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

In the Java world, different technologies are meant to be modular and "pluggable" into an application server. There is then a difference between

• the application server
  • [SPI]
• the pluggable technology
  • [API]
• the end user application

Two examples of such technologies are JTA (the transaction manager) and JCA (adapter for JMS or database). But there are others.

Implementer of such a pluggable technology must then implement the SPI to be pluggable in the app. server and provide an API to be used by the end-user application. An example from JCA is the ManagedConnection interface which is part of the SPI, and the Connection that is part of the end-user API.