接口

@SPI
public interface MyTestRegistry {
    /**
     * 注册服务
     */
    @Adaptive
    String register(URL url);
}

两个实现

@Activate(group = {"GROUP_A", "GROUP_B"}, order = 999)
public class EtcdRegistry implements MyTestRegistry {
    @Override
    public String register(URL url) {
        return "Etcd register already! ";
    }
}

@Activate(group = {"GROUP_B"})
public class ZookeeperRegistry implements MyTestRegistry {
    @Override
    public String register(URL url) {
        return "Zookeeper register already! ";
    }
}

配置

etcd=com..xxx.xxx.xxx.EtcdRegistry
zookeeper=com.xxx.xxx.xxx.ZookeeperRegistry

测试函数

public class SpiTest {
    public static void main(String[] args) {
        URL url = URL.valueOf("test://localhost/test")
                .addParameter("my.test.registry", "zookeeper");

        MyTestRegistry registry = ExtensionLoader
                .getExtensionLoader(MyTestRegistry.class)
                .getExtension("etcd");

        System.out.println("=================");
        System.out.println(registry.register(url));
        System.out.println("=================");

        registry = ExtensionLoader
                .getExtensionLoader(MyTestRegistry.class)
                .getAdaptiveExtension();

        registry.register(url);

        System.out.println(registry.register(url));
        System.out.println("=================");

        ExtensionLoader
                .getExtensionLoader(MyTestRegistry.class)
                .getActivateExtension(url, new String[]{}, "GROUP_B")
                .forEach(p -> System.out.println(p.register(url)));
        System.out.println("=================");
    }
}

输出

=================
Etcd register already! 
=================
Zookeeper register already! 
=================
Zookeeper register already! 
Etcd register already! 
=================

总结

@SPI 标注是一个扩展点，各种实现配置在配置文件里（key是实现名字，类似spring里的bean name，value是实现类）
@Adaptive 是自适应注解，根据使用时url里的参数，动态寻找实现类
@Activate 是激活注解，可以激活一串实现，常用于filter

注解参数说明

@SPI 的 value （默认使用的extension，可填配置里的etcd或zookeeper）
@Adaptive 的 value （调用时，动态匹配url里key的名字，不填为类的驼峰转换xxx.xx.xx）
@Activate 的 group （把所有实现，进行分组，激活时，激活整个组）
@Activate 的 value （类似@Adaptive的value）
@Activate 的 order （激活时排序）