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.
// Service provider framework sketch // Service interface public interface Service { ... // Service-specific methods go here } // Service provider interface public interface Provider { Service newService(); } // Noninstantiable class for service registration and access public class Services { private Services() { } // Prevents instantiation (Item 4) // Maps service names to services private static final Map<String, Provider> providers = new ConcurrentHashMap<String, Provider>(); public static final String DEFAULT_PROVIDER_NAME = "<def>"; // Provider registration API public static void registerDefaultProvider(Provider p) { registerProvider(DEFAULT_PROVIDER_NAME, p); } public static void registerProvider(String name, Provider p){ providers.put(name, p); } // Service access API public static Service newInstance() { return newInstance(DEFAULT_PROVIDER_NAME); } public static Service newInstance(String name) { Provider p = providers.get(name); if (p == null) throw new IllegalArgumentException( "No provider registered with name: " + name); return p.newService(); } }