Java 9 Module System Introduction

Java 9 Module System Introduction
Dan Stine
Software Architecture, Development and Integration Meetup
Newton, MA
October 19, 2017

About Me
• Software Architect
• Library Author and Maintainer
• Build Process Designer
• Enterprise Dependency Graph Wrangler
dstine at copyright.com
sw at stinemail.com
github.com/dstine
10/20/20172

About You
• Java version
• Java build tool
• Any familiarity with new module system?
10/20/20173

Java 9
• GA on September 21, 2017
– Exactly four weeks ago
• Not immediately enticing for many individual coders
– Compared to Java 8: Streams! Lambdas!!
• Java Platform Module System (JPMS)
– Driving feature of Java 9
– Project Jigsaw initiated in 2008
– Promises to impact system architecture and software evolution
10/20/20175

Why Does Java Need a Module System?
• Multitude of open-source libraries and frameworks
– Covering immense number of use cases
• Organizations develop internal shared libraries
– Consumed by systems managed by other people
• Build tools and artifact repositories to manage all of this
– Easily (?)
10/20/20176

Exposed Internals
10/20/20177
That type is in the
“internal” package;
I’m sure no one will
import it
That class uses the
“Impl” suffix; I’m sure
everyone will code to
the interface instead

API Management Problems
• No mechanism to clearly define a library’s API
• No mechanism to enforce a library’s API
• Library evolution suffers
• www.semver.org
Clause 1:
Software using Semantic Versioning MUST declare a public API.
This API could be declared in the code itself or exist strictly in
documentation. However it is done, it should be precise and
comprehensive.
10/20/20178

Applications
• These issues also apply to applications
• Layering and partitioning are common techniques
– UI  services  repositories  data access
– Bounded contexts for various subdomains
– Client libraries for service consumers
– Shared utilities
• Layering and partitioning = modularization
• How do you ensure “the rules” are followed?
10/20/20179

Collaboration Difficulty
10/20/201710
It’s covered in
the project
wiki
We wrote some
custom static
analysis
Alice reviews
everyone’s
code

Lack of Encapsulation
• Traditional JAR
• All types and packages exported to all consumers
• All dependencies exported to all consumers
• All runtime dependencies exported to compile
classpath of all consumers
10/20/201711

Access levels
• Four traditional levels
10/20/201712
Level Accessible By
Private Just this class
Package-private … and this package
Protected … and any subclasses
Public … and everyone else

Dependency Configuration
• At build time, our tooling uses a non-trivial degree of
sophistication to manage dependencies
• But much of the information known at build time is
discarded
• All JARs are on the runtime classpath; all classes are
available to all others
10/20/201713

Modules
• Java 9 provides a new abstraction – the module
• “A set of packages designed for reuse” – Oracle
• Stronger encapsulation than traditional JAR files
– Finer-grained access control
– At the package and module levels
• Reliable dependency configuration
– Explicit notion of dependencies between modules
10/20/201715

Module Declaration
• New special file: module-info.java
• Module name
– Reverse domain name convention strongly encouraged
– Single global namespace
• Declares dependencies on other modules
• Declares exported packages
module [MODULE_NAME] {
requires [OTHER_MODULE_NAME];
exports [PACKAGE_NAME];
}
10/20/201716

Example: Internal Package
10/20/201717
com.example.lib
Foo
of(String s)
of(Integer i)
com.example.lib.internal
StringToFoo
IntegerToFoo
com.example.app
uses com.example.lib
module lib {
exports com.example.lib
}
module app {
requires lib
}
app cannot compile against

Example: Internal Third-Party Dependency
10/20/201718
com.example.app
module app {
requires lib
}
com.example.lib
Foo
of(String s)
of(Integer i)
StringToFoo
uses Guava
IntegerToFoo
module lib {
requires com.google.common
}
app cannot compile against
com.google.common

Example: Third-Party Dependency in API
10/20/201719
com.example.app
module app {
requires lib
}
com.example.lib
Foo
of(String s)
of(Integer i)
of(UnsignedInteger ui)
StringToFoo
IntegerToFoo
UnsignedIntegerToFoo
module lib {
requires transitive com.google.common
}
app may compile against
com.google.common

Module Path
• Classpath replaced by module path
– Supported by javac (compile time) and java (runtime)
• module-info.java is compiled to module-info.class
• Placed at root of modular JAR
10/20/201720

Module System Guarantees
• All module dependencies are satisfied
• Declared package visibility is enforced
• No split packages
• No cyclic dependencies
10/20/201721

Access Levels
• Two new levels; protected and public effectively redefined
(Qualified exports enables another two variations beyond these)
10/20/201722
Level Accessible
Protected … and any subclasses in this module
Protected (exported) … and any subclasses in other modules
Public … and any classes in this module
Public (exported) … and any classes in other modules

Additional Benefits
• Faster class loading
– Build time: fewer dependencies to load
– Runtime: don’t need to scan every JAR on classpath
• Shorter (re-)compile times
– Recompile only if API of a dependency has changed
– API is now clearly defined (packages and dependencies)
• More responsive IDEs
– Auto-complete, “Open Type” dialog, suggested imports
10/20/201723

JDK Itself is Now Modular
• Monolithic JDK had tangled complex dependency graph
• Refactored (starting in Java 7 timeframe)
• Now officially partitioned into modules
– http://openjdk.java.net/projects/jigsaw/doc/jdk-modularization.html
• User modules must now explicitly declare a dependency
on each required JDK platform module
10/20/201725

JDK Platform Modules
• Base platform module
– Name = java.base
– Packages = java.io, java.lang, java.net, java.util, ….
– Implicit in all module declarations
• Other platform modules
– Names = java.sql, java.logging, java.xml, …
– Must be explicitly declared
• JDK documentation
– Lists all modules with exported packages and dependencies
10/20/201726

Example: JDK Platform Module Dependency
10/20/201727
module app {
requires lib
}
com.example.app
com.example.lib
Foo
of(String s)
of(Integer i)
of(Document doc)
StringToFoo
IntegerToFoo
DocumentToFoo
module lib {
requires transitive java.xml
}

Adopting Modules
• Could wait
– Classpath has not been removed
– Classpath ignores module-info.class
• Could slowly work dependency graph from the bottom up
– Wait for all dependencies to modularize
– Shared company libraries
– All third-party libraries, and their dependencies
10/20/201729

Top Down Adoption
• Start modularizing at the top of the dependency graph
• Take advantage of automatic modules (next slide)
• Reap some benefits immediately
• Gain more as the ecosystem catches up
10/20/201730

Automatic Modules
• Put traditional (non-modular) JARs on the module path
• These are treated as automatic modules
• Receive auto-generated module name
– Based on file name without version
– E.g. commons-lang3-3.1.jar  commons-lang3
– Use this in module-info.java
• Like JARs on the classpath, automatic modules:
– Export all their packages
– “Require” all other modules
10/20/201731

Other Components of the Module System
• Qualified exports
• jdeps command line tool
• Automatic-Module-Name JAR manifest entry
• Illegal access & JDK internals
• Services (java.util.ServiceLoader)
• Modular runtime images (including jlink tool)
• Layers
10/20/201732

Summary
• Module system addresses several pain points in building
and maintaining software systems
– Strong encapsulation via explicit package exports
– Reliable configuration via explicit module dependencies
• Module is “a set of packages designed for reuse”
• Module path replaces classpath
• JDK itself is now composed of platform modules
• Can begin top-down adoption now
10/20/201733

References
• JDK 9 javadoc
– http://download.java.net/java/jdk9/docs/api/overview-
summary.html
• State of the Module System
– http://openjdk.java.net/projects/jigsaw/spec/sotms/
– Slightly outdated; update is promised
• Jigsaw / JPMS / JSR 376
– http://openjdk.java.net/projects/jigsaw/spec/
– Includes JLS and JVMS diffs
10/20/201734

Qualified Exports
• Share (some of) API with a known set of other modules
• Package export can be qualified with to keyword
10/20/201736

Access Levels
• Eight levels
10/20/201737
Level Accessible By
Protected … and any subclasses in this module
Protected (exported to) … and any subclasses in particular modules
Protected (exported) … and any subclasses in all modules
Public … and any classes in this module
Public (exported to) … and any classes in particular modules
Public (exported) … and any classes in all modules

jdeps
• Command line tool for dependency analysis
• Appeared in Java 8, improved in Java 9
• Report on JDK internal dependencies
• Report on module graph
• Generate module-info.java
• Etc.
10/20/201738

Automatic-Module-Name
• Modularization can take some effort
• Library maintainer can declare the intended module name
immediately
• Add Automatic-Module-Name entry to JAR manifest
• For example, Guava has declared com.google.common
10/20/201739

Illegal Access & JDK Internals
• One goal of JPMS is to properly hide JDK internals
– Allow for faster / better evolution
• But many libraries use the internals
– Who has heard of sun.misc.Unsafe
• Command line flag --illegal-access
– Default is permit to provide more gradual cutover
– Default will eventually be deny
– http://mail.openjdk.java.net/pipermail/jigsaw-dev/2017-
June/012841.html
10/20/201740

Services
• java.util.ServiceLoader introduced in Java 6
• Module A declares use of an interface
• Modules B and C can declare implementations
• Module A does not have to declare dependency on B or C
– Doesn’t know about them
– B and C are managed independently
10/20/201741

Service Example
module java.sql {
...
uses java.sql.Driver;
}
module com.mysql.jdbc {
...
provides java.sql.Driver with com.mysql.jdbc.Driver;
}
module org.postgresql {
...
provides java.sql.Driver with org.postgresql.Driver;
}
10/20/201742

Modular Runtime Images
• Java has always had dynamic (runtime) linking
• Now Java has optional static linking
• New tool jlink creates a custom runtime image
– All required JAR modules
– Only the required JDK modules
• More efficient use of compute resources
– Scale up: large public web service resource requirements
– Scale down: mobile devices
10/20/201743

Layers
• Application servers host applications
• Hosted applications might require different versions of
modules than those used by the server
• Hosted applications might require new or different
services than those used by the server
• Application servers can create new layers in which to
host individual applications
10/20/201744

Formal Syntax (JLS § 7.7)
ModuleDeclaration:
{Annotation} [open] module Identifier {. Identifier}
{ {ModuleDirective} }
ModuleDirective:
requires {RequiresModifier} ModuleName;
exports PackageName [to ModuleName {, ModuleName}] ;
opens PackageName [to ModuleName {, ModuleName}] ;
uses TypeName;
provides TypeName with TypeName {, TypeName} ;
RequiresModifier:
(one of)
transitive static
10/20/201745

Java 9 Module System Introduction

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