dubbo-cluster/src/main/java/com/alibaba/dubbo/rpc/cluster/loadbalance/RoundRobinLoadBalance.java

/*
 * Copyright 1999-2011 Alibaba Group.
 *  
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *  
 *      http://www.apache.org/licenses/LICENSE-2.0
 *  
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.dubbo.rpc.cluster.loadbalance;

import java.util.List;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import com.alibaba.dubbo.common.Extension;
import com.alibaba.dubbo.common.utils.AtomicPositiveInteger;
import com.alibaba.dubbo.rpc.Invoker;
import com.alibaba.dubbo.rpc.Invocation;

/**
 * Round robin load balance.
 *
 * @author qian.lei
 * @author william.liangf
 */
@Extension(RoundRobinLoadBalance.NAME)
public class RoundRobinLoadBalance extends AbstractLoadBalance {
    public static final String NAME = "roundrobin"; 
    
    private final ConcurrentMap<String, AtomicPositiveInteger> sequences = new ConcurrentHashMap<String, AtomicPositiveInteger>();

    protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, Invocation invocation) {
        String key = invokers.get(0).getInterface().getName() + "." + invocation.getMethodName();// + System.identityHashCode(invokers);
        AtomicPositiveInteger sequence = sequences.get(key);
        if (sequence == null) {
            sequences.putIfAbsent(key, new AtomicPositiveInteger());
            sequence = sequences.get(key);
        }
        // 取模轮循
        return invokers.get(sequence.getAndIncrement() % invokers.size());
    }

}