The document discusses Project Jigsaw and the Java Platform Module System. It provides an overview of the key aspects of modular development in Java 9 such as modules, module types, module declarations with exports and requires, and services. It also demonstrates a sample modular application structure with multiple modules - m1, m2 and app module. The document discusses how modules provide reliable configuration and strong encapsulation of types and packages in Java 9.

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Jigsaw Modularity
The current state of the Module System
Srinivasan Raghavan
Senior Member of Technical Staff
Java Platform Group
srinivasan.raghavan@oracle.com
Sep 24, 2016

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Safe Harbor Statement
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remains at the sole discretion of Oracle.
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Program Agenda
Project Jigsaw Introduction
Reliable configuration and Strong encapsulation
Module Types
Modules and Services
Q and A
1
2
3
4
5
4

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Project Jigsaw
5
• JEP 200: The Modular JDK
• JEP 201: Modular Source Code
• JEP 220: Modular Run-Time Images
• JEP 260: Encapsulate Most Internal APIs
• JEP 261: Module System
• JEP 282: jlink: The Java Linker
• JSR 376: Java Platform Module System

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• java.activation@9-ea
• java.annotations.comm
on@9-ea
• java.base@9-ea
• java.compact1@9-ea
• java.compact2@9-ea
• java.compact3@9-ea
• java.compiler@9-ea
• java.corba@9-ea
• java.datatransfer@9-ea
• java.desktop@9-ea
• java.httpclient@9-ea
• java.instrument@9-ea
• java.jnlp@9-ea
• java.logging@9-ea
• java.management@9-ea
• java.naming@9-ea
• java.prefs@9-ea
• java.rmi@9-ea
• java.scripting@9-ea
• java.se@9-ea
• java.se.ee@9-ea
• java.security.jgss@9-ea
• java.security.sasl@9-ea
• java.smartcardio@9-ea
• java.sql@9-ea
• java.sql.rowset@9-ea
• java.transaction@9-ea
• java.xml@9-ea
• java.xml.bind@9-ea
• java.xml.crypto@9-ea
• java.xml.ws@9-ea
• javafx.base@9-ea
• javafx.controls@9-ea
• javafx.deploy@9-ea
• javafx.fxml@9-ea
• javafx.graphics@9-ea
• javafx.media@9-ea
• javafx.swing@9-ea
• javafx.web@9-ea
• jdk.accessibility@9-ea
• jdk.attach@9-ea
• jdk.charsets@9-ea
• jdk.compiler@9-ea
• jdk.crypto.ec@9-ea
• jdk.crypto.pkcs11@9-ea
• jdk.deploy@9-ea
• jdk.deploy.controlpanel
@9-ea
• jdk.deploy.controlpanel
.fx@9-ea
• jdk.dynalink@9-ea
• jdk.hotspot.agent@9-ea
• jdk.httpserver@9-ea
• jdk.internal.le@9-ea
• jdk.internal.opt@9-ea
• jdk.jartool@9-ea
• jdk.javadoc@9-ea
• jdk.javaws@9-ea
• jdk.jcmd@9-ea
• jdk.jconsole@9-ea
• jdk.jdeps@9-ea
• jdk.jdi@9-ea
• jdk.jdwp.agent@9-ea
• jdk.jfr@9-ea
• jdk.jlink@9-ea
• jdk.jshell@9-ea
• jdk.jsobject@9-ea
• jdk.jstatd@9-ea
• jdk.jvmstat@9-ea
• jdk.localedata@9-ea
• jdk.management@9-ea
• jdk.management.cmm@
9-ea
• jdk.management.jfr@9-
ea
• jdk.management.resour
ce@9-ea
• jdk.naming.dns@9-ea
• jdk.naming.rmi@9-ea
• jdk.net@9-ea
• jdk.pack200@9-ea
• jdk.packager@9-ea
• jdk.packager.services@9
-ea
• jdk.plugin@9-ea
• jdk.plugin.dom@9-ea
• jdk.plugin.server@9-ea
• jdk.policytool@9-ea
• jdk.rmic@9-ea
• jdk.scripting.nashorn@9
-ea
• jdk.scripting.nashorn.sh
ell@9-ea
• jdk.sctp@9-ea
• jdk.security.auth@9-ea
• jdk.security.jgss@9-ea
• jdk.snmp@9-ea
• jdk.unsupported@9-ea
• jdk.vm.cds@9-ea
• jdk.vm.ci@9-ea
• jdk.xml.bind@9-ea
• jdk.xml.dom@9-ea
• jdk.xml.ws@9-ea
• jdk.zipfs@9-ea
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Let’s take a configuration
java.base
m1 m2
App

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package com.m1.p1;
public class Type1 {
public String whoAmI() {
return " I am " + Type1.class.getSimpleName()
+" from the module " + Type1.class.getModule().getName()
+ " from package " + Type1.class.getPackageName()
+ " Loaded by " + Type1.class.getModule().getClassLoader().toString();
}
}
package com.m1.p2;
public class Type2 {
public String whoAmI() {
return " I am " + Type2.class.getSimpleName()
+" from the module " + Type2.class.getModule().getName()
+ " from package " + Type2.class.getPackageName()
+ " Loaded by " + Type2.class.getModule().getClassLoader().toString();
}
}
The module m1 code

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package com.m2.p3;
import com.m1.p1.Type1;
import com.m1.p2.Type2;
public class Type3 {
private final Type1 type1 = new Type1();
private final Type2 type2 = new Type2();
public Type1 getMeType1() {
return type1;
}
public String whoAmI() {
return " I am " + Type3.class.getSimpleName()
+" from the module " + Type3.class.getModule().getName()
+ " from package " + Type3.class.getPackageName()
+ " Loaded by " + Type3.class.getModule().getClassLoader().toString();
}
public String whoisType2() {
return type2.whoAmI();
}
}
The module m2 code

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module m1 {
exports com.m1.p1;
exports com.m1.p2 to m2;
}
module m2 {
requires transitive m1;
exports com.m2.p3;
}
The module m1 and m2 module-info.java

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package com.app.p1;
import com.m1.p1.Type1;
import com.m2.p3.Type3;
class App1 {
static final Type3 type3 = new Type3();
public static void main(String[] args) {
System.out.println(type3.whoAmI());
System.out.println(type3.whoisType2());
System.out.println(type3.getMeType1().whoAmI());
}
}
module app{
requires m2;
exports com.app.p1;
}
The app module code

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$find src -name “*.java"
src//app/com/app/p1/App1.java
src//app/module-info.java
src//m1/com/m1/p1/Type1.java
src//m1/com/m1/p2/Type2.java
src//m1/module-info.java
src//m2/com/m2/p3/Type3.java
src//m2/module-info.java
How its structured

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$javac -d mods --module-source-path src/ $
(find src -name "*.java")
$java -p mods -m app/com.app.p1.App1
I am Type3 from the module m2 from package com.m2.p3 Loaded by jdk.internal.loader.ClassLoaders
$AppClassLoader@5ae9a829
I am Type2 from the module m1 from package com.m1.p2 Loaded by jdk.internal.loader.ClassLoaders
$AppClassLoader@5ae9a829
I am Type1 from the module m1 from package com.m1.p1 Loaded by jdk.internal.loader.ClassLoaders
$AppClassLoader@5ae9a829
Build and run

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$jlink --module-path $JDKMODS:mods --add-modules app --compress=2 --strip-
debug --output myappimage
$myappimage/bin/java --list-modules
app
java.base@9-ea
m1
m2
$ myappimage/bin/java -m app/com.app.p1.App1
I am Type3 from the module m2 from package com.m2.p3 Loaded by jdk.internal.loader.ClassLoaders
$AppClassLoader@5ae9a829
I am Type2 from the module m1 from package com.m1.p2 Loaded by jdk.internal.loader.ClassLoaders
$AppClassLoader@5ae9a829
I am Type1 from the module m1 from package com.m1.p1 Loaded by jdk.internal.loader.ClassLoaders
$AppClassLoader@5ae9a829
Create your own JDK

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What we can infer
15
• As we can see from the code and module-info Type2 is exported
only to m2 which means app module cannot directly access it
• Type1 is return value of a method in Type3 so Type1 is transitively
exported to app module
• If the app says Type2 type = new Type2() would fail to compile
• Type2 class code can be only accessible to Type3 which in m2
• This ensures a reliable configuration. It also ensures the Type2 is
encapsulated and its used only where it is meant to.

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$javac -d mods --module-source-path src/ $(find src -
name “*.java")
src/app/com/app/p1/App2.java:8: error: Type2 is not visible because package com.m1.p2 is not visible
import com.m1.p2.Type2;
^
src/app/com/app/p1/App2.java:18: error: cannot find symbol
Type2 type = new Type2();
^
symbol: class Type2
location: class App2
src/app/com/app/p1/App2.java:18: error: cannot find symbol
Type2 type = new Type2();
^
symbol: class Type2
location: class App2
3 errors
Type2 accessed by the App module

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public Unsafe getUnsafe() throws Exception {
Unsafe unsafe =Unsafe.getUnsafe();
}
public Unsafe someHowGetUnsafe() throws Exception {
Field field = Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
Unsafe unsafe = (Unsafe) field.get(null);
return unsafe;
}
Problems with un encapsulated code

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Problems with un encapsulated code
18
• com.sun.misc.Unsafe is important code widely used inside JDK but not
not meant for out use
• In Java8 getUnsafe() would fail with java.lang.SecurityException:
Unsafe . There would be clever check in code to see whether only Jdk
classes call unsafe
• But with some clever reflection hacks someHowGetUnsafe() would
pass
• In Java 9 both would fail with
java.lang.IllegalAccessError:jdk.internal.misc.Unsafe (in module
java.base) because module java.base does not export
jdk.internal.misc to unnamed module

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Modules Types
19
• Name Module - A module With module-info.java with both exports
and requires specified
• Automatic Module - Any Jar in ModulePath. The module info is auto-
generated which exports all package and requires all other mods
• Unnamed Module- All the jars which reside in the class-path
togethers is Unnamed Module
• Weak module - A module which has a module-info.java which
defines what is requires but exports all the package

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Modules Types
20
• Name Module - A module With module-info.java with both exports
and requires specified
• Automatic Module - Any Jar in ModulePath. The module info is auto-
generated which exports all package and requires all other mods
• Unnamed Module- All the jars which reside in the class-path
togethers is Unnamed Module
• Weak module - A module which has a module-info.java which
defines what is requires but exports all the package

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Modules And Services
21
• Services are first class citizens in the Module System
• The provide the basic loose-coupling between the Service Types and
the their Implementation
• Any Interface or abstract class be a Service Type
• Service Types and the their Implementation can be in any module and
they follow the following semantics
• when a module use a Service the module-info carries a “use” Service
• when a module provide an Implementation for the Service the
module-info carries “provides” service “with” Impl

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Modules And Services
module m1 {
exports com.m1.p1;
exports com.m1.p2 to m2;
}
module m2{
requires public m1;
exports com.m2.p3;
provides com.m1.p1.Service1 with com.m2.p3.Impl1;
provides com.m1.p1.Service1 with com.m2.p3.Impl2;
}
module app{
requires m2;
exports com.app.p1;
uses com.m1.p1.Service1;
}

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Modules And Services
public static void main(String[] args) {
getImpl("com.m2.p3.Imp1");
}
public static Service getImpl(String impl) {
Service service = null;
Optional<Provider<Service>> oprovider
= ServiceLoader.load(Service.class)
.stream()
.fiter(p -> p.type().getName().equals(impl)
.map(Provider::get)
.findFirst();
if (oprovider.isPresent()) {
Provider<Service> provider = oprovider.get();
service = provider.get();
}
return service;
}

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Some of Design issues and solution
24
• ReflectiveAccessToNonExportedTypes.
• Some libraries require reflective access to members of the non-
exported types of other modules
• Examples include dependency injection (Guice)
• Solution proposed is a weak module
• Weak Module provides Reilable Configuration but skips the
encapsulation part so that DI frameworks can reflect upon

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Weak modules
weak module app{
requires m2;
}

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References
26
• http://openjdk.java.net/projects/jigsaw/spec/issues/
#ReflectiveAccessToNonExportedTypes
• http://openjdk.java.net/projects/jigsaw/spec/reqs/
• http://hg.openjdk.java.net/jigsaw/jake
• http://download.java.net/java/jigsaw/docs/api/index.html
• http://openjdk.java.net/projects/jigsaw/talks/
• https://www.youtube.com/channel/UCdDhYMT2USoLdh4SZIsu_1g/
videos

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Question and Answers
27