负载均衡轮询算法

随机算法简单可行，但不够均衡，在极端情况下会造成一台服务器一直收到请求，另一个服务器一直没收到请求。所以这时候就需要轮询算法。通过依次按顺序调用服务器列表中的服务器即可。例如服务器列表中有ABC三台服务器，一个自增数字，每次自增完取3的余数，0的话取服务器A，1的话取服务器B，2的话取服务器C即可。

public class RandomRobin {
    private static AtomicInteger index = new AtomicInteger(0);
    public static String getServer(){
        ArrayList<String> serverList = new ArrayList<>(ServerIps.LIST);
        if (index.get() >= serverList.size()) {
            index.set(0);
        }
        return serverList.get(index.getAndIncrement());
    }
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            System.out.println(getServer());
        }
    }
}

缺点：循环的时间复杂度是O(n)，性能浪费。
多线程情况：

public class RandomRobin {
    private static AtomicInteger index = new AtomicInteger(0);
    static ReentrantLock lock = new ReentrantLock();
    private static ArrayList<String> serverList = new ArrayList<>(ServerIps.LIST);
    public static String getServer() {
        if (index.get() >= serverList.size()) {
            index.set(0);
        }
        return serverList.get(index.getAndIncrement());
    }
    public static void main(String[] args) {
        CountDownLatch count = new CountDownLatch(5);
        CyclicBarrier barrier = new CyclicBarrier(5);
        ExecutorService pool = Executors.newFixedThreadPool(5);
        for (int i = 0; i < 100; i++) {
            pool.execute(() -> {
                try {
                    barrier.await();
                    lock.lock();
                    System.out.println("线程" + Thread.currentThread().getName() + "开始执行");
                    System.out.println(getServer());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                } finally {
                    lock.unlock();
                    count.countDown();
                }
            });
        }
        try {
            count.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        pool.shutdown();
    }
}