Modularity of The Java Platform Javaday (http://javaday.org.ua/)

Modularity of the Java Platform
Martin Toshev

Who am I
Software engineer @ EPAM Bulgaria
BG JUG governance board member (http://jug.bg)
OpenJDK contributor

Agenda
Modularity 101
Modularity on top of the platform: OSGi
Modularity of the platform: Jigsaw
OSGi and Jigsaw interoperability: Penrose

Modularity 101
Standard Java libraries are modules - Hibernate,
log4j and any library you can basically think of …
Build systems like Maven provide transparent
management of modules

Modularity 101
• Benefits of modularization:
– smaller modules are typically tested easier than a
monolithic application
– allows for easier evolution of the system - modules
evolve independently

Modularity 101
• Benefits of modularization:
– development of the system can be split easier between
teams/developers
– increased maintainability of separate modules

Modularity 101
The dependency mechanism used by the JDK
introduces a number of problems that modular
systems aim to solve:
–The "JAR hell" problem caused by shortcomings of the
classloading process

Modularity 101
The dependency mechanism used by the JDK
introduces a number of problems that modular
systems aim to solve:
–The lack of dynamicity in managing dependent modules
–The lack of loose coupling between modules

Modularity 101
Module systems aim to solve the mentioned
problems and typically provide:
• module management
• module deployment
• versioning
• dependency management
• module repositories
• configuration management

OSGi (Open Service Gateway iniatiave) provides a
specification for module systems implemented in
Java
It is introduced as JSR 8 and JSR 291 to the Java
platform

Q: So what is an OSGi runtime ?

A: An OSGi runtime (module system) makes use of the Java
classloading mechanism in order to implement a container for
modular units (bundles) and is based on the OSGi spec - a series
of standards by the OSGi Alliance. Many application servers are
implemented using OSGi as a basis - it is also used in systems
from a diversity of areas

A: An OSGi bundle is a just a JAR file that contains source code,
bundle metadata and resources. A bundle may provide various
services and components to the OSGi runtime. An OSGi runtime
allows for bundles to be installed, started, stopped, updated and
uninstalled without requiring a reboot

A: The OSGi Core spec defines a layered architecture that
determines what is supported by the runtime – each layer
defines a particular functionality supported by the runtime and
the bundles
OSGi logical units:
 bundles
 services
 services registry
 life-cycle
 modules
 security
 execution environment
OSGi logical layers:

A: Bundles may export packages for use by other bundles or
import packages exported by other bundles - this dependency
mechanism is referred to as wire protocol and is provided by the
Module layer of OSGi.

A: Bundles may publish services to the runtime and use already
published services from the runtime – this dependency
mechanism is provided by the Service layer of OSGI.

A: The MANIFEST.MF file of the bundle’s JAR file describes the
metadata of the bundle
Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Sample
Bundle-SymbolicName: com.sample
Bundle-Version: 1.0.0.qualifier
Bundle-Activator: ua.org.javaday.seminar.demo.Activator
Bundle-Vendor: javaday.org.ua
Require-Bundle: org.java.jigsaw.core,
org.java.jigsaw.extensions
Bundle-RequiredExecutionEnvironment: JavaSE-1.7
Service-Component: OSGI-INF/service.xml
Import-Package: ua.org.javaday.seminar.jigsaw.shared;version="1.0.0“
Export-Package: ua.org.javaday.seminar.demo.utils

A: The runtime may implement extensions based on the OSGi
Compendium spec that extends the OSGi Core spec. These could
be:  remote services
 log service
 HTTP service
 device access service
 configuration admin
 metatype service
 preferences service
 user admin
 wire admin
 DMT admin service
 IO connector service
 provisioning service
 UPnP device service
 configuration admin
 declarative services
 event admin service
 deployment admin
 XML parser service
 monitoring service
 others

• OSGi continues to evolve …

Live Demo

Modularity of the platform:
Jigsaw
When speaking of modularity we should also
consider the entire runtime (rt.jar) and the JDK core
libraries …
… and built-in support for improved "OSGi-like"
modules in the Java platform

Jigsaw
The JDK is monolithic …

Jigsaw
• JDK 8 compact profiles provide smaller versions …
(javac -profile <profile_name> or
make profiles for an OpenJDK build)
compact 1 compact 2 compact 3

Jigsaw
The aim of project Jigsaw is to provide a module
system for the Java platform
Although deferred to JDK 9 some additional effort
such as Compact Profiles and removed/ deprecated
inter-library dependencies have been introduced in
JDK8 as an intermediate solution

Jigsaw
Modularization of the Java platform is a significant
change that impacts the entire ecosystem - may
even break existing projects

Jigsaw
Draft 3 of Jigsaw requirements states that
applications using only standard APIs must remain
compatible
Draft 3 of Jigsaw requirements also mentions that
some JDK-specific APIs must remain compatible but
no concrete JDK APIs are listed yet

Jigsaw
Q: So what is exactly project Jigsaw ?

Jigsaw
A: Project Jigsaw provides the basis for JSR 376: Java Platform
Module System. Currently work on Jigsaw is being done for:
JEP 200 - define JDK modules (in a modules.xml file in the JDK repo)
JEP 201 - reorganize JDK sources into modules and modify JDK build
JEP 220 - restructure JRE/JDK images to accommodate modules

Jigsaw
A: Structure of JDK sources as defined by JEP 201:
old
src/{share,$OS}/{classes,native}/$PACKAGE/*.{java,c,h,cpp,hpp}
•new
src/$MODULE/{share,$OS}/classes/$PACKAGE/*.java
native/include/*.{h,hpp}
$LIBRARY/*.{c,cpp} conf/*

Jigsaw
A: In the early exploratory phase of project Jigsaw there was a
proof-of-concept implementation with JDK8 early-access builds
(now dropped).

Jigsaw
Q: So what was exactly the exploratory phase of Jigsaw ?
A: Jigsaw POC provided a modularized version of JDK along with
additional tools and language support for creating Jigsaw
modules. JDK8 early-access builds provided two types of JDK:
JDK modules image - all components are preinstalled as
modules
JDK base image + jmod packages - base JDK installation along
with additional Jigsaw modules of the JDK that can be installed
on-demand using the jmod tool

Jigsaw
Q: What is a Jigsaw module ?

Jigsaw
A: A collection of Java classes, native libraries and other
resources along with metadata that provides name and version
of the module and dependencies on other modules

Jigsaw
A: Jigsaw resolves modules during build and installation.
Jigsaw has no dynamics, no module lifecycle. The module system
assumes the existence of a foundational module named
java.base

Jigsaw
Q: What is a Jigsaw module in terms of the exploratory phase ?
A: Module can use ("require") other modules and additionally
specify version or version ranges for the module dependency -
modules are loaded with different module classloaders
module ua.org.javaday.A @ 1.0 {
requires ua.org.javaday.B @ [2.0, 3.0);
}
module ua.org.javaday.A {
requires ua.org.javaday.B @ >= 1.0;
requires ua.org.javaday.C @ < 2.0;
}

Jigsaw
A: Module versions are compared using a similar approach as the
one used for Debian package versions … The intent is to be able
to package a module as an OS-specific package (not applicable
for Windows/Mac OS)

Jigsaw
A: Modules can also "require" optional modules - meaning that
compilation succeeds even if the required module is missing
requires optional jdk-corba@8-ea;
}

Jigsaw
A: Modules can also "require" local modules - meaning that the
local module is a kind of a "mixin" - it is loaded in the same
classloader as the requiring module
requires local ua.org.javaday.B @ [2.0, 3.0);

Jigsaw
A: A module may export packages and classes
requires ua.org.javaday.B @ [2.0, 3.0);
exports ua.org.javaday.A.seminar.Sample;
exports ua.org.javaday.A.seminar.samples.*;
}

Jigsaw
A: Modules can explicitly specify which other modules can
"require" them
exports ua.org.javaday.A.seminar;
permits ua.org.javaday.B;
}

Jigsaw
A: Modules can logically provide other module names (aliases):
provides ua.org.javaday.First @ 2.0;
}

Jigsaw
A: Modules can have a single entry point
the main() method ua.org.javaday.A.Main is called when invoking:
java -m ua.org.javaday.A
permits ua.org.javaday.B;
class ua.org.javaday.A.Main;
}

Jigsaw
A: Modules can define multiple views
the main() method ua.org.javaday.A.Main is called when invoking:
java -m ua.org.javaday.A
exports ua.org.javaday.A.seminar;
view ua.org.javaday.internal.view {
permits ua.org.javaday.B
}
}

Jigsaw
A: A module can declare that it provides a service
provides service ua.org.javaday.A.external.TestService
with ua.org.javaday.A.external.TestServiceImpl;
}

Jigsaw
A: A module can require a service
module ua.org.javaday.B {
requires service ua.org.javaday.A.external.TestService
}

Jigsaw
A: A service can also be required optionally
module ua.org.javaday.B {
requires optional service ua.org.javaday.A.TestService;
}

Jigsaw
Q: So why not adopt OSGi (at least the module layer) for
modularization of the Java platform ?

Jigsaw
A: As chief architect of the Java platform Mark Reinhold states:
The OSGI module layer is not operative at compile time - it
addresses modularity only during packaging, deployment and
execution. It is also strictly build on top of the platform so it
cannot be used to modularize it.

Jigsaw
A: However - according to Peter Kriens (former technical director
at the OSGi Alliance and one of the key drivers behind the OSGi
spec) Jigsaw is not typically needed an will introduce burden to
the platform and modules can be introduced much easier:
The only thing we need to add to the existing system is
versioning information on the package and record this version in
the class file

Live Demo

When speaking of JDK modules we should consider
interoperability with existing module systems such
as OSGi
The purpose of project Penrose is to explore
interoperability between OSGi and Jigsaw

Penrose is still in early stage of development …
Current work on project Penrose is deprecated due
to the dropped proof-of-concept version of Jigsaw …

Penrose goals:
–ensuring OSGi frameworks run unmodified in a Jigsaw-
enabled runtime
–create modules/bundles that have both OSGi and
Jigsaw metadata in them
–Jigsaw metadata can be extended with OSGi concepts
–extend OSGi to read Jigsaw metadata

Penrose goals:
–mapping Jigsaw metadata to OSGi metadata
–resolve Jigsaw modules in an OSGi runtime
–enhance OSGi to use Jigsaw APIs
–more cross delegation between the two systems …

Summary
Q: Project Jigsaw - when ?

Summary
Q: Project Jigsaw - when ?
A: Maybe when the Armagedon comes …
… or maybe in 2016 …

Summary
Q: Projects Jigsaw/Penrose - when ?
A: Unless you …
… discuss:
http://mail.openjdk.java.net/mailman/listinfo/jigsaw-dev
… and code:
http://hg.openjdk.java.net/jigsaw

References
OSGi Alliance
http://www.osgi.org/Main/HomePage
Jigsaw Project
http://openjdk.java.net/projects/jigsaw/
Penrose Project
http://openjdk.java.net/projects/penrose/

References
Modularity - what is it ?
http://www.infoq.com/articles/modular-java-what-is-it/
Java modularity - why ?
http://java.dzone.com/articles/java-modularity-2-why
Java JAR hell problem
http://en.wikipedia.org/wiki/Java_Classloader#JAR_hell

References
Project Jigsaw: Phase Two
http://mreinhold.org/blog/jigsaw-phase-two
Project Jigsaw: Goals & Requirements Draft 3
http://openjdk.java.net/projects/jigsaw/goals-reqs/03
JSR 376: Java Platform Module System
https://www.jcp.org/en/jsr/proposalDetails?id=376

References
Project Jigsaw: Exploratory Phase
http://openjdk.java.net/projects/jigsaw/exploratory-
phase

References
Java Module System Requirements
http://openjdk.java.net/projects/jigsaw/doc/draft-java-
module-system-requirements-12
Project Jigsaw: The Big Picture
http://cr.openjdk.java.net/~mr/jigsaw/notes/jigsaw-big-
picture-01
Java 8 Modules Jigsaw and OSGi
http://www.slideshare.net/mfrancis/java-8-modules-
jigsaw-and-osgi-neil-bartlett

References
Project Jigsaw: Late for the train
http://mreinhold.org/blog/late-for-the-train-qa
Unbearable lightness of Jigsaw
http://blog.osgi.org/2011/05/unbearable-lightness-of-
jigsaw.html
Netbeans discussion on Jigsaw
http://wiki.netbeans.org/Jigsaw

References
Java Modularity - OSGi and Project Jigsaw
http://techdistrict.kirkk.com/2009/06/12/java-
modularity-osgi-and-project-jigsaw/
The Modular Java Platform & Project Jigsaw
http://www.jfokus.se/jfokus14/preso/Jigsaw.pdf
JAX 2013: A Project Jigsaw primer
http://jaxenter.com/a-project-jigsaw-primer-50029.html

References
JavaOne 2013: The Modular Java Platform and Project
Jigsaw
http://parleys.com/play/52549d02e4b0a43ac12124be/a
bout
OpenJDK Penrose JavaOne 2012
http://www.slideshare.net/bosschaert/open-jdk-
penrose-javaone-2012

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