Java SPI用法案例详解

public interface UploadCDN {
    void upload(String url);
}

public class QiyiCDN implements UploadCDN {  //上传爱奇艺cdn
    @Override
    public void upload(String url) {
        System.out.println("upload to qiyi cdn");
    }
}

public class ChinaNetCDN implements UploadCDN {//上传网宿cdn
    @Override
    public void upload(String url) {
        System.out.println("upload to chinaNet cdn");
    }
}

public static void main(String[] args) {
    ServiceLoader<UploadCDN> uploadCDN = ServiceLoader.load(UploadCDN.class);
    for (UploadCDN u : uploadCDN) {
        u.upload("filePath");
    }
}

public final class ServiceLoader<S> implements Iterable<S> {


    //扫描目录前缀
    private static final String PREFIX = "META-INF/services/";

    // 被加载的类或接口
    private final Class<S> service;

    // 用于定位、加载和实例化实现方实现的类的类加载器
    private final ClassLoader loader;

    // 上下文对象
    private final AccessControlContext acc;

    // 按照实例化的顺序缓存已经实例化的类
    private LinkedHashMap<String, S> providers = new LinkedHashMap<>();

    // 懒查找迭代器
    private java.util.ServiceLoader.LazyIterator lookupIterator;

    // 私有内部类，提供对所有的service的类的加载与实例化
    private class LazyIterator implements Iterator<S> {
        Class<S> service;
        ClassLoader loader;
        Enumeration<URL> configs = null;
        String nextName = null;

        //...
        private boolean hasNextService() {
            if (configs == null) {
                try {
                    //获取目录下所有的类
                    String fullName = PREFIX + service.getName();
                    if (loader == null)
                        configs = ClassLoader.getSystemResources(fullName);
                    else
                        configs = loader.getResources(fullName);
                } catch (IOException x) {
                    //...
                }
                //....
            }
        }

        private S nextService() {
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                //反射加载类
                c = Class.forName(cn, false, loader);
            } catch (ClassNotFoundException x) {
            }
            try {
                //实例化
                S p = service.cast(c.newInstance());
                //放进缓存
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                //..
            }
            //..
        }
    }
}

@SPI("random")
public interface LoadBalance {
    @Adaptive({"loadbalance"})
    <T> Invoker<T> select(List<Invoker<T>> var1, URL var2, Invocation var3) throws RpcException;
}

public class DemoLoadBalance implements LoadBalance {

    @Override
    public <T> Invoker<T> select(List<Invoker<T>> invokers, URL url, Invocation invocation) throws RpcException {
        System.out.println("my demo loadBalance is used, hahahahh");
        return invokers.get(0);//选择第一个
    }
}

<dubbo:reference id="helloService" interface="com.dubbo.spi.demo.api.IHelloService" loadbalance="demo" />