简单分析JDK中「SPI」的原理

public interface Animal {
    String animalName () ;
}

public class CatAnimal implements Animal {
    @Override
    public String animalName() {
        System.out.println("Cat-Animal：布偶猫");
        return "Ragdoll";
    }
}

public class DogAnimal implements Animal {
    @Override
    public String animalName() {
        System.out.println("Dog-Animal：哈士奇");
        return "husky";
    }
}

butte.program.basics.spi.impl.CatAnimal
butte.program.basics.spi.impl.DogAnimal

public class SpiAnaly {
    public static void main(String[] args) {
        ServiceLoader<Animal> serviceLoader = ServiceLoader.load(Animal.class);
        Iterator<Animal> animalIterator = serviceLoader.iterator();
        while(animalIterator.hasNext()) {
            Animal animal = animalIterator.next();
            System.out.println("animal-name:" + animal.animalName());
        }
    }
}

Cat-Animal：布偶猫 \n animal-name:ragdoll
Dog-Animal：哈士奇 \n animal-name:husky

public final class ServiceLoader<S> implements Iterable<S> {
    // 配置文件目录
    private static final String PREFIX = "META-INF/services/";
    // 表示正在加载的服务的类或接口
    private final Class<S> service;
    // 类加载器用来定位，加载，实例化服务提供方
    private final ClassLoader loader;
    // 创建ServiceLoader时采用的访问控制上下文
    private final AccessControlContext acc;
    // 按实例化的顺序缓存服务提供方
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
    // 惰性查找迭代器
    private LazyIterator lookupIterator;
    /**
     * service：表示服务的接口或抽象类
     * loader: 类加载器
     */
    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }
    /**
     * ServiceLoader构造方法
     */
    private ServiceLoader(Class<S> svc, ClassLoader cl) {
        loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
        acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
        reload();
    }
    public void reload() {
        providers.clear();
        // 实例化迭代器
        lookupIterator = new LazyIterator(service, loader);
    }
    public static <S> ServiceLoader<S> load(Class<S> service,ClassLoader loader) {
        return new ServiceLoader<>(service, loader);
    }

    private class LazyIterator implements Iterator<S> {
        // 服务接口
        Class<S> service;
        // 类加载器
        ClassLoader loader;
        // 实现类URL
        Enumeration<URL> configs = null;
        // 实现类全名
        Iterator<String> pending = null;
        // 下个实现类全名
        String nextName = null;
    }
}

public Iterator<S> iterator() {
    return new Iterator<S>() {
        Iterator<Map.Entry<String,S>> knownProviders = providers.entrySet().iterator();
        public boolean hasNext() {
            if (knownProviders.hasNext())
                return true;
            return lookupIterator.hasNext();
        }
        public S next() {
            if (knownProviders.hasNext())
                return knownProviders.next().getValue();
            return lookupIterator.next();
        }
        public void remove() {
            throw new UnsupportedOperationException();
        }
    };
}

private class LazyIterator implements Iterator<S> {
    private boolean hasNextService() {
        // 1、拼接名称
        String fullName = PREFIX + service.getName();
        // 2、加载资源文件
        configs = loader.getResources(fullName);
        // 3、解析文件内容
        pending = parse(service, configs.nextElement());
        nextName = pending.next();
        return true;
    }
}

private class LazyIterator implements Iterator<S> {
    private S nextService() {
        // 1、通过全限定命名获取Class对象
        String cn = nextName;
        Class<?> c = Class.forName(cn, false, loader);
        // 2、实例化对象
        S p = service.cast(c.newInstance());
        // 3、放入缓存并返回该对象
        providers.put(cn, p);
        return p;
    }
}

public class DriverManager {
    private static void loadInitialDrivers() {
        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(java.sql.Driver.class);
                Iterator<Driver> driversIterator = loadedDrivers.iterator();
            }
        });
    }
}