Jigsaw is a project that started 8 years ago to address issues with Java's module system. It aims to avoid classpath hell, improve encapsulation across packages, remove manual security, and enable faster startup. The Java Platform Module System (JPMS) specified in JSR 376 addresses these goals. Key changes in Java 9 include modularizing the JDK itself into modules like java.base and representing dependencies between modules. The module system enforces encapsulation through access controls and provides reflection capabilities. While disruptive, the new modular structure in Java 9 provides benefits over the current and OSGi systems and developers are encouraged to start adopting modules now ahead of the Java 9 release.

1. Jigsaw
A long winding road started almost 8
years ago...

2. About the speakers
Rikard Thulin Fredrik Jansson

3. History: It started with a war
jar cvf jigsaw.war
SUN Oracle OSGi

4. Obvious solution
Apply a well know pattern!
SUN Oracle OSGi

5. We lost a few years… But now, close to eight years later we are on track!

6. Jigsaw motivation
● avoid JAR/Classpath Hell
● Encapsulation across packages
● Remove manual security
● Faster startup
JSR 376: JavaTM
Platform Module System
Spec lead: Mark Reinhold - Chief Architect of the Java Platform group at Oracle.

7. Java 7 dependency graph

8. Java 9 dependency graph by modules

9. The spaghetti monster and cyclic dependencies

10. We have been warned since 1996 of sun packages

11. sun internal API:s
deprecated in JDK 9
planned to be removed in JDK10
workaround with command line flag
An application that uses only standard Java SE APIs, and possibly also JDK-
specific APIs, must work the same way […] as it does today. [1]
To find problems, use the Java Dependency Analysis Tool (jdeps -jdkinternals)
[1] Project Jigsaw: Goals & Requirements

12. Binary structure of JRE/JDK 9
● Disruptive change
● Be warned if you
○ Written your own class loader
○ Do things with rt.jar or tools.jar
○ Use the endorsed mechanism
○ Using the extension mechanism
○ Jar URL Schema
■ jar:file:<path-to-jar>!<path-to-file-in-jar>
■ jrt:/<module-name>/<path-to-file-in-module>

13. Binary structure of JRE/JDK 9
● Mostly affect tools vendors
○ Has been in JDK 9 builds since late 2014
● Good change, as the previous approach was as-is-works
● Need more details? - google JEP 261

14. Version-string schema
● Will change, but is not in JDK 9 builds yet
if (System.getProperty(“java.version”).startsWith(“1.7”)) {
System.out.println(“This will not work…”);
}

15. Modules
The file module-info.java contains the modules definition
module-info.java:
module com.squeed.javaforum {
}

16. Dependencies
module com.squeed.javaforum {
requires com.squeed.nforum;
requires java.logging;
}
All modules depends on java.base

17. Encapsulation
module com.squeed.javaforum {
exports com.squeed.javaforum.agenda;
exports com.squeed.javaforum.party to somemodule;
}
Second export is a qualified export

18. Encapsulation
public
protected
<package>
private

19. Encapsulation
public to everyone
public to specific modules
public within a module
public
protected
<package>
private

20. Reflection
setAccessible(true)
Is enforced by the compiler, VM and core reflection

21. Is enforced by the compiler, VM and core reflection
Classloaders are unchanged
Compared to today's module systems like OSGi - “classloaders on steroids”
Image by Alex Buckley
Jigsaw vs OSGi

22. Summary
● Start now
○ If you build heavy stuff (IDE’s, build tools, application servers)
● Wait a year for the Java 9 release
○ If you build vanilla (aka. “regular”)
● Safe harbor - do not wait too long
○ Dependencies to 3PP may cause problems
Java 9 might change a lot of stuff
● How maven / IDEs / application servers works

23. Q & A(maybe)