An overview ofProject Jigsaw

src/main/java

api/Foo.java

internal/Qux.java

<project> <modelVersion>4.0.0</modelVersion> <groupId>example</groupId> <artifactId>foo</artifactId> <version>1</version> </project>

<project> <modelVersion>4.0.0</modelVersion> <groupId>example</groupId> <artifactId>bar</artifactId> <version>1</version> </project>

src/main/java

api/Bar.java

internal/Baz.java

pom.xml pom.xml

<project> <modelVersion>4.0.0</modelVersion> <groupId>example</groupId> <artifactId>bar</artifactId> <version>1</version> <dependencies> <dependency> <groupId>example</groupId> <artifactId>foo</artifactId> <version>1</version> </dependency> </dependencies> </project>

module-info.java example.foo

api/Foo.java

internal/Qux.java

module-info.java example.bar

api/Bar.java

internal/Baz.java

module example.foo { exports api; }

module example.bar { requires example.foo; }

Scope

Task module descriptor

module descriptordeployment descriptorbuild execution descriptorAPI for custom tasks

runtimecompile-time build-time

Enforcement obligatory (with opt-out)

optional(provided/optional dependency)

Unit Java package (sub-)project

api/Foo.class internal/Qux.class api/Bar.class internal/Baz.class

api/Foo.class internal/Qux.class api/Bar.class internal/Baz.class

class path (system class loader)

Modularity prior to Java 9 is emulated by class loader hierarchies. By tweaking classloaders to define multiple parents, it is also possible to run several versions of thesame module (e.g. OSGi).

api/Foo.class internal/Qux.class api/Bar.class internal/Baz.class

api/Foo.class internal/Qux.class api/Bar.class internal/Baz.class

module loader

example.foo

exam

ple.

bar

“example.bar reads example.foo”

The integrity of the module graph is verified by the Java 9 runtime. Jars without amodule-info.class are bundled in an unnamed module that reads all modules (fallback).

try { Class<?> pluginType = Class.forName("example.Foo"); runAppWithPlugin(pluginType); } catch(ClassNotFoundException exception) { runAppWithoutPlugin(); }

void runAppWithPlugin(Class<?> pluginType) { activate(pluginType.newInstance()); // and run the app... }

When code is bundled as a module, looking up a class is still possible but any execution of foreign code is no longer permitted.

In order to overcome this, a new reflection API for modules is introduced that allowsfor the dynamic attachment of “read edges” at runtime. At compile time, the currentdraft does however not foresee a possibility of defining optional dependencies.

module example.overview {

exports pkg.name; exports pkg.other.name to example.friend, example.other;

requires mdl.name; requires public mdl.name.exposed;

provides service.type.name with service.impl.name; uses service.other.type.name;}

Jigsaw recommends crossing of module boundaries by using service interfaces.Implementations of such interfaces can then be retreived by the ServiceLoader API.

Finally, modules can – similarly to class loaders – be organized in layers. This allowsThe migration of class loader hierarchy systems into a modularized formats. Layerscan be created programmatically via a reflection API.

Java 9: more than Jigsaw! (biased selection)

• New process API• HTTP2 client• Improved contended locking• Unified JVM logging• Compiler control• Variable handles• Segmented code cache• Smart Java compilation, phase 2• Project Coin extensions

(private interface methods)• Unicode 7.0/8.0• Convenience collection factories

• Annotation pipeline enhancements• Multi-version jar files• Compact strings• “Indifying” string concatenation• Stack-walking API• Currency and money API• Enhanced method handles• Platform specific desktop features• javac performance (sjavac, type inference)• Jshell• javadoc improvements (HTML5, search)• Improved concurrency

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