The document outlines Java 9 features, particularly focusing on Project Jigsaw which introduces modularity, discussing its benefits such as improved encapsulation, security, and maintenance. It addresses migration issues developers may encounter when transitioning from earlier Java versions to Java 9, detailing specific changes and APIs that have been removed or modified. Key directives for module management are provided, along with recommendations for resolving common migration challenges.

1. Java 9: modularity &
migration issues
Oleh Krupenia
Senior Software Engineer, Consultant

2. Agenda
1. What is Project Jigsaw;
2. Which benefits does modularity bring;
3. module-info.java contents;
4. Which migration issues can be and how to resolve them;
5. Let’s migrate a small project to Java 9
2

3. Release GA Date End of Public
Updates
8 Mar 2014 Jan 2019
9 Sep 2017 Mar 2018
10 (18.3) Mar 2018 Sep 2018
11(18.9 LTS) Sep 2018 TBA
3

4. Project Jigsaw
4

5. Project Jigsaw
5

6. Module
A uniquely named, reusable group of related packages, as well as resources
(such as images and XML files) and a module descriptor specifying:
- the module’s name
- the module’s dependencies
- the packages it explicitly makes available to other modules
- the services it offers
- the services it consumes
- to what other modules it allows reflection
6

7. Module
module com.foo {
requires com.google.common;
}
7

8. Advantages
- Improved encapsulation
- Better security
- Easier construction and maintenance of large
applications
- Performance
8

9. Access levels
- public to everyone
- public to specific modules
- public within current module
- protected
- package
- private
9

10. 10

11. JDK 8
bin
*.exe
jre lib
tools.jar
bin
*.exe
lib
rt.jar
11

12. JDK 8 JDK 9
bin
*.exe
jre lib
tools.jar
bin
*.exe
lib
rt.jar
bin
*.exe
conf libjmods
12

13. JEP 260: Encapsulate Most
Internal APIs
Public
- java.*
- javax.*
- com.sun.*
Internal API – sun.*
13

14. JEP 260: Encapsulate Most
Internal APIs
Internal API
- non-critical
sun.misc.BASE64Decoder
- critical
sun.misc.Unsafe
14

15. --module-path --classpath
Modular JAR application module unnamed module
Non-Modular JAR automatic module unnamed module
--module-path
15

16. Type Origin Exports packages Can read modules
Platform provided by JDK explicitly -
Application any JAR containing
module-info.class
on the module path
explicitly platform
application
automatic
Automatic any JAR without
module-info.class
on the module path
all platform
application
automatic
unnamed
Unnamed all JARs and classes
on the classpath
all platform
application
automatic
Module readability
16

17. Requires
A requires module directive specifies that this
module depends on another module.
module com.foo {
requires com.google.common;
requires transitive com.foo.bar;
}
17

18. exports and exports…to
An exports module directive specifies one of the
module’s packages whose public types (and their
nested public and protected types) should be
accessible to code in all other modules.
module com.foo {
requires com.google.common;
exports com.baz;
exports com.bar to foo.bar;
}
18

19. provides…with
A provides … with module directive specifies that a
module provides a service implementation—making
the module a service provider.
module com.foo {
requires com.google.common;
exports com.foo;
provides com.google.common.collect.BiMap
with com.foo.TestBiMap;
}
19

20. uses
A uses module directive specifies a service used by this
module—making the module a service consumer. A
service is an object of a class that implements the
interface or extends the abstract class specified in the
uses directive.
module foo.bar {
requires com.google.common;
uses com.google.common.collect.BiMap;
}
20

21. opens, opens ... to
Before Java 9, reflection could be used to learn about
all types in a package and all members of a type—even
its private members—whether you wanted to allow this
capability or not. Thus, nothing was truly encapsulated.
module foo.bar {
requires com.google.common;
opens com.foo to foo.bar;
open com.foo
}
21

22. 22
Migration issues
Linking
22

23. Migration issues
23

24. The underscore character ("_") is not
valid identifier
Error: as of release 9, '_' is a keyword, and may not be
used as a legal identifier.
JEP 302: Lambda Leftovers
BiFunction<Integer, String, String> biss = (i, _) ->
String.valueOf(i);
24

25. Removed GC Options
The following GC combinations will cause your
application to fail to start in JDK 9:
- DefNew + CMS
- ParNew + SerialOld
- Incremental CMS
25

26. Use CLDR Locale Data by Default
In JDK 9, the Unicode Consortium’s Common Locale Data
Repository (CLDR) data is enabled as the default locale data.
Code that uses locale-sensitive services such as date, time,
and number formatting may produce different results with the
CLDR locale data. Remember that even
System.out.printf() is locale-aware.
26

27. -source and -target
In JDK 8, -source and -target values of 1.5/5 and
earlier were deprecated and caused a warning to be
generated. In JDK 9, those values cause an error.
If possible, use the new --release flag instead of the -
source and -target options.
27

28. Removed java.* APIs
java.* APIs that have been removed in JDK 9 include the
previously deprecated methods from the
java.util.logging.LogManager and java.util.jar.Pack200 packages:
java.util.logging.LogManager.addPropertyChangeListener
java.util.logging.LogManager.removePropertyChangeListener
java.util.jar.Pack200.Packer.addPropertyChangeListener
java.util.jar.Pack200.Packer.removePropertyChangeListener
28

29. java.awt.peer Not Accessible
The java.awt.peer and java.awt.dnd.peer packages
aren’t accessible in JDK 9. The packages were never
part of the Java SE API, despite being in the java.*
namespace.
Replace this with Component.isDisplayable()
29

30. Removed com.sun.image.codec.jpeg
Package
The nonstandard package com.sun.image.codec.jpeg
has been removed.
Use the Java Image I/O API instead.
30

31. Removed Tools Support for Compact
Profiles
Profiles, introduced in Java SE 8, define subsets of the Java SE
Platform API that can reduce the static size of the Java runtime
on devices that have limited storage capacity.
In JDK 9, the -profile option is supported by javac only in
conjunction with the --release 8 option, and isn’t supported
by java.
31

32. Access To Internal APIs
If you must use an internal API that has been made
inaccessible by default, then you can break encapsulation
using the --add-exports command-line option.
The syntax of the --add-exports option is:
--add-exports <source-module>/<package>=<target-
module>(,<target-module>)*
32

33. Access To Internal APIs
where <source-module> and <target-module> are
module names and <package> is the name of a
package.
--add-exports
java.management/sun.management=ALL-UNNAMED
33

34. Run jdeps on Your Code
Run the jdeps tool on your application to see what
packages and classes your applications and libraries
depend on.
34

35. Run jdeps on Your Code
> jdeps guava-20.0.jar
guava-20.0.jar -> java.base
guava-20.0.jar -> java.logging
guava-20.0.jar -> jdk.unsupported
com.google.common.annotations -> java.base
com.google.common.base -> java.logging
com.google.common.cache -> JDK internal API
35

36. --add-opens
If you have to allow code on the class path to do deep
reflection to access non public members, then use the --add-
opens runtime option.
--add-opens module/package=target-module(,target-module)*
--add-opens java.management/sun.management=ALL-
UNNAMED
36

37. Dependencies On Java EE Modules
In JDK 9, the modules that contain CORBA or the APIs
shared between Java SE and Java EE are not resolved by
default when you compile or run code on the class path.
37

38. Dependencies On Java EE Modules
java.corba — CORBA
java.transaction — The subset of the Java Transaction API to support
CORBA Object Transaction Services
java.activation — JavaBeans Activation Framework
java.xml.bind — Java Architecture for XML Binding (JAXB)
java.xml.ws — Java API for XML Web Services (JAX-WS), Web Services
Metadata for the Java Platform, and SOAP with Attachments for Java
(SAAJ)
java.xml.ws.annotation — The subset of the JSR-250 Common
Annotations to support web services
38

39. Dependencies On Java EE Modules
1. Use the --add-modules command-line option to ensure that
the module with the API is resolved at startup. For example,
specify --add-module java.xml.bind to ensure that the
java.xml.bind module is resolved.
1. Deploy the standalone version of the API (and implementation
if needed) on the class path. Each of the Java EE APIs are
standalone technologies with projects published in Maven
Central.
39

40. Split packages
Module is not allowed to read the same package from two
different modules.
Possible solutions:
1. Rename packages
2. The option --patch-module $module=$artifact will merge all
classes from $artifact into $module
3. Combine the two jars into a single jar
4. Use unnamed modules
40

41. Example
41

42. Questions?
42

43. Thank you!
oleh.krupenia@globallogic.com