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

The difference between API and SPI comes when an API additionally provides some concrete implementations. In that case, the service provider has to implement a few APIs (called SPI)

An example is JNDI:

JNDI provides interfaces & some classes for context lookup. The default way to lookup a context is provided in IntialContext. This class internally will use SPI interfaces (using NamingManager) for provider specific implementations.

See the JNDI Architecture below for better understanding.