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Java并發(fā)編程service層處理并發(fā)事務(wù)加鎖可能會(huì)無(wú)效問(wèn)題

更新時(shí)間：2023年07月27日 09:43:58 作者：煙雨樓臺(tái)笑江湖

這篇文章主要介紹了Java并發(fā)編程service層處理并發(fā)事務(wù)加鎖可能會(huì)無(wú)效問(wèn)題，具有很好的參考價(jià)值，希望對(duì)大家有所幫助。如有錯(cuò)誤或未考慮完全的地方，望不吝賜教

問(wèn)題描述

近期寫了一個(gè)單體架構(gòu)秒殺的功能，在對(duì)商品庫(kù)存進(jìn)行扣減，有線程安全問(wèn)題，因此加了Lock鎖進(jìn)行同步，但發(fā)現(xiàn)加鎖后并沒有控制住庫(kù)存線程安全的問(wèn)題，導(dǎo)致庫(kù)存仍被超發(fā)。

輸出一下代碼：

@Override
@Transactional(rollbackFor = Exception.class)
public Result startSeckillLock(long seckillId, long userId) {
	/**
	  * 這里加鎖，還是會(huì)出現(xiàn)超賣
      *
      * 因?yàn)檫M(jìn)入service方法中時(shí)，spring事務(wù)已經(jīng)開啟，隔離級(jí)別默認(rèn)是可重復(fù)讀，
      * 因?yàn)槭聞?wù)先開啟，后加鎖，隔離級(jí)別為可重復(fù)讀的情況下，當(dāng)前線程讀不到其他線程更新的數(shù)據(jù)，
      * 所以就會(huì)出現(xiàn)超賣的情況
      *
      * 下面方法通過(guò)aop加鎖，order = 1，在事務(wù)開啟之前加鎖
      *
      * 還有就是直接在controller中加鎖
      */
	lock.lock();
    try {
		//校驗(yàn)庫(kù)存
        String nativeSql = "SELECT number FROM seckill WHERE seckill_id = ?";
        Object object =  dynamicQuery.nativeQueryObject(nativeSql, new Object[]{seckillId});
        Long number =  ((Number) object).longValue();
        System.out.println(">>>>>>>>>>>>>>>>>>>>>> number : {}" + number);
        if(number > 0){
            //扣庫(kù)存
            nativeSql = "UPDATE seckill  SET number=? WHERE seckill_id = ?";
            dynamicQuery.nativeExecuteUpdate(nativeSql, new Object[]{number - 1, seckillId});
            //創(chuàng)建訂單
            SuccessKilled killed = new SuccessKilled();
            killed.setSeckillId(seckillId);
            killed.setUserId(userId);
            killed.setState((short)0);
            killed.setCreateTime(new Timestamp(System.currentTimeMillis()));
            dynamicQuery.save(killed);
            return Result.ok(SeckillStatEnum.SUCCESS);
            //支付
        }else{
            return Result.error(SeckillStatEnum.END);
        }
    } finally {
        lock.unlock();
    }
//        https://cloud.tencent.com/developer/article/1630866
//        finally 在 return 之后時(shí)，先執(zhí)行 finally 后，再執(zhí)行該 return；
//        finally 內(nèi)含有 return 時(shí)，直接執(zhí)行其 return 后結(jié)束；
//        finally 在 return 前，執(zhí)行完 finally 后再執(zhí)行 return。
//        return Result.ok(SeckillStatEnum.SUCCESS);
}

問(wèn)題分析

由于spring事務(wù)是通過(guò)AOP實(shí)現(xiàn)的，所以在startSeckillLock()方法執(zhí)行之前會(huì)開啟事務(wù)，之后會(huì)有提交事務(wù)的邏輯。

而lock的動(dòng)作是發(fā)生在事務(wù)之內(nèi)。

數(shù)據(jù)庫(kù)默認(rèn)的事務(wù)隔離級(jí)別為可重復(fù)讀（repeatable-read）。

因?yàn)槭鞘聞?wù)先開啟后加鎖，隔離級(jí)別為可重復(fù)讀的情況下，當(dāng)前線程是讀取不到其他線程更新的數(shù)據(jù)，也就是說(shuō)其他線程雖然更新了庫(kù)存且事務(wù)也提交了，但是因?yàn)楫?dāng)前線程已經(jīng)開啟了事務(wù)（可重復(fù)讀的隔離級(jí)別），所以當(dāng)前線程在事務(wù)中獲取到的仍然是開啟事務(wù)時(shí)的庫(kù)存，所以就會(huì)出現(xiàn)超賣的情況。

問(wèn)題解決

一：在controller層加鎖

二：在service層自己定義事務(wù)的開啟和提交，加鎖的代碼方到開啟事務(wù)之前，解鎖在提交事務(wù)之后

三：AOP+鎖

自定義注解ServiceLock：

@Target({ElementType.PARAMETER, ElementType.METHOD})    
@Retention(RetentionPolicy.RUNTIME)    
@Documented    
public  @interface Servicelock { 
     String description()  default "";
}

自定義切面LockAspect：

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Component;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
@Component
@Aspect
@Order(1)
public class LockAspect {
    private static final Lock lock = new ReentrantLock();
    @Pointcut("@annotation(com.wjy.seckill.common.aop.ServiceLock)")
    public void lockAspect() {
    }
    @Around("lockAspect()")
    public Object around(ProceedingJoinPoint joinPoint) {
        lock.lock();
        Object result = null;
        try {
            result = joinPoint.proceed();
        } catch (Throwable e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        } finally {
            lock.unlock();
        }
        return result;
    }
}

切入秒殺方法:

@Override
@ServiceLock
@Transactional(rollbackFor = Exception.class)
public Result startSeckillAopLock(long seckillId, long userId) {
	//校驗(yàn)庫(kù)存
    String nativeSql = "SELECT number FROM seckill WHERE seckill_id = ?";
    Object object =  dynamicQuery.nativeQueryObject(nativeSql, new Object[]{seckillId});
    Long number =  ((Number) object).longValue();
    System.out.println(">>>>>>>>>>>>>>>>>>>>>> number : {}" + number);
    if(number > 0){
        //扣庫(kù)存
        nativeSql = "UPDATE seckill  SET number=? WHERE seckill_id = ?";
        dynamicQuery.nativeExecuteUpdate(nativeSql, new Object[]{number - 1, seckillId});
        //創(chuàng)建訂單
        SuccessKilled killed = new SuccessKilled();
        killed.setSeckillId(seckillId);
        killed.setUserId(userId);
        killed.setState((short)0);
        killed.setCreateTime(new Timestamp(System.currentTimeMillis()));
        dynamicQuery.save(killed);
        return Result.ok(SeckillStatEnum.SUCCESS);
        //支付
    }else{
    	return Result.error(SeckillStatEnum.END);
    }
}

至此問(wèn)題解決

表結(jié)構(gòu)

/*
 Navicat Premium Data Transfer
 Source Server         : localhost
 Source Server Type    : MySQL
 Source Server Version : 50732
 Source Host           : localhost:3306
 Source Schema         : spring-boot-seckill
 Target Server Type    : MySQL
 Target Server Version : 50732
 File Encoding         : 65001
 Date: 05/01/2022 15:51:06
*/
SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;
-- ----------------------------
-- Table structure for seckill
-- ----------------------------
DROP TABLE IF EXISTS `seckill`;
CREATE TABLE `seckill`  (
  `seckill_id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '商品庫(kù)存id',
  `name` varchar(120) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL COMMENT '商品名稱',
  `number` int(11) NOT NULL COMMENT '庫(kù)存數(shù)量',
  `start_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '秒殺開啟時(shí)間',
  `end_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '秒殺結(jié)束時(shí)間',
  `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創(chuàng)建時(shí)間',
  `version` int(11) NOT NULL COMMENT '版本號(hào)',
  PRIMARY KEY (`seckill_id`) USING BTREE,
  INDEX `idx_start_time`(`start_time`) USING BTREE,
  INDEX `idx_end_time`(`end_time`) USING BTREE,
  INDEX `idx_create_time`(`create_time`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 1004 CHARACTER SET = utf8 COLLATE = utf8_general_ci COMMENT = '秒殺庫(kù)存表' ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of seckill
-- ----------------------------
INSERT INTO `seckill` VALUES (1000, '1000元秒殺iphone8', 100, '2018-05-10 15:31:53', '2018-05-10 15:31:53', '2018-05-10 15:31:53', 0);
INSERT INTO `seckill` VALUES (1001, '500元秒殺ipad2', 100, '2018-05-10 15:31:53', '2018-05-10 15:31:53', '2018-05-10 15:31:53', 0);
INSERT INTO `seckill` VALUES (1002, '300元秒殺小米4', 100, '2018-05-10 15:31:53', '2018-05-10 15:31:53', '2018-05-10 15:31:53', 0);
INSERT INTO `seckill` VALUES (1003, '200元秒殺紅米note', 100, '2018-05-10 15:31:53', '2018-05-10 15:31:53', '2018-05-10 15:31:53', 0);
-- ----------------------------
-- Table structure for success_killed
-- ----------------------------
DROP TABLE IF EXISTS `success_killed`;
CREATE TABLE `success_killed`  (
  `seckill_id` bigint(20) NOT NULL COMMENT '秒殺商品id',
  `user_id` bigint(20) NOT NULL COMMENT '用戶Id',
  `state` tinyint(4) NOT NULL COMMENT '狀態(tài)標(biāo)示：-1指無(wú)效，0指成功，1指已付款',
  `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創(chuàng)建時(shí)間',
  PRIMARY KEY (`seckill_id`, `user_id`) USING BTREE,
  INDEX `idx_create_time`(`create_time`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci COMMENT = '秒殺成功明細(xì)表' ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of success_killed
-- ----------------------------
SET FOREIGN_KEY_CHECKS = 1;