We've already seen SPI mentioned before when we looked at the Preferences class, and how the implementation is selected and loaded, but what exactly is it? The Service Provider Interface is a somewhat generic term for an interface that a third party can implement (or a class, abstract or not, that can be extended) to provide extra functionality, replace existing components, and more.
In a nutshell, the author of the target system (for example, the JDK itself in our previous example) defines and publishes an interface. Ideally, this system would provide a default implementation, but that's not necessary in all cases. Any interested third party could then implement this interface, register ...
