一 接口声明

@SPI(FailoverCluster.NAME)
public interface Cluster {

    /**
     * Merge the directory invokers to a virtual invoker.
     *
     * @param <T>
     * @param directory
     * @return cluster invoker
     * @throws RpcException
     */
    @Adaptive
    <T> Invoker<T> join(Directory<T> directory) throws RpcException;

}

默认实现是FailoverCluster，扩展实现名称是cluster

二 自适应扩展类

package com.alibaba.dubbo.rpc.cluster;

import com.alibaba.dubbo.common.extension.ExtensionLoader;

public class Cluster$Adaptive implements com.alibaba.dubbo.rpc.cluster.Cluster {
    public com.alibaba.dubbo.rpc.Invoker join(com.alibaba.dubbo.rpc.cluster.Directory arg0) throws com.alibaba.dubbo.rpc.RpcException {
        if (arg0 == null)
            throw new IllegalArgumentException("com.alibaba.dubbo.rpc.cluster.Directory argument == null");
        if (arg0.getUrl() == null)
            throw new IllegalArgumentException("com.alibaba.dubbo.rpc.cluster.Directory argument getUrl() == null");
        com.alibaba.dubbo.common.URL url = arg0.getUrl();
        String extName = url.getParameter("cluster", "failover");
        if (extName == null)
            throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.cluster.Cluster) name from url(" + url.toString() + ") use keys([cluster])");
        com.alibaba.dubbo.rpc.cluster.Cluster extension = (com.alibaba.dubbo.rpc.cluster.Cluster) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.cluster.Cluster.class).getExtension(extName);
        return extension.join(arg0);
    }
}