Java SPI

通过一个案例来看SPI

public interface DemoSPI {
    void echo();
}
public class FirstImpl implements DemoSPI{
    @Override
    public void echo() {
        System.out.println("first echo");
    }
}
public class SecondImpl implements DemoSPI{
    @Override
    public void echo() {
        System.out.println("second echo");
    }
}
public class Main {
    public static void main(String[] args) {
        ServiceLoader<DemoSPI> loader = ServiceLoader.load(DemoSPI.class);
        Iterator<DemoSPI> iterator = loader.iterator();
        while (iterator.hasNext()) {
          iterator.next().echo();
        }
        System.out.println("加载完毕！！！！！");
    }
}

SPI :Service Provider Interface, 服务提供接口。
JDK SPI：ServiceLoader.load
使用load方法频率高，容易影响IO吞吐和内存消耗, 可以看到回去静态资源目录下读取文件。
使用load方法想要获取指定的实现类，需要自己进行遍历并编写各种比较代码，可能实现有多个所以需要比对。

Dubbo SPI: ApplicationModel.defaultModel().getExtensionLoader
增加缓存，来降低磁盘IO访问及减少对象的生成
使用Map的hash查找，来提升检索指定实现类的性能。

@SPI
public interface DubboSPI {
    void echo();
}
public class DubboFirstSPI implements DubboSPI{
    @Override
    public void echo() {
        System.out.println("first dubbo spi");
    }
}
public class DubboSecondSPI implements DubboSPI{
    @Override
    public void echo() {
        System.out.println("second spi");
    }
}

public class DubboMain {
    public static void main(String[] args) {
        ExtensionLoader<DubboSPI> loader = ExtensionLoader.getExtensionLoader(DubboSPI.class);
        DubboSPI firstf = loader.getExtension("first");
        DubboSPI second = loader.getExtension("second");
        firstf.echo();
        second.echo();
    }
}

com.liyong.leran.dubbospi.DubboSPI里面的内容：

first=com.liyong.leran.dubbospi.DubboFirstSPI
second=com.liyong.leran.dubbospi.DubboSecondSPI

public T getExtension(String name, boolean wrap) {
      if (StringUtils.isEmpty(name)) {
          throw new IllegalArgumentException("Extension name == null");
      }
      if ("true".equals(name)) {
          return getDefaultExtension();
      }
      final Holder<Object> holder = getOrCreateHolder(name);
      Object instance = holder.get();
      if (instance == null) {
          synchronized (holder) {
              instance = holder.get();
              if (instance == null) {
                  instance = createExtension(name, wrap);
                  holder.set(instance);
              }
          }
      }
      return (T) instance;
  }

// 走缓存的方法
private Holder<Object> getOrCreateHolder(String name) {
      Holder<Object> holder = cachedInstances.get(name);
      if (holder == null) {
          cachedInstances.putIfAbsent(name, new Holder<>());
          holder = cachedInstances.get(name);
      }
      return holder;
  }