有个项目没有引入redis, 又要使用到分布式锁. 所以打算用mysql实现一下;

建表语句:

```sql
SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;-- ----------------------------
-- Table structure for resource_lock
-- ----------------------------
DROP TABLE IF EXISTS `resource_lock`;
CREATE TABLE `resource_lock`  (`id` bigint(20) NOT NULL,`resource_key` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL COMMENT '需要加锁的key,如:文件目录key等',`expire_time` bigint(20) NULL DEFAULT NULL COMMENT '过期时间戳',`create_time` datetime(0) NULL DEFAULT NULL,PRIMARY KEY (`id`) USING BTREE,UNIQUE INDEX `resource_key`(`resource_key`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_bin ROW_FORMAT = Dynamic;SET FOREIGN_KEY_CHECKS = 1;

对应实体类

package org.information.harbor.data.company.main.entity;import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableId;
import com.baomidou.mybatisplus.annotation.TableName;
import lombok.Builder;
import lombok.Data;import java.io.Serializable;
import java.util.Date;/*** * @TableName resource_lock*/
@TableName(value ="resource_lock")
@Data
@Builder
public class ResourceLock implements Serializable {/*** */@TableIdprivate Long id;/*** 需要加锁的key,如:文件目录key等*/private String resourceKey;/*** 过期时间戳*/private Long expireTime;/*** */private Date createTime;@TableField(exist = false)private static final long serialVersionUID = 1L;@Overridepublic boolean equals(Object that) {if (this == that) {return true;}if (that == null) {return false;}if (getClass() != that.getClass()) {return false;}ResourceLock other = (ResourceLock) that;return (this.getId() == null ? other.getId() == null : this.getId().equals(other.getId()))&& (this.getResourceKey() == null ? other.getResourceKey() == null : this.getResourceKey().equals(other.getResourceKey()))&& (this.getExpireTime() == null ? other.getExpireTime() == null : this.getExpireTime().equals(other.getExpireTime()))&& (this.getCreateTime() == null ? other.getCreateTime() == null : this.getCreateTime().equals(other.getCreateTime()));}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((getId() == null) ? 0 : getId().hashCode());result = prime * result + ((getResourceKey() == null) ? 0 : getResourceKey().hashCode());result = prime * result + ((getExpireTime() == null) ? 0 : getExpireTime().hashCode());result = prime * result + ((getCreateTime() == null) ? 0 : getCreateTime().hashCode());return result;}@Overridepublic String toString() {StringBuilder sb = new StringBuilder();sb.append(getClass().getSimpleName());sb.append(" [");sb.append("Hash = ").append(hashCode());sb.append(", id=").append(id);sb.append(", resourceKey=").append(resourceKey);sb.append(", expireTime=").append(expireTime);sb.append(", createTime=").append(createTime);sb.append(", serialVersionUID=").append(serialVersionUID);sb.append("]");return sb.toString();}
}

总体思路是:

1. 去表中插入一个数据(key). 这个数据建立了唯一索引
2. 插入成功.则返回true. 加锁成功
3. 插入失败,则说明可能已经存在. 但是还有可能是之前解锁失败的.等特殊情况. 所以需要查询并验证一下过期时间. 如果过期了, 就用乐观锁更新时间, 自己占有锁.
4. 解锁没啥特殊的, 就是直接删除这个key

具体实现代码:

package org.information.harbor.data.company.main.util;import cn.hutool.core.collection.CollUtil;
import cn.hutool.core.util.ObjectUtil;
import cn.hutool.core.util.StrUtil;
import lombok.extern.slf4j.Slf4j;
import org.information.harbor.data.company.main.entity.ResourceLock;
import org.information.harbor.data.company.main.service.ResourceLockService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;import java.util.List;
import java.util.Optional;/*** 数据库实现的分布式锁** @author: riyueming* @date: 2021/10/15* @description:*/
@Slf4j
@Component
public class LockUtil {/*** 定时任务锁的key, 也可约定为其他值*/public static final String INDEX__TASK_LOCK="INDEX_TASK_LOCK";@AutowiredResourceLockService resourceLockService;/*** 过期时间 : 5分钟.  可以改成配置项*/public static final Long TIME_OUT = 5 * 60 * 1000L;public boolean tryLock(String resourceKey) {ResourceLock build =ResourceLock.builder().resourceKey(resourceKey).expireTime(System.currentTimeMillis() + TIME_OUT).build();//直接插入try {if (resourceLockService.save(build)) {return true;}} catch (Exception e) {log.error(StrUtil.format("主键重复或保存出错resourceKey:[{}]", resourceKey), e);}//失败或报错. 查询存在值. 看过期时间.List<ResourceLock> resourceLocks = resourceLockService.selectAllByResourceKey(resourceKey);ResourceLock resourceLock = CollUtil.emptyIfNull(resourceLocks).stream().findFirst().orElse(null);Long oldExpireTime = Optional.ofNullable(resourceLock).map(ResourceLock::getExpireTime).orElse(null);if (ObjectUtil.isNotNull(oldExpireTime)) {if (System.currentTimeMillis() > oldExpireTime) {//过期//更新. 返回 乐观锁int upCount =resourceLockService.updateExpireTimeByExpireTime(System.currentTimeMillis() + TIME_OUT,oldExpireTime);return upCount > 0;}}return false;}public void unLock(String resourceKey) {//直接删除resourceLockService.deleteByResourceKey(resourceKey);}}

操作表的接口都是通过mybatis-plus自动生成的, 就不贴了;

再来一个具体使用的代码. 注意解锁放在finally中. 还要加flag防止误解锁

 public void generateIndexAndReport() {log.info("started generateIndexAndReportJob");boolean lockFlag = false;//加锁:文件夹 解锁try {lockFlag = lockUtil.tryLock(LockUtil.INDEX__TASK_LOCK);if (!lockFlag) {log.info("generateIndexAndReportJobHandler.本次调用加锁失败,当前时间:[{}]", DateUtil.now());return;}//doSomeThings} catch (Exception e) {log.error(StrUtil.format("generateIndexAndReportJobHandler定时任务执行异常,当前时间:[{}]", DateUtil.now()), e);} finally {//解锁if (lockFlag) {lockUtil.unLock(LockUtil.INDEX__TASK_LOCK);}}log.info("finished generateIndexAndReportJob");}

这样好像效率不怎么高. 如果有高效的方式欢迎留言, 一起探讨下