前言

累加器也是Redis場見的應用之一。值得注意的是Redis內的命令諸如incr等命令皆為原子操作。但是多命令組合而成的操作，為非原子操作。（如abc=incr(hello) get(hello) set(hello,abc)。此類操作需要使用Multi或Lua指令碼保證Redis內操作的正確性。

Demos

/**
 * 記錄Redis累加器。
 * 
 * */

public class RedisCounterDemo {

    // demo1: first not perfect
    public static Jedis jedis = new JedisPoolManager().getJedis();

    public
 
 static long INCR_END_COUNT = 10;
    public static long DECR_END_COUNT = 0;


    // 建議操作為 自增在前 操作在後。
    // 如果 操作在前 自增在後。會導致先執行操作，後判斷。易導致資料總值變多。(即秒殺時，顯示無庫存。仍然認為有。造成損失。)
    public static boolean increase(String key){
        Long count = jedis.incr(key);
        if(count > INCR_END_COUNT){
            // do something
 

            return true;
        }
        return false;
    }

    public static boolean decrease(String key){
        Long count = jedis.decr(key);
        if(count > DECR_END_COUNT){
            // do something
            return true;
        }
        return false;
    }

    // 使用LUA 判斷 
    // < LUA ERROR ? why? warn the type
 

    // have the string key first. 
    public static boolean execLua(String key){
        String script = "if tonumber(redis.call('get', KEYS[1])) > 100  then return redis.call('incr',KEYS[1]) else return 0 end";
        Object result = jedis.eval(script, Collections.singletonList(key), Collections.singletonList(key));
        if ((long)result > 0l) {
            // do something
            System.out.println("Hello");
            return true;
        }
        return false;
    }

    // 使用multi事務進行處理
    public static boolean execMulti(String key){
        return false;
    }

    public static void main(String[] args) {
        execLua("hello");
    }

}

//to number
//https://blog.csdn.net/u013916933/article/details/54962768

//https://stackoverflow.com/questions/21707021/lua-script-for-redis-which-sums-the-values-of-keys

//Exception in thread "main" redis.clients.jedis.exceptions.JedisDataException: ERR Error running script (call to f_1fb04376fc5dc524aeac5a008daa880db5104604): @user_script:1: user_script:1: attempt to compare number with nil 
//at redis.clients.jedis.Protocol.processError(Protocol.java:127)
//at redis.clients.jedis.Protocol.process(Protocol.java:161)
//at redis.clients.jedis.Protocol.read(Protocol.java:215)
//at redis.clients.jedis.Connection.readProtocolWithCheckingBroken(Connection.java:340)
//at redis.clients.jedis.Connection.getOne(Connection.java:322)
//at redis.clients.jedis.Jedis.getEvalResult(Jedis.java:2731)
//at redis.clients.jedis.Jedis.eval(Jedis.java:2671)
//at redis.clients.jedis.Jedis.eval(Jedis.java:2719)
//at com.yanxml.redis.demos.count.RedisCounterDemo.execLua(RedisCounterDemo.java:47)
//at com.yanxml.redis.demos.count.RedisCounterDemo.main(RedisCounterDemo.java:62)

Reference