package com.hiwei.spi.demo;
public interface Animal {
    void speak();
}

package com.hiwei.spi.demo;
public class Cat implements Animal {
    @Override
    public void speak() {
        System.out.println("喵喵喵！");
    }
}

package com.hiwei.spi.demo;
public class Dog implements Animal {
    @Override
    public void speak() {
        System.out.println("汪汪汪！");
    }
}

package com.hiwei.spi;
import com.hiwei.spi.demo.Animal;
import java.sql.SQLException;
import java.util.ServiceLoader;
public class SpiDemoApplication {
    public static void main(String[] args){
    	//會根據文件找到對應的實現(xiàn)類
        ServiceLoader<Animal> load = ServiceLoader.load(Animal.class);
        //執(zhí)行實現(xiàn)類方法
        for (Animal animal : load) {
            animal.speak();
        }
    }
}

package com.example.demo;
import com.hiwei.spi.demo.Animal;
public class Pig implements Animal {
    @Override
    public void speak() {
        System.out.println("哼哼哼！");
    }
}

package com.example.demo;
import com.hiwei.spi.SpiDemoApplication;
public class DemoApplication {
    public static void main(String[] args) {
        SpiDemoApplication.main(args);
    }
}

<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.27</version>
</dependency>

DriverManager.getConnection("url", "username", "password");

    static {
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }

AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {
				//獲取Driver.class的實現(xiàn)類
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator<Driver> driversIterator = loadedDrivers.iterator();

                /* Load these drivers, so that they can be instantiated.
                 * It may be the case that the driver class may not be there
                 * i.e. there may be a packaged driver with the service class
                 * as implementation of java.sql.Driver but the actual class
                 * may be missing. In that case a java.util.ServiceConfigurationError
                 * will be thrown at runtime by the VM trying to locate
                 * and load the service.
                 *
                 * Adding a try catch block to catch those runtime errors
                 * if driver not available in classpath but it's
                 * packaged as service and that service is there in classpath.
                 */
                try{
                    while(driversIterator.hasNext()) {
                        driversIterator.next();
                    }
                } catch(Throwable t) {
                // Do nothing
                }
                return null;
            }
        });

//生成默認主鍵生成策略
private ShardingKeyGenerator createDefaultKeyGenerator(final KeyGeneratorConfiguration keyGeneratorConfiguration) {
     //SPI服務發(fā)現(xiàn)
     ShardingKeyGeneratorServiceLoader serviceLoader = new ShardingKeyGeneratorServiceLoader();
     return containsKeyGeneratorConfiguration(keyGeneratorConfiguration)
             ? serviceLoader.newService(keyGeneratorConfiguration.getType(), keyGeneratorConfiguration.getProperties()) : 					serviceLoader.newService();
    }

    static {
        //SPI: 加載主鍵生成策略
        NewInstanceServiceLoader.register(ShardingKeyGenerator.class);
    }

    public static <T> void register(final Class<T> service) {
    	//服務發(fā)現(xiàn)
        for (T each : ServiceLoader.load(service)) {
            registerServiceClass(service, each);
        }
    }

	private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
		Map<String, List<String>> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}
		result = new HashMap<>();
		try {
			//加載文件中的類
			Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
			while (urls.hasMoreElements()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
				for (Map.Entry<?, ?> entry : properties.entrySet()) {
					String factoryTypeName = ((String) entry.getKey()).trim();
					String[] factoryImplementationNames =
							StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
					for (String factoryImplementationName : factoryImplementationNames) {
						result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
								.add(factoryImplementationName.trim());
					}
				}
			}
			// Replace all lists with unmodifiable lists containing unique elements
			result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
					.collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
			cache.put(classLoader, result);
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
		return result;
	}