Presentation by Neil Bartlett (Paremus) from OSGi DevCon 2012 BOF (22 March, 2012)
Video recording of the presentation is available at http://youtu.be/QJaSW5dW5c0
Abstract: The idea of integrating a module system into the core Java runtime has been proposed and discussed for many years, and in Java SE 8 it may finally be happening. How will this affect OSGi developers and users? This talk will describe the main differences between OSGi and Jigsaw, the prototype OpenJDK module system. Pros and cons of each in different environments will be discussed. Finally, opportunities and challenges for interoperability: from the perspective of both application developers (who may need to integrate modules from both kinds) and from library module developers (who may need to target both module systems).
3. WHY MODULARISE THE JDK?
• The JRE is monolithic
• Download time and start-up time are directly affected by
number of types available at runtime
• Start-uptime includes a linear search through the class path to
find system and application code
• Oracle 1.7 Windows boot classpath nearly 20k classes
• rt.jar index alone is approx 1Mb
4. OSGi
• If only Java had a module system that could help with that!
• As we all know, OSGi will not be used for JDK modularity
5. JIGSAW MOTIVATION
• JRElibraries evolved organically and haphazardly over 13+
years
• Many cyclic dependencies
• Many weird/unexpected dependencies
• E.g.: java.util is an incoherent mess
• Splitting packages is unavoidable
7. SINGLE CLASSLOADER
• Yeah but there’s the single classloader assumption...
• Many parts of the JRE assume a single boot classloader
• Package-private (“default”) accessibility requires whole
packages in single classloader
• Need to split packages across modules but not across
classloaders
• Break the one-to-one mapping of modules to classloaders
9. DEPENDENCY DIRECTION
• Yeah, but
the dependency goes in the “wrong” direction
(fragment depends on host).
• Jigsaw
wants the “host” to depend on the “fragment”: host
shouldn’t resolve if fragment unavailable.
13. Pretty sure we could have
supported this, if only you
had talked to us.
14. REFACTORING
• Usual and best solution: move classes to the correct, most
logical place
• Obviously impossible in the JRE.
•9 million Java developers (Sun estimate, 2009) and billions
of apps depend on this library.
16. MODULES
• Module declared in module-info.java (module-info.class)
• All new keywords are “scoped”
module B @ 1.0 {
...
}
17. REQUIRES
• Modules require other modules by name (and optionally,
version)
module B @ 1.0 {
require A @ [2.0,3.0);
}
18. LOCAL
• Requirements can be marked “local”
• Target module is loaded in same classloader
module B @ 1.0 {
require local A @ [2.0,3.0);
}
19. EXPORTS
• Modules list their exports, at package and type level
• May include re-exported contents of required modules
module B @ 1.0 {
require A @ [2.0,3.0);
export org.foo.ClassFoo;
export org.bar.*;
}
20. FRIENDS
• Modules can control which other modules require them
• Compare with “friend” classes in C++
• N.B. permit clause is not versioned.
module A @ 2.0 {
permit B;
}
21. PROVIDES
• Modules can logically “provide” other modules names
• Compare with “virtual packages” in Debian
• Supports substitution, but not refactoring (splits or joins)
module com.ibm.stax @ 1.0 {
provide jdk.stax @ 2.0;
}
22. ENTRY POINT
• Modules can have a single entry-point class
• Compare with Main-Class header in Jar manifests.
module A @ 2.0 {
permit B;
class org.foo.Main;
}
23. VERSIONS
• Modules will be versioned
• Requirements use exact version or a range
• No version semantics beyond ordering
25. LIFECYCLE
• Resolver solves dependencies in the face of multiple valid
alternatives
• OSGi resolver finds best fit for currently installed bundles at
runtime
• Jigsaw resolves during build and installation
• Jigsaw has no dynamics, no module lifecycle
27. COMPARISON
• Whole-module dependencies and split packages are the
biggest differences
• Jigsaw
will suffer all the same problems seen in OSGi with
Require-Bundle (e.g. Eclipse Core Runtime refactoring mess)
• While Jigsaw will technically achieve JDK modularity it will
increase developer maintenance burden
29. INTEROP
• Both are here to stay... try to get the best of both worlds!
• OSGi is established and will continue to be used widely
• Jigsaw is underway and a key component of Java 8
• It need not be a zero-sum game
30. JAVA 8 REQUIREMENTS
• “It must be demonstrated by prototype to be feasible to
modify an OSGi micro-kernel such that OSGi bundles running
in that kernel can depend upon Java modules. The kernel must
be able to load Java modules directly and resolve them using
its own resolver, except for core system modules. Core system
modules can only be loaded using the module system’s
reification API.”
• http://openjdk.java.net/projects/jigsaw/doc/draft-java-module-
system-requirements-12
33. LEVEL 0: TOLERATE
• Ensure that OSGi frameworks continue to run unmodified on
Jigsaw-enabled runtime.
• Creating
modules/bundles that have both Jigsaw and OSGi
metadata on the same JAR.
34. LEVEL 1: UNDERSTAND
• Teach OSGi to read Jigsaw module info
• Mapping Jigsaw metadata into OSGi concepts e.g. requires =
Require-Bundle, exports = Export-Package.
• Resolve Jigsaw modules using the OSGi resolver.
35. LEVEL 2: EXPLOIT
• OSGi implementation exploits Jigsaw modularity
• E.g. use Jigsaw publication repositories.
37. ACHIEVEMENT 1
• Passed the OSGi tests on a Jigsaw-enabled runtime
• Equinox 3.7, the OSGi Reference Implementation.
• OSGi R4.3 Compliance Tests
38. ACHIEVEMENT 2
• Run a Java application as either OSGi or Jigsaw modules
• Took a Java 2D demo
• Brokeinto multiple functional units, run demo as either
OSGi bundles or Jigsaw modules
39. GOAL 1
• Map Jigsaw module metadata to OSGi equivalents
• Discussion: how to interpret Jigsaw directives
40. GOAL 2
• Modify OSGi (Equinox) to use Jigsaw reification APIs
• Load a 3rd-party module from the Jigsaw repository
• Resolve it using the OSGi resolver
41. TIMELINE
• Java 8: summer 2013? Code freeze early 2013.
• Jigsaw seems to be basically “done”, with little recent activity
• No JSR yet, or perhaps ever.
• If we need changes in Jigsaw, we need to push them soon.
42. GET INVOLVED
• Penrose home & mailing list:
• http://openjdk.java.net/projects/penrose/
• Hg repository:
• http://hg.openjdk.java.net/penrose/jigsaw/
