Difference between SPI and API?

Question

What is the difference between Service Provider Interface (SPI) and Application Programming Interface (API)?

More specifically, for Java libraries, what makes them a

Answer 1

I suppose an SPI slots into a larger system by implementing certain features of an API, and then registering itself as being available via service lookup mechanisms. An API is used by the end-user application code directly, but may integrate SPI components. It's the difference between encapsulation and direct usage.