Dynamic Applications

Why ?

● Standard: reference implementation for JSR 330

● Flexible: can map all sorts of metadata to bindings

● Type-safe: detailed messages when things go wrong

● Modular: multiple extensions available, OSGi-ready

● Fluent Java binding API

● Records generic information lost during erasure

● Can be driven by Java / annotations / XML / ...etc...

JSR 330

● New standard for Java dependency injection

// Constructor injection

// Setter injection

// Field injection

Extending JSR 330

● JSR 330 tells us how to mark dependencies

● and qualify them (just like with Plexus hints)

● But it does not say how to mark components

Identifying JSR 330 components

● Wrap the Class-Path up as a ClassSpace and scan it● Look for classes with qualifiers such as @Named

● Empty @Named means "use class name" instead● Binding type found by analysing class hierarchy

What is a dynamic application?

● OSGi lets us dynamically add / remove bundles● ... without needing to restart the application● This may add / remove qualified components

● ... but Guice bindings are static

Dynamic application vs. static bindings

● How can we resolve this mismatch?

● Need support for dynamic component collections!

Injector Graph

Bean Locator

Dynamic Collections

● Backed by dynamic Iterable of qualified beans

● Iterables updated as injectors come and go

● Copy-on-iteration avoids synchronization overhead

● Weak-ref detects when Iterable not used anymore

Qualified Dependencies

● Qualified Map

● Qualified Instance

Hinted Dependencies

● Hinted Map

● Hinted List

Dynamic Updates

● Passive collections are not enough

● Sometimes need active notifications

● Should we force use of an explicit API?

● ... or use reflection to scan for methods?

Mediator Pattern

POJO

Mediator

Bean Locator add bean

remove bean

Mediator Benefits

● Watching application bean can remain a POJO● Mediators detected just like any other component

● Created on-demand as bean instances created● Removed when the watching bean is GC'd

Bootstrapping

● Just wrap your Class-Path as a ClassSpace

● ... install the QualifiedScannerModule

● and kick-start Guice injection as usual

Time for a Demonstration!

What about Peaberry?

● Majority of plugin bindings tied to its lifecycle

● Can simply create one injector per plugin

● For plugin services that are not tied to thelifecycle we need something more dynamic!

● Proxies backed by the OSGi Service Registry

Links

JSR 330 http://code.google.com/p/atinject/

Guice http://code.google.com/p/google-guice/

Peaberry http://code.google.com/p/peaberry/

Spice-Inject http://svn.sonatype.org/spice/trunk/spice-inject

OSGi http://www.osgi.org/About/HowOSGi

Blog Updates http://www.sonatype.com/people/author/mcculls/

Questions?

Tr an sfo r m in g th e w ay so ftw are is m ad e .