Commons Discovery: Service Discovery component

The Discovery component is about discovering, or finding, implementations for pluggable interfaces. It provides facilities for instantiating classes in general, and for lifecycle management of singleton (factory) classes.

Fundamentally, Discovery locates classes that implement a given Java interface. The discovery pattern, though not necessarily this package, is used in many projects including JAXP (SaxParserFactory and others) and commons-logging (LogFactory). By extracting this pattern, other projects can (re)use it and take advantage of improvements to the pattern as Discovery evolves.

Discovery improves over previous implementations by establishing facilities for working within managed environments. These allow configuration and property overrides without appealing to the global System properties (which are scoped across an entire JVM).


This is not indended to be a replacement to be used strictly by the user, but rather a replacement to be used directly by projects. Use by the user is also reasonable, but limited due to 'keeping this simple'. In particular, there is no configuration for this discovery service, it relies only on usage patterns.

Given a java.lang.Class parameter 'package.Class' that represents a fundamental service as either an interface, an abstract class (or even a regular class), find an implementation of that class:

  • Look for system property named 'package.Class', the value of which is the name of an implementation.
  • Look for a 'local' property named 'package.Class'...
  • Attempt JDK 1.3+ style discovery
  • Attempt to load a default implementation
In all cases, verify that the discovered implementation implements 'package.Class'.

The package aims to :

  • Have an API which should be as simple to use as possible
  • Be based on usage patterns, not specific configuration file
  • Represent proper discovery & class-loading principles
  • Be reasonably portable across JDK levels (1.1.8+?)