《Redis应用实例》Java实现（27）：定长队列和淘汰队列

定长队列和淘汰队列都是固定长度的队列。
不同之处是定长队列超过定长后后面的元素不添加了，而淘汰队列超过定长后面添加的元素添加后最前面的元素会删掉。
 

package com.foxbill.redisinaction;

import redis.clients.jedis.Jedis;
import redis.clients.jedis.Pipeline;
import java.util.List;

/**
 * 定长队列和淘汰队列
 */
public class Chapter27 {
    static private String KEY_QUEUE = "Chapter27:fixedlenqueue";
    static private int MAX_QUEUE_SIZE = 10;

    static public void start(Jedis jedis) {
        cleanData(jedis);
        testFixedLen(jedis);
        cleanData(jedis);
        testEliminate(jedis);

    }

    //清除测试数据
    private static void cleanData(Jedis jedis) {
        jedis.del(KEY_QUEUE);
    }

    //测试定长队列
    private static void testFixedLen(Jedis jedis) {
        for (int i = 0; i <12; i++) {
            addFixedLen(jedis,"item:"+String.valueOf(i));
        }
        //打印队列
        List<String> list = jedis.lrange(KEY_QUEUE, 0, -1);
        String result = String.join(",", list);
        // 输出结果
        System.out.println(result);
    }

    //测试淘汰队列
    private static void testEliminate(Jedis jedis) {
        for (int i = 0; i <12; i++) {
            addEliminate(jedis,"item:"+String.valueOf(i));
        }
        //打印队列
        List<String> list = jedis.lrange(KEY_QUEUE, 0, -1);
        String result = String.join(",", list);
        // 输出结果
        System.out.println(result);
    }

    //定长队列添加元素
    private static void addFixedLen(Jedis jedis,String item) {
        Pipeline pipelined = jedis.pipelined();
        //先添加，后截断
        pipelined.rpush(KEY_QUEUE, item);
        //保留最旧的MAX_QUEUE_SIZE个元素
        pipelined.ltrim(KEY_QUEUE, 0, MAX_QUEUE_SIZE-1);
        pipelined.syncAndReturnAll();
    }

    //淘汰队列添加元素
    private static void addEliminate(Jedis jedis,String item) {
        Pipeline pipelined = jedis.pipelined();
        //先添加，后截断
        pipelined.rpush(KEY_QUEUE, item);
        //保留最新的MAX_QUEUE_SIZE个元素
        pipelined.ltrim(KEY_QUEUE, 0-MAX_QUEUE_SIZE, -1);
        pipelined.syncAndReturnAll();
    }

}