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 API is the description of classes/interfaces/methods/... that you call and use to achieve a goal, and
• the SPI is the description of classes/interfaces/methods/... that you extend and implement to achieve a goal.

Put differently, the API tells you what a specific class/method does for you, and the SPI tells you what you must do to conform.

Usually API and SPI are separate. For example, in JDBC the Driver class is part of the SPI: If you simply want to use JDBC, you don't need to use it directly, but everyone who implements a JDBC driver must implement that class.

Sometimes they overlap, however. The Connection interface is both SPI and API: You use it routinely when you use a JDBC driver and it needs to be implemented by the developer of the JDBC driver.