Android-SPI学习笔记

public interface IPrinter {
  void print();
}

// module_a -- implementation project(':module_common')
// com.hearing.modulea.APrinter
public class APrinter implements IPrinter {
  @Override
  public void print() {
    Log.d("LLL", "APrinter");
  }
}
// src/main/resources/META-INF/services/com.hearing.common.IPrinter
// 可以配置多个实现类
com.hearing.modulea.APrinter

// ----------------------------------------------------------------//

// module_b -- implementation project(':module_common')
// com.hearing.moduleb.BPrinter
public class BPrinter implements IPrinter {
  @Override
  public void print() {
    Log.d("LLL", "BPrinter");
  }
}
// src/main/resources/META-INF/services/com.hearing.common.IPrinter
com.hearing.moduleb.BPrinter

// implementation project(':module_common')
ServiceLoader<IPrinter> printers = ServiceLoader.load(IPrinter.class);
for (IPrinter printer : printers) {
  printer.print();
}

public static <S> ServiceLoader<S> load(Class<S> service) {
  // 获取当前线程的类加载器
  ClassLoader cl = Thread.currentThread().getContextClassLoader();
  return ServiceLoader.load(service, cl);
}

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

private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

private ServiceLoader(Class<S> svc, ClassLoader cl) {
  service = Objects.requireNonNull(svc, "Service interface cannot be null");
  loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
  reload();
}

// Clear this loader's provider cache so that all providers will be reloaded.
public void reload() {
  providers.clear();
  // 创建了一个懒迭代器
  lookupIterator = new LazyIterator(service, loader);
}

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() {
      // 如果 knownProviders 缓存中已经存在，则直接返回，否则加载
      if (knownProviders.hasNext()) return knownProviders.next().getValue();
      return lookupIterator.next();
    }

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

private static final String PREFIX = "META-INF/services/";

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;
  }

  private boolean hasNextService() {
    if (nextName != null) {
      return true;
    }
    if (configs == null) {
      try {
        // 获取服务配置文件
        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;
      }
      // 解析服务配置
      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 {
      // 反射通过类加载器加载指定服务
      c = Class.forName(cn, false, loader);
    } catch (ClassNotFoundException x) {
      // throw Exception
    }
    if (!service.isAssignableFrom(c)) {
      // throw Exception
    }
    try {
      S p = service.cast(c.newInstance());
      providers.put(cn, p);
      return p;
    } catch (Throwable x) {
      // throw Exception
    }
    throw new Error();   // This cannot happen
  }

  public boolean hasNext() {
    return hasNextService();
  }

  public S next() {
    return nextService();
  }

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