The Java Module System

Getting Started

Let’s get started!

(Please share responsibly)

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


  • 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


first discussions about modularity in JDK


Project Jigsaw is created


exploratory phase; ends with JSR 376


prototype is released


Java 9 gets released with module system


  • Reliable Configuration

  • Strong Encapsulation

  • Scalable Systems (esp. the JDK)

  • Security, Performance, Maintainability


  • Version Selection

  • Multiple Versions

  • Package Isolation


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...


Pieces of a puzzle

These are the nodes in our graph.



  • have a unique name

  • express their dependencies

  • export specific packages
    (and hide the rest)


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

  • gets generated from

    module $module_name {
    	requires $other_module;
    	exports $api_package;
  • this is called a Module Declaration or a
    Module Descriptor


Putting the pieces together

Readability brings edges into our graph.

It is the basis for Reliable Configuration.


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


  • 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


Hiding internals

Accessibility governs which types a module can see.

It builds on top of Readability.

It is the basis for Strong Encapsulation.


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


  • great boost for maintainability

  • major reason for community unrest

  • critical APIs survive longer
    (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!


advent calendar structure


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

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


No Module

Compilation, Packaging, Execution

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

A single module


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

(Still Boring...)

A single module

Compilation, Packaging, Execution

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

A single module

The Module Path

  • used by java, javac, and other tools
    to look up dependencies

  • accepts a list of artifacts or folders

  • intended for modular artifacts

Example use:

# `mods` is a directory with JARs
$ java --module-path mods # ...

A single module

Readability & Accessibility

advent calendar readability accessibility

Multiple Modules


Multiple Modules

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;

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
# launch the application
$ java --module-path mods --module advent

Multiple Modules

The Module Source Path

With multi-module compilation, several
modules can be compiled at once.

  • used by javac to look up source files

  • defines a folder structure

  • uses * as placeholder for module name


--module-source-path "./*/src/main/java"

Remember This?

Benefiting From JDK Modularization

Create a JDK install with just the code you need:

  • know which modules your app uses (⇝ jdeps)

  • create an image with those modules (⇝ jlink)

This is about jlink.

A Minimal JDK Image

Create with jlink:

$ jlink
#   define output folder for the image
    --output jdk-minimal
#   where to find modules? (obsolete in ⑪)
    --module-path $JAVA_HOME/jmods
#   which modules to add (includes dependencies!)
    --add-modules java.base

Try it out:

$ jdk-minimal/bin/java --list-modules
> java.base

Image For A Backend

Say you use JAXP, JDBC, and JUL:

$ jlink
	--output jdk-backend
	--module-path $JAVA_HOME/jmods # obsolete in ⑪
	--add-modules java.xml,java.sql,java.logging

Image For A Backend

$ jdk-backend/bin/java --list-modules
> java.base
> java.logging
> java.sql
> java.transaction.xa
> java.xml

Image Including Your App And Dependencies

To create an image for your app:

  • all JARs need to be modularized!

  • including dependencies

Unless you use Gunnar Morling’s ModiTect,
which creates module descriptors on the fly.

Linking And Launching

Creating the image:

$ jlink
	--output app-image
	--module-path $JAVA_HOME/jmods:mods
	--add-modules advent
#   services are not resolves automatically
	--add-modules factory.surprise,factory.chocolate

Launching the app:

app-image/bin/java --module advent

Creating A Launcher

You can even create a launcher:

$ jlink
    --output app-image
    --module-path $JAVA_HOME/jmods:mods
    --add-modules advent,...
#   --launcher <name>=<module>[/<mainclass>]
    --launcher calendar=advent

Launching the app:


More Features

  • automatic service binding
    (with --bind-services)

  • various optimizations
    (size and launch performance)

  • plugin API (not yet public)

  • cross OS image generation


You can use jlink to:

  • create a runtime image
    with just the right modules

  • create an application image
    including your code

This should make certain kinds of deploys
smaller and easier.

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 11
(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

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 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?

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

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 9.0.1

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 11 and try it!

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>)*
    --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 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/factory.quotes=advent
    --module advent

Use target ALL-UNNAMED to open to all modules.

(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
#   --patch-module <module>=<JAR>
    --module advent

All classes from jsr305.jar are put

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.


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

  • --patch-module to add classes to module

The latter two accept ALL-UNNAMED as target.

More at

Analyzing Dependencies With JDeps

Helping You Modularize!

About JDeps

  • a command line tool

  • analyzes bytecode to
    detect dependencies

  • exists since Java 8

  • really shines with modules

Simplest Invocation

$ jdeps 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
# ... truncated more module dependencies ...
> -> com.beust.jcommander  not found
> ->       sh-2.6.3.jar
> ->        sh-2.6.3.jar
> ->   sh-2.6.3.jar
> ->       sh-2.6.3.jar
> ->               java.base
> -> java.lang             java.base
> -> javax.swing           java.desktop
# ... truncated many more package dependencies ...

Analyzing Dependencies

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

# ... truncated split package warnings ...
# ... truncated some module/JAR dependencies...
> sh-2.6.3.jar -> libs/commons-codec-1.6.jar
> sh-2.6.3.jar -> libs/commons-io-2.4.jar
> sh-2.6.3.jar -> libs/dom4j-1.6.1.jar
> sh-2.6.3.jar -> libs/exp4j-0.1.38.jar
> sh-2.6.3.jar -> libs/guava-18.0.jar
# ... truncated more module/JAR dependencies...
# ... truncated many, many package dependencies ...

Creating Graphs

$ jdeps
	--class-path 'libs/*' -recursive
	--dot-output dots
# sed -i 's/-1.0-SNAPSHOT.jar//g'
# sed -i '/java.base/d'
$ dot -Tpng -O dots/

JDK-Internal APIs

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

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

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 calendar {
    	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 calendar {
    	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.

Java Module System Beyond The Basics

Transitive Dependencies
Optional Dependencies
Qualified Exports
Reflective Access

Java Module System Beyond The Basics

Transitive Dependencies
Optional Dependencies
Qualified Exports
Reflective Access

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) {
	// ...
advent requires transitive problem

⇝ 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;
	// ...
advent requires transitive solution

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 factory.api;
	requires transitive factory.chocolate;
	requires transitive factory.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 API usable without further dependencies

  • aggregator modules

  • splitting, merging, renaming modules

More at

Java Module System Beyond The Basics

Transitive Dependencies
Optional Dependencies
Qualified Exports
Reflective Access

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.

Adapting Code

Use case:

  • a library may be absent from some deployments

  • code is aware and does not call absent modules


  • each SurpriseFactory implementation
    has its own module

  • advent works without any specific factory

Adapting Code

advent requires static

Enhancing A Module

Use case:

  • a project may provide usability functions
    for other libraries

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


  • hypothetical library uber-lib

  • provides usability functions for various libraries

Enhancing A Module

uber lib requires static


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

For advent and the two factories:

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

Adapting Code

Checking whether module is present:

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

Enhancing A Module

For uber-lib:

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

Two kinds of applications:

  • modules with code adapting to absence

  • utility libraries that aren’t called
    without that dependency

More at

Java Module System Beyond The Basics

Transitive Dependencies
Optional Dependencies
Qualified Exports
Reflective Access

Dependency Inversion?

advent dependency inversion

Service Locator Pattern

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

Services in JPMS

In the JPMS:

  • modules declare which services they use

  • modules declare which services they provide

  • ServiceLoader is the registry

  • code can interact with it to load services

Service Declarations

Module declarations:

// consumer
module A {
	uses some.Service;

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

(A and B need access to some.Service)

Loading Services

  • A never "sees" providers like B

  • module system picks up all providers

  • A can get providers from ServiceLoader


Factory Services

module advent {
	requires surprise;
	uses surprise.SurpriseFactory;

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

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

Factory Services

advent services

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:


Java Module System Beyond The Basics

Transitive Dependencies
Optional Dependencies
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;


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
Qualified Exports
Reflective Access

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!


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.

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