The Java Module System

Nicolai Parlog

codefx.org / @nipafx

Manning

@ManningBooks

Let’s get started!

there’s much to talk about
⇝ this can only cover parts of the JPMS
⇝ we have to skip some details
slides at slides.codefx.org/jpms

Java Module System Basics

Beyond The Basics

Command Line Options

Migration Challenges

Incremental Modularization

Java Module System Basics

The very short version

Modules

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

These information

are defined in module-info.java
get compiled to module-info.class
end up in JAR root folder

Readability

Modules express dependencies
with requires directives:

module A {
	requires B;
}

module system checks all dependencies
(⇝ reliable configuration)
lets module read its dependencies

Accessibility

Modules export packages
with exports directives

module B {
	exports p;
}

Code in module A can only access Type in module B if:

Type is public
Type is in an exported package
A reads B

(⇝ strong encapsulation)

Jigsaw Advent Calendar

A simple example

Structure

Code

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

Module Structure

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 with module-info.java
$ javac -d classes ${*.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

Java Module System Beyond The Basics

Transitive Dependencies

Optional Dependencies

Services

Qualified Exports

Reflective Access

Java Module System Beyond The Basics

Transitive Dependencies

Optional Dependencies

Services

Qualified Exports

Reflective Access

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

Use to share sensible code between modules.

Java Module System Beyond The Basics

Transitive Dependencies

Optional Dependencies

Services

Qualified Exports

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` directives
}

The same as open packages
but for all of them!

Summary

With open modules or open packages:

code can be made accessible
at run time only
particularly valuable to open
for reflection

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

Command Line Options

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 options to the rescue!
(I often call them "escape hatches".)

Command Line Options

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 java.xml.ws.annotation
    --module advent

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/factory.quotes=advent
    --module advent

Use target ALL-UNNAMED to export
to code on the class path.

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/factory.quotes=advent
    --module advent

Use target ALL-UNNAMED to open
to code on the class path.

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

Patch Modules

Maybe you have a package split…

Mend it with --patch-module:

$ java --module-path mods
    --add-modules java.xml.ws.annotation
#   --patch-module <module>=<JAR>
    --patch-module java.xml.ws.annotation=jsr305.jar
    --module advent

All classes from jsr305.jar are put
into java.xml.ws.annotation.

Patch Module

By putting JAR content into a module A:

split packages can be mended
A needs to read JAR’s dependencies,
which need to export used packages
modules using JAR content need to read A
and A needs to export used packages

Often used with --add-reads and --add-exports.

Summary

Edit module graph with:

--add-modules to add modules
--add-reads to add readability edges
--patch-module to add classes to module
--add-exports to export packages to modules
--add-opens to open packages to modules

The latter two accept ALL-UNNAMED as target.

More at codefx.org:

Five Command Line Options To Hack The JPMS

Migration Challenges

What to look out for
when running on JDK 11

Break Stuff

Some internal changes break existing code!

Just by running on JDK 11
(even without modularizing the application).

Of Modules And JARs

Modularized JDK and legacy JARs have to cooperate.

Two requirements:

for the module system to work,
everything needs to be a module
for compatibility, the class path
and regular JARs have to keep working

The Unnamed Module

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.

Challenges

internal APIs
JEE modules
split packages
runtime images

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/*'
	scaffold-hunter-2.6.3.jar

> 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?

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

JEE Modules

java.activation (javax.activation)
java.corba (CORBA packages)
java.transaction (javax.transaction)
java.xml.bind (javax.xml.bind.*)
java.xml.ws (JAX-WS packages)
java.xml.ws.annotation (javax.annotation)

These were

deprecated for removal in ⑨
removed in ⑪

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.

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?

JDeps reports split packages:

$ jdeps -summary
	-recursive --class-path 'libs/*'
	project.jar

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

What to do?

Your artifacts:

rename one of the packages
merge package into the same artifact
merge the artifacts
place both artifacts on the class path

Otherwise:

upgrade the JDK module with the artifact
--patch-module with the artifact’s content

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

Obsolete

Compact Profiles — jlink
Endorsed Standards Override Mechanism,
Extension Mechanism,
Boot Class Path Override — --upgrade-module-path
JRE selection -version:N — jlink?
Web Start — openwebstart.com
JavaFX — openjfx.io/

But wait, there’s more!

Yes, yes, there’s more:

Java 11 Migration Guide

Background:

And there are new version strings:

goodbye 1.9.0_31, hello 9.0.1

General Advice I

The most relevant for most applications:

internal APIs
split packages
JEE 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 11 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.

Enablers

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"

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 calendar {
	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.

name defined by manifest entry
AUTOMATIC-MODULE-NAME or
derived from JAR 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 calendar {
	requires guava;
}
```

Example

What Goes Where?

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

Class Path

Module 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

Bottom-Up

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

Required for projects with
unmodularized dependencies
(applications).

turn project JARs into modules

Top-Down

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

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

What about published projects with
unmodularized dependencies
(libraries)?

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

Inside-Out

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

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

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

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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)

Incremental Modularization

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)

Migration Challenges

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 Module System

Let’s get started!

Table of contents

Java Module System Basics

The very short version

Modules

Readability

Accessibility

Jigsaw Advent Calendar

A simple example

Structure

Code

Module Structure

Module Structure

Module Creation

Compilation, Packaging, Execution

Java Module System Beyond The Basics

Java Module System Beyond The Basics

Qualified Exports

Known Problem

Qualified Exports

Factory Utilities

Summary

Java Module System Beyond The Basics

Reflective Access

Open Packages

Open Modules

Summary

Command Line Options

Command Line Options

Add Modules

Add Readability Edges

Add Exports

Open Packages

Patch Modules

Patch Module

Summary

Migration Challenges

What to look out forwhen running on JDK 11

Break Stuff

Of Modules And JARs

The Unnamed Module

Challenges

Internal APIs

What to look for?

What else to look for?

What to do?

JEE Modules

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

General Advice II

Incremental Modularization

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?

Modularization Strategies

Bottom-Up

Top-Down

Top-Down

What to look out for
when running on JDK 11