Dubbo 系列JDK SPI 原理解析

private static final ProxyFactory PROXY_FACTORY = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();
private static final Protocol PROTOCOL = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
……
Invoker<?> invoker = PROXY_FACTORY.getInvoker(ref, (Class) interfaceClass, url);
DelegateProviderMetaDatainvoker wrapperInvoker = new DelegateProviderMetaDataInvoker(invoker, this);
Exporter<?> exporter = PROTOCOL.export(wrapperInvoker);
exporters.add(exporter);
……

public interface MySPI {
    void say();
}
public class HelloMySPI implements MySPI{
    @Override
    public void say() {
        System.out.println("HelloMySPI say:hello");
    }
}
public class GoodbyeMySPI implements MySPI {
    @Override
    public void say() {
        System.out.println("GoodbyeMySPI say:Goodbye");
    }
}
public static void main(String[] args) {
    ServiceLoader<MySPI> serviceLoader = ServiceLoader.load(MySPI.class);
    Iterator<MySPI> iterator = serviceLoader.iterator();
  // 开始迭代执行
    while (iterator.hasNext()) {
        MySPI spi = iterator.next();
        spi.say();
    }
}
//配置文件 org.daley.spi.demo.MySPI
org.daley.spi.demo.GoodbyeMySPI
org.daley.spi.demo.HelloMySPI
//输出如下：
//GoodbyeMySPI say:Goodbye
//HelloMySPI say:hello

public static <S> ServiceLoader<S> load(Class<S> service) {
    ClassLoader cl = Thread.currentThread().getContextClassLoader();
    return ServiceLoader.load(service, cl);
}
public static <S> ServiceLoader<S> load(Class<S> service,
                                            ClassLoader loader)
{
    return new ServiceLoader<>(service, loader);
}
private ServiceLoader(Class<S> svc, ClassLoader cl) {
  // 绑定接口和类加载器
    service = Objects.requireNonNull(svc, "Service interface cannot be null");
    loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
    acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
  // 加载
    reload();
}

private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
public void reload() {
    providers.clear();
    lookupIterator = new LazyIterator(service, loader);
}
private class LazyIterator implements Iterator<S>{
  private LazyIterator(Class<S> service, ClassLoader loader) {
        this.service = service;
        this.loader = loader;
  }
}

public boolean hasNext() {
    if (acc == null) {
        return hasNextService();
    } else {
      ……
    }
}
// pengding中保存了配置文件中的实现类名，按行迭代
Iterator<String> pending = null;
private boolean hasNextService() {
      ……
    if (configs == null) {
        try {
          // 设置配置文件路径 META-INF/services/org.daley.spi.demo.MySPI
            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);
        }
    }
  // 第一次遍历 pendind=null
    while ((pending == null) || !pending.hasNext()) {
        if (!configs.hasMoreElements()) {
            return false;
        }
        pending = parse(service, configs.nextElement());
    }
  // 按行拿到实现类名
    nextName = pending.next();
    return true;
}

public S next() {
    if (acc == null) {
        return nextService();
    } else {
        ……
    }
}
private S nextService() {
    if (!hasNextService())
        throw new NoSuchElementException();
        //当前正在迭代的实现类名
    String cn = nextName;
    nextName = null;
    Class<?> c = null;
    try {
      // 根据类路径加载class
        c = Class.forName(cn, false, loader);
    ……
      // 实例化实现类，并且保存在providers中
        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
}