An Overview of Project Jigsaw
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The document provides an overview of Project Jigsaw in Java 9, which introduces a modular system and module declarations. Key points include the structure of modules and module descriptors, module readability and dependencies, and how modularity addresses issues with the prior class loader-based approach. It also discusses features like services, layers, and the module reflection API.
An Overview of Project Jigsaw
- 1. An overview of Project Jigsaw
- 3. 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; }
- 4. Scope Task module descriptor module descriptor deployment descriptor build execution descriptor API for custom tasks runtime compile-time build-time Enforcement obligatory (with opt-out) optional (provided/optional dependency) Unit Java package (sub-)project
- 5. 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 class loaders to define multiple parents, it is also possible to run several versions of the same module (e.g. OSGi).
- 6. 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 example.bar “example.bar reads example.foo” The integrity of the module graph is verified by the Java 9 runtime. Jars without a module-info.class are bundled in an unnamed module that reads all modules (fallback).
- 7. 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 allows for the dynamic attachment of “read edges” at runtime. At compile time, the current draft does however not foresee a possibility of defining optional dependencies.
- 8. 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 allows The migration of class loader hierarchy systems into a modularized formats. Layers can be created programmatically via a reflection API.
- 11. 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