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Java SPI

通过一个案例来看SPI

public interface DemoSPI {void echo();
}
public class FirstImpl implements DemoSPI{@Overridepublic void echo() {System.out.println("first echo");}
}
public class SecondImpl implements DemoSPI{@Overridepublic 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{@Overridepublic void echo() {System.out.println("first dubbo spi");}
}
public class DubboSecondSPI implements DubboSPI{@Overridepublic 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;}