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Discover class that implements a given service interface, with discovery and configuration features similar to that employed by standard Java APIs such as JAXP.

In the context of this package, a service interface is defined by a Service Provider Interface (SPI). The SPI is expressed as a Java interface, abstract class, or (base) class that defines an expected programming interface.

DiscoverClass provides the find methods for locating a class that implements a service interface (SPI). Each form of find varies slightly, but they all perform the same basic function. The DiscoverClass.find methods proceed as follows:

• Get the name of an implementation class. The name is the first non-null value obtained from the following resources:
  • The value of the (scoped) system property whose name is the same as the SPI's fully qualified class name (as given by SPI.class.getName()). The ScopedProperties class provides a way to bind properties by classloader, in a secure hierarchy similar in concept to the way classloader find class and resource files. See ScopedProperties for more details.

    If the ScopedProperties are not set by users, then behaviour is equivalent to System.getProperty().

  • The value of a Properties properties property, if provided as a parameter, whose name is the same as the SPI's fully qualifed class name (as given by SPI.class.getName()).
  • The value obtained using the JDK1.3+ 'Service Provider' specification (http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html) to locate a service named SPI.class.getName(). This is implemented internally, so there is not a dependency on JDK 1.3+.

• If the name of the implementation class is non-null, load that class. The class loaded is the first class loaded by the following sequence of class loaders:
  • Thread Context Class Loader
  • DiscoverSingleton's Caller's Class Loader
  • SPI's Class Loader
  • DiscoverSingleton's (this class or wrapper) Class Loader
  • System Class Loader
  An exception is thrown if the class cannot be loaded.
• If the name of the implementation class is null, AND the default implementation class name (defaultImpl) is null, then an exception is thrown.
• If the name of the implementation class is null, AND the default implementation class (defaultImpl) is non-null, then load the default implementation class. The class loaded is the first class loaded by the following sequence of class loaders:
  • SPI's Class Loader
  • DiscoverSingleton's (this class or wrapper) Class Loader
  • System Class Loader

  This limits the scope in which the default class loader can be found to the SPI, DiscoverSingleton, and System class loaders. The assumption here is that the default implementation is closely associated with the SPI or system, and is not defined in the user's application space.

  An exception is thrown if the class cannot be loaded.

• Verify that the loaded class implements the SPI: an exception is thrown if the loaded class does not implement the SPI.

IMPLEMENTATION NOTE - This implementation is modelled after the SAXParserFactory and DocumentBuilderFactory implementations (corresponding to the JAXP pluggability APIs) found in Apache Xerces.

Discover names of SPI implementation Classes from properties. The names are the non-null values, in order, obtained from the following resources:

• ManagedProperty.getProperty(SPI.class.getName());
• properties.getProperty(SPI.class.getName());

Discover names of SPI implementation Classes from properties. The names are the non-null values, in order, obtained from the following resources:

• ManagedProperty.getProperty(SPI.class.getName());
• properties.getProperty(SPI.class.getName());