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Discover singleton service providers. This

DiscoverSingleton instances are cached by the Discovery service, keyed by a combination of

• thread context class loader,
• groupContext, and
• SPI.

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.

DiscoverSingleton provides the find methods for locating and instantiating a singleton instance of an implementation of a service (SPI). Each form of find varies slightly, but they all perform the same basic function. The simplest find methods are intended for direct use by components looking for a service. If you are not sure which finder(s) to use, you can narrow your search to one of these:

• static Object find(Class spi);
• static Object find(Class spi, Properties properties);
• static Object find(Class spi, String defaultImpl);
• static Object find(Class spi, Properties properties, String defaultImpl);
• static Object find(Class spi, String propertiesFileName, String defaultImpl);
• static Object find(String groupContext, Class spi, Properties properties, String defaultImpl);
• static Object find(String groupContext, Class spi, String propertiesFileName, String defaultImpl);

• Examine an internal cache to determine if the desired service was previously identified and instantiated. If found in cache, return it.
• 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 (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.
• Create an instance of the class.

Variances for various forms of the find methods are discussed with each such method. Variances include the following concepts:

• rootFinderClass - a wrapper encapsulating a finder method (factory or other helper class). The root finder class is used to determine the 'real' caller, and hence the caller's class loader - thereby preserving knowledge that is relevant to finding the correct/expected implementation class.
• propertiesFileName - Properties may be specified directly, or by property file name. A property file is loaded using the same sequence of class loaders used to load the SPI implementation:
  • Thread Context Class Loader
  • DiscoverSingleton's Caller's Class Loader
  • SPI's Class Loader
  • DiscoverSingleton's (this class) Class Loader
  • System Class Loader
• groupContext - differentiates service providers for different logical groups of service users, that might otherwise be forced to share a common service and, more importantly, a common configuration of that service.

  The groupContext is used to qualify the name of the property file name: groupContext + '.' + propertiesFileName. If that file is not found, then the unqualified propertyFileName is used.

  In addition, groupContext is used to qualify the name of the system property used to find the service implementation by prepending the value of groupContext to the property name: groupContext> + '.' + SPI.class.getName(). Again, if a system property cannot be found by that name, then the unqualified property name is used.

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