SPI和API的区别

     今天在阅读Dubbo的开发者文档时，看到两个名词：SPI和API。API（Application Programming Interface）这个名词常常会遇到，咱们通常称之为“接口”。可是SPI是什么呢？跟API有什么区别？带着这两个问题，我google一下：

What is the difference between Service Provider Interface (SPI) and Application Programming Interface (API)?
More specifically, for Java libraries, what makes them an API and/or SPI?
the API is the description of classes/interfaces/methods/... that you call and use to achieve a goal
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.
Sometimes SPI and API overlap. 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.
The Connection interface on the other hand 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。