difference between SPI and API?

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();
    }
}

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