Seminar "Modularity of the Java Platform" of the Bulgarian Java User Group. Topics of the seminar: Modularity 101 Modularity on top of the platform: OSGi Modularity of the platform: Jigsaw OSGi and Jigsaw interoperability: Penrose

1. Modularity of the Java
Platform
Martin Toshev

2. BG JUG mailing list:
https://groups.google.com/forum/#
!forum/bg-jug

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

4. Modularity 101

5. 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

6. 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

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

8. 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

9. 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

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

11. Modularity on top of the
platform: OSGi

12. Modularity on top of the platform:
OSGi
• OSGi (Open Service Gateway initiave) provides a
specification for module systems implemented in
Java
• It is introduced as JSR 8 and JSR 291 to the Java
platform

13. Modularity on top of the platform:
OSGi
Q: So what is an OSGi runtime ?

14. Modularity on top of the platform:
OSGi
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

15. Modularity on top of the platform:
OSGi
Q: So what is an OSGi runtime ?
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

16. Modularity on top of the platform:
OSGi
Q: So what is an OSGi runtime ?
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:

17. Modularity on top of the platform:
OSGi
Q: So what is an OSGi runtime ?
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. 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.

18. Modularity on top of the platform:
OSGi
Q: So what is an OSGi runtime ?
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: sample.Activator
Bundle-Vendor: QIVICON
Require-Bundle: org.eclipse.smarthome.core,
com.qivicon.extensions
Bundle-RequiredExecutionEnvironment: JavaSE-1.7
Service-Component: OSGI-INF/service.xml
Import-Package: com.qivicon.services.hdm;version="3.0.0“
Export-Package: com.sample.utils

19. Modularity on top of the platform:
OSGi
Q: So what is an OSGi runtime ?
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

20. Modularity on top of the platform:
OSGi
Q: What about Maven support for OSGi bundles ?

21. Modularity on top of the platform:
OSGi
Q: What about Maven support for OSGi bundles ?
A: Such a support is provided by the Tycho Maven plug-ins that
provide support for packaging types, target platform definitions,
interoperability with the Maven dependency mechanism and so
on …

22. Modularity on top of the platform:
OSGi
• OSGi continues to evolve …

23. Live Demo
Modularity on top of the platform: OSGi

24. Modularity of the platform:
Jigsaw

25. 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

26. Modularity of the platform:
Jigsaw
• The JDK is monolithic …

27. Modularity of the platform:
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

28. Modularity of the platform:
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

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

30. Modularity of the platform:
Jigsaw
Q: So what is exactly project Jigsaw ?

31. Modularity of the platform:
Jigsaw
Q: So what is exactly project Jigsaw ?
A: Jigsaw will provide the basis for a Java Module System JSR

32. Modularity of the platform:
Jigsaw
Q: So what is exactly project Jigsaw ?
A: Project Jigsaw provides a modularized version of JDK along
with additional tools and language support for creating Jigsaw
modules. Currently early-access builds provide 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

33. Modularity of the platform:
Jigsaw
Q: So what is exactly project Jigsaw ?
A: No jre directory exists anymore in the JDK installation, rt.jar
and tools.jar no longer exist. Modular JDK must be compatible
with existing applications to a greater extend. Legacy
applications are expected to run on a Jigsaw JDK if:
– they don't depend upon the internal structure of the
JDK/JRE
– they use only supported APIs

34. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?

35. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
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

36. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
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

37. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
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 org.bgjug.A @ 1.0 {
requires org.bgjug.B @ [2.0, 3.0);
}
module org.bgjug.A {
requires org.bgjug.B @ >= 1.0;
requires org.bgjug.C @ < 2.0;
}

38. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: Module versions are compared using a similar approach as the
one used for Debian package versions …

39. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: Modules can also "require" optional modules - meaning that
compilation succeeds even if the required module is missing
module org.bgjug.A {
requires optional jdk-corba@8-ea;
}

40. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
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
module org.bgjug.A @ 1.0 {
requires local org.bgjug.B @ [2.0, 3.0);
}

41. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: A module may export packages and classes
module org.bgjug.A @ 1.0 {
requires org.bgjug.B @ [2.0, 3.0);
exports org.bgjug.A.seminar.Sample;
exports org.bgjug.A.seminar.samples.*;
}

42. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: Modules can explicitly specify which other modules can
"require" them
module org.bgjug.A @ 2.0 {
exports org.bgjug.A.seminar;
permits org.bgjug.B;
}

43. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: Modules can logically provide other module names (aliases):
module com.bgjug.A @ 1.0 {
provides com.bgjug.First @ 2.0;
}

44. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: Modules can have a single entry point
the main() method org.bgjug.A.Main is called when invoking:
java -m org.bgjug.A
module org.bgjug.A @ 1.0 {
permits org.bgjug.B;
class org.bgjug.A.Main;
}

45. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: Modules can define multiple views
the main() method org.bgjug.A.Main is called when invoking:
java -m org.bgjug.A
module org.bgjug.A {
exports org.bgjug.A.seminar;
view org.bgjug.internal.view {
permits org.bgjug.B
}
}

46. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: A module can declare that it provides a service
module org.bgjug.A {
provides service org.bgjug.A.external.TestService
with org.bgjug.A.external.TestServiceImpl;
}

47. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: A module can require a service
module org.bgjug.B {
requires service org.bgjug.A.external.TestService
}

48. Modularity of the platform:
Jigsaw
Q: What is a Jigsaw module ?
A: A service can also be required optionally
module org.bgjug.B{
requires optional service org.bgjug.A.TestService;
}

49. Modularity of the platform:
Jigsaw
Q: How are modules compiled ?

50. Modularity of the platform:
Jigsaw
Q: How are modules compiled ?
A: The javac compiler is extended with a ModulePath - the
equivalent of ClassPath but for module dependencies
(-modulepath option)
For javac the -d option (that specifies the output directory):
– outputs to legacy single-module structure if ClassPath is
set
– outputs to multi-module structure if ModulePath is set

51. Modularity of the platform:
Jigsaw
Q: How are modules managed ?

52. Modularity of the platform:
Jigsaw
Q: How are modules managed ?
A: Modules can be added to module libraries. Various tools are
added to support creation of modules, module libraries and
module repositories and packaging of artifacts:
– jmod - for managing modules and module libraries
– jpkg - for packaging modules (uses pack200 compression)

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

54. Modularity of the platform:
Jigsaw
Q: So why not adopt OSGi (at least the module layer) for
modularization of the Java platform ?
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.
For compile-time resolution of OSGi dependencies during
development IDEs (such as Eclipse) and build tooling (such as
Maven) provide a mechanism for specifying a target platform
which is collection of bundles used to provide compile-time
dependencies

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

56. Modularity of the platform:
Jigsaw
Q: What about IDE support for Jigsaw module development ?

57. Modularity of the platform:
Jigsaw
Q: What about IDE support for Jigsaw module development ?
A: It is still an early stage to consider possible IDE support - IDE
vendors outline use cases for Jigsaw usage in order to consider
how to extend their IDEs with Jigsaw support (valid mostly for
NetBeans - Eclipse and IntelliJ are still idle).
However - according to Peter Kriens the structure of a JDK module
(especially the module-info.java file) will increase effort needed to
support modules in the various IDEs and build tools

58. Modularity of the platform:
Jigsaw
Q: What about Maven support for Jigsaw module development ?

59. Modularity of the platform:
Jigsaw
Q: What about Maven support for Jigsaw module development ?
A: Still at a very early stage of discussion - but most probably
Maven will provide build-in support for Jigsaw modules and
module dependency resolution from upstream Jigsaw repositories

60. Modularity of the platform:
Jigsaw
Q: What about Maven support for Jigsaw module development ?
A: According the Peter Kriens:
And not to forget the build tools, they will start having to interpret
the module-info file and link to the appropriate module system to
find their class path. Today, a build tool tells the compiler its class
path, in the future it would first have to compile or interpret the
Java file. This alone will probably kill 90% of the ant scripts because
the class path is used in other places then compiling. Also maven
will have to start to interact with this

61. Live Demo
Modularity of the platform: Jigsaw

62. OSGi and Jigsaw interoperability:
Penrose

63. OSGi and Jigsaw interoperability:
Penrose
• 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

64. OSGi and Jigsaw interoperability:
Penrose
• Penrose is still in early stage of development …

65. OSGi and Jigsaw interoperability:
Penrose
• Penrose goals:
– ensuring OSGi frameworks run unmodified in an
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 module info

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

67. OSGi and Jigsaw interoperability:
Penrose
• Since module-info.java is compiled to module-
info.class this implies that class file must be
parsed in order to read metadata
• Penrose proposes a simpler JSON format
(module-info.json) that can be used to generate
both Jigsaw and OSGi metadata

68. OSGi and Jigsaw interoperability:
Penrose
MANIFEST.MF (OSGi)
module-info.java (Jigsaw)
module org.bgjug @ 1.0 {
exports org.bgjug.external;
}
Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: org.bgjug
Bundle-SymbolicName: org.bgjug
Bundle-RequiredExecutionEnvironment: JavaSE-1.7
Export-Package: org.bgjug.external;version="1.0.0"
module-info.json (Penrose)
"module" : {
"name" : "org.bgjug",
"version": 1.0,
"exports" : [{
"name" :
"org.bgjug.external",
"org.osgi.version" :
"1.0.0"
}]
}
module-info.json (Penrose)

69. Summary
Q: Projects Jigsaw/Penrose - when ?

70. Summary
Q: Projects Jigsaw/Penrose - when ?
A: Maybe when the Armagedon comes …

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

72. Q&A
Thank you
Bulgarian JUG mailing list:
https://groups.google.com/forum/#!forum/bg-jug

73. 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/

74. 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#JA
R_hell

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

76. 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

77. 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

78. References
JavaOne 2013: The Modular Java Platform and Project
Jigsaw
http://parleys.com/play/52549d02e4b0a43ac12
124be/about
OpenJDK Penrose JavaOne 2012
http://www.slideshare.net/bosschaert/open-jdk-
penrose-javaone-2012