The document discusses the introduction of modules in JDK 9 and how they address issues such as platform scalability, jar hell, and performance. It elaborates on the Java Linker (jlink) used for packaging and creating optimized runtime images from Java modules, along with examples of its plugins and functionalities. The aim is to enhance the modular structure of applications while providing step-by-step instructions on module creation and linking.

1. JDK-9
Modules and Java Linker (JLink)
G. Bhanu Prakash
Java Platform Team
Oracle India Private Limited
bhanu.prakash.gopularam@oracle.com
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2. Agenda
1. Modules
2. Module Dependencies
3. Jlink and Packaging
4. Jlink Plugins
5. Example Plugins
1. System Module Descriptor Plugin
2. Compress Plugin
3. Release-Info Plugin
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3. SAFE HARBOR STATEMENT
The following is intended to outline our general product direction. It is
intended for information purpose only, and may not be incorporated in
any contract. It is not a commitment to deliver any material, code, or
functionality, and should not be relied upon in making purchasing
decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole discretion
of Oracle
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4. Jigsaw Problems
Problems
 Platform scalability (small devices)
 Jar Hell or Classpath Hell
• Java runtime crashes with NoClassDefFoundError
• Shadowing of classes
 Performance
• Startup
• Download
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5. Jigsaw Solutions
Problems
 Platform scalability
• Modularize JDK
 Jar Hell or Classpath Hell
• Replace classpath with module dependencies
 Performance
• Startup – install optimizations in module library
• Download - incremental modules in demand
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6. • What is Module
• Module-Info.java
– Visibility Rules
• REQUIRES
• EXPORTS
• REQUIRES PUBLIC
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Modules

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“public” no longer means “accessible”
Modules

8. com.foo.app
module-info.java
module com.foo.app {
requires com.foo.bar;
requires java.sql;
}
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Example: com.foo.app
Modules Example
com.foo.bar
module com.foo.app {
exports com.foo.bar;
}

9. Actual Module Dependencies (com.foo.app)
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Transitive Dependencies (com.foo.app)

11. JDK Platform Modules
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12. How to compile modules
Inputs
 Jmods, Modular jar files
 Exploded modules
1. Compile com.foo.bar (base)
javac -d modscom.foo.bar
srccom.foo.barmodule-info.java
srccom.foo.barcomfoobar*.java
2. Compile com.foo.app (main application)
javac -modulepath mods
-d modscom.foo.app
srccom.foo.appmodule-info.java
srccom.foo.appcomfooapp*.java
Or
javac –modulesourcepath src –d mods …
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13. Create Modules
 Create module com.foo.bar
jmod create
--class-path modscom.foo.bar
com.foo.bar
 Create module com.foo.app
jmod create
--class-path modscom.foo.app
com.foo.app
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14. Module Information
• jmod describe com.foo.bar
com.foo.bar
requires mandated java.base
exports com.foo.bar
• jmod describe com.foo.app
com.foo.app
requires com.foo.bar
requires mandated java.base
requires java.sql
conceals com.foo.app
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16. Linking
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17. Jlink and Packaging (JEP-282)
Brief History
 Jrecreate was introduced in Java 8 with EJDK (JavaSE Embedded)
 Instead of binaries we ship the EJDK
Jlink Usage: Command line tool to link module
 modules into jimage file
 generate runtime images
JEP-282: “Link time is an opportunity to do whole-world
optimizations that are otherwise difficult at compile or costly
at runtime”
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18. Jlink Packaging
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Legacy JDK image
JDK-9 Generated Image

19. Jlink Packaging
Create image file
jlink --output myimage
--addmods com.foo.app
--modulepath jdk-9b131jmods;mods
>ls myimage
bin lib conf release
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20. App execution from Generated image
>myimagebinjava -listmods
com.foo.app
com.foo.bar
java.base@9-ea
java.logging@9-ea
java.sql@9-ea
java.xml@9-ea
>java -m com.greetings/com.greetings.Main
Output: Greetings (com.foo.app.Main) : World (from com.foo.bar)
>java -m com.foo.app/com.foo.app.App2
Output: Exception:oracle.jdbc.driver.OracleDriver
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-modulepath mods
Not required

21. Jlink Plugins
 Image customization enabled using a set of predefined plugins
 12 builtin plugins. Example:
 compress-resources
 release-info
 sort-resources
 exclude-files
 gen-installed-modules
 replace-file
 Programmatic access to jlink features
 As per the JEP, the plugin API is strictly experimental
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Plugins and Image creation - WorkFlow

23. Optimizing Java startup
Problem: Significant time spent parsing and validating
module-info.class for each module in system image
itself
Solution: Implement jlink plugin to generate a pre-
validated system module graph
SystemModuleDescriptorPlugin
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--installed-modules Startup time
Disabled 510 ms
Enabled 220 ms

24. • Reduces memory use by a sizable amount
• This plugin is ON by default (--installed-modules on)
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25. Compress Plugin
LEVEL_0
StringSharingPlugin - Scans image classes constant pool (UTF-8 Strings)
LEVEL_1:
ZipPlugin(resFilter) - Zip compression of image classes
LEVEL_2:
StringSharingPlugin(resFilter)
ZipPlugin(resFilter)
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Compression
level
Size (~)
None 28.5 MB
LEVEL_0 17.3 MB
LEVEL_1 11.7 MB
LEVEL_2 11.5 MB
Table: Jlink generated image size

26. Release-info Plugin
Normal JDK
myimage>cat release
#Wed Aug 24 09:19:38 IST 2016
MODULES=java.sql,com.foo.bar,com.foo.app,java.logging,java.xml,java.base
OS_VERSION=5.2
OS_ARCH=amd64
OS_NAME=Windows
JAVA_VERSION=9-ea
With Release Plugin:
jlink --release-info
add:build_type=fastdebug,source=oraclejdk,java_version=9+101 …
myimage2>cat release
#Wed Aug 24 09:32:36 IST 2016
MODULES=java.sql,com.foo.bar,com.foo.app,java.logging,java.xml,java.base
OS_VERSION=5.2
OS_ARCH=amd64
OS_NAME=Windows
JAVA_VERSION=9-ea
build_type=fastdebug,source=oraclejdk,java_version=9+101
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27. Summary
• Accessibility is enforced by the compiler, VM, and Core
Reflection
• Freedom to adopt modules at your own pace
– Modularize the application
– Modularize the libraries
• Some libraries may require more changes to work as modules
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28. More Information
 OpenJDK Project Jigsaw page
 http://openjdk.java.net/project/jigsaw
 mailto: jigsaw-dev@openjdk.java.net
 Module Usage at Compile Time slides
http://openjdk.java.net/projects/jigsaw/doc/ModulesAndJavac.pdf
 Javadoc for Java Module java APIs:
http://cr.openjdk.java.net/~mr/jigsaw/api/
 Modules in the Java Language and VM
http://openjdk.java.net/projects/jigsaw/doc/lang-vm.html
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29. Thank You. Questions?
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