SPI

public class ProviderInterceptorFactory {
   //通过ExtensionLoader来加载interceptor
   private static List<ProviderInterceptor> interceptors = ExtensionLoader
         .getExtensionList(ProviderInterceptor.class);
   // 也可以手动调用这个方法来注册
   public static boolean registerInterceptor(ProviderInterceptor providerContextInterceptor) {
      if (!interceptors.contains(providerContextInterceptor)) {
         return interceptors.add(providerContextInterceptor);
      }
      return false;
   }
   public static boolean unregisterInterceptor(ProviderInterceptor providerContextInterceptor) {
      return interceptors.remove(providerContextInterceptor);
   }
   public static List<ProviderInterceptor> getInterceptors() {
      return interceptors;
   }
}

public final class ExtensionLoader {

   //使用map来存储接口对应的实现，如果map中没有，则通过serviceloader来加载
   private static Map<Class<?>, Object> extensionMap = new ConcurrentHashMap<Class<?>, Object>();

   private static Map<Class<?>, List<?>> extensionListMap = new ConcurrentHashMap<Class<?>, List<?>>();

   private ExtensionLoader() {
   }

   public static <T> T getExtension(Class<T> clazz) {
      T extension = (T) extensionMap.get(clazz);
      if (extension == null) {
         extension = newExtension(clazz);
         if (extension != null) {
            extensionMap.put(clazz, extension);
         }
      }
      return extension;
   }

   public static <T> List<T> getExtensionList(Class<T> clazz) {
      List<T> extensions = (List<T>) extensionListMap.get(clazz);
      if (extensions == null) {
         extensions = newExtensionList(clazz);
         if (!extensions.isEmpty()) {
            extensionListMap.put(clazz, extensions);
         }
      }
      return extensions;
   }

   public static <T> T newExtension(Class<T> clazz) {
      //  here
      ServiceLoader<T> serviceLoader = ServiceLoader.load(clazz);
      for (T service : serviceLoader) {
         return service;
      }
      return null;
   }

   public static <T> List<T> newExtensionList(Class<T> clazz) {
      ServiceLoader<T> serviceLoader = ServiceLoader.load(clazz);
      List<T> extensions = new ArrayList<T>();
      for (T service : serviceLoader) {
         extensions.add(service);
      }
      return extensions;
   }
}

重点就是ServiceLoader.load

// ServiceLoader 实现了Iterable
public final class ServiceLoader<S>
    implements Iterable<S>
{
    //目录路径
    private static final String PREFIX = "META-INF/services/";

    // 正在加载的类或接口
    private Class<S> service;

    //用来实例化的classloader
    private ClassLoader loader;

    // 按实例化的顺序缓存实现类
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

    //ServiceLoader自定义的一个内部类，主要实现了懒加载
    private LazyIterator lookupIterator;

    
    public void reload() {
        providers.clear();
        lookupIterator = new LazyIterator(service, loader);
    }

    private ServiceLoader(Class<S> svc, ClassLoader cl) {
        service = svc;
        loader = cl;
        reload();
    }

    private static void fail(Class service, String msg, Throwable cause)
        throws ServiceConfigurationError
    {
        throw new ServiceConfigurationError(service.getName() + ": " + msg,
                                            cause);
    }

    private static void fail(Class service, String msg)
        throws ServiceConfigurationError
    {
        throw new ServiceConfigurationError(service.getName() + ": " + msg);
    }

    private static void fail(Class service, URL u, int line, String msg)
        throws ServiceConfigurationError
    {
        fail(service, u + ":" + line + ": " + msg);
    }

    //解析文件，将文件中的name放到list中
    private int parseLine(Class service, URL u, BufferedReader r, int lc,
                          List<String> names)
        throws IOException, ServiceConfigurationError
    {
        String ln = r.readLine();
        if (ln == null) {
            return -1;
        }
        int ci = ln.indexOf('#');
        if (ci >= 0) ln = ln.substring(0, ci);
        ln = ln.trim();
        int n = ln.length();
        if (n != 0) {
            if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
                fail(service, u, lc, "Illegal configuration-file syntax");
            int cp = ln.codePointAt(0);
            if (!Character.isJavaIdentifierStart(cp))
                fail(service, u, lc, "Illegal provider-class name: " + ln);
            for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
                cp = ln.codePointAt(i);
                if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
                    fail(service, u, lc, "Illegal provider-class name: " + ln);
            }
            // 加载过的不放在这个list里
            if (!providers.containsKey(ln) && !names.contains(ln))
                names.add(ln);
        }
        return lc + 1;
    }

   
    private Iterator<String> parse(Class service, URL u)
        throws ServiceConfigurationError
    {
        InputStream in = null;
        BufferedReader r = null;
        ArrayList<String> names = new ArrayList<>();
        try {
            in = u.openStream();
            r = new BufferedReader(new InputStreamReader(in, "utf-8"));
            int lc = 1;
            while ((lc = parseLine(service, u, r, lc, names)) >= 0);
        } catch (IOException x) {
            fail(service, "Error reading configuration file", x);
        } finally {
            try {
                if (r != null) r.close();
                if (in != null) in.close();
            } catch (IOException y) {
                fail(service, "Error closing configuration file", y);
            }
        }
        return names.iterator();
    }

    //懒加载的实现
    private class LazyIterator
        implements Iterator<S>
    {

        Class<S> service;
        ClassLoader loader;
        Enumeration<URL> configs = null;
        Iterator<String> pending = null;
        String nextName = null;

        private LazyIterator(Class<S> service, ClassLoader loader) {
            this.service = service;
            this.loader = loader;
        }

        public boolean hasNext() {
            if (nextName != null) {
                return true;
            }
            if (configs == null) {
                try {
                    String fullName = PREFIX + service.getName();
                    if (loader == null)
                        configs = ClassLoader.getSystemResources(fullName);
                    else
                        configs = loader.getResources(fullName);
                } catch (IOException x) {
                    fail(service, "Error locating configuration files", x);
                }
            }
            while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();
            return true;
        }

        public S next() {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                c = Class.forName(cn, false, loader);
            } catch (ClassNotFoundException x) {
                fail(service,
                     "Provider " + cn + " not found");
            }
            if (!service.isAssignableFrom(c)) {
                fail(service,
                     "Provider " + cn  + " not a subtype");
            }
            try {
                //在next到某个实现类时，才真正的加载实例
                S p = service.cast(c.newInstance());
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                fail(service,
                     "Provider " + cn + " could not be instantiated",
                     x);
            }
            throw new Error();          // This cannot happen
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }

    }

    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();
            }

        };
    }
    public static <S> ServiceLoader<S> load(Class<S> service,
                                            ClassLoader loader)
    {
        return new ServiceLoader<>(service, loader);
    }

  
    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

    public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
        ClassLoader cl = ClassLoader.getSystemClassLoader();
        ClassLoader prev = null;
        while (cl != null) {
            prev = cl;
            cl = cl.getParent();
        }
        return ServiceLoader.load(service, prev);
    }
}