定义一组接口：public interface MyDriver 

实现为：public class JdDriver implements MyDriver  public class AliDriver implements MyDriver 

public void invoker(){    ServiceLoader<MyDriver>  serviceLoader = ServiceLoader.load(MyDriver.class);    Iterator<MyDriver> drivers = serviceLoader.iterator();    boolean isNotFound = true;    while (drivers.hasNext()){        isNotFound = false;        drivers.next().load();    }    if(isNotFound){        throw new RuntimeException("一个驱动实现类都不存在");    }}

public class App {    public static void main( String[] args )    {        DriverFactory factory = new DriverFactory();        factory.invoker();    }}

ServiceLoader<MyDriver>  serviceLoader = ServiceLoader.load(MyDriver.class);Iterator<MyDriver> drivers = serviceLoader.iterator();

public final class ServiceLoader<S> implements Iterable<S>{//配置文件的路径 private static final String PREFIX = "META-INF/services/";    // 代表被加载的类或者接口    private final Class<S> service;    // 用于定位，加载和实例化providers的类加载器    private final ClassLoader loader;    // 创建ServiceLoader时采用的访问控制上下文    private final AccessControlContext acc;    // 缓存providers，按实例化的顺序排列    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();    // 懒查找迭代器，真正加载服务的类    private LazyIterator lookupIterator;   //服务提供者查找的迭代器    private class LazyIterator        implements Iterator<S>    { .....private boolean hasNextService() {            if (nextName != null) {                return true;            }            if (configs == null) {                try {//全限定名：com.xxxx.xxx                    String fullName = PREFIX + service.getName();                    if (loader == null)                        configs = ClassLoader.getSystemResources(fullName);                    else                        configs = loader.getResources(fullName);                }            }            while ((pending == null) || !pending.hasNext()) {                if (!configs.hasMoreElements()) {                    return false;                }                pending = parse(service, configs.nextElement());            }            nextName = pending.next();            return true;        }

        private S nextService() {            if (!hasNextService())                throw new NoSuchElementException();            String cn = nextName;            nextName = null;            Class<?> c = null;            try {//通过反射获取                c = Class.forName(cn, false, loader);            }            if (!service.isAssignableFrom(c)) {                fail(service, "Provider " + cn  + " not a subtype");            }            try {                S p = service.cast(c.newInstance());                providers.put(cn, p);                return p;            }        }........