSGi