再谈使用java实现分布式锁redis篇

当前网上可以找到许多基于redis使用java实现的分布式锁的代码&＃xff0c;其主要实现方式主要有以下几种&＃xff1a;

1.      SETNX、GETSET、GET、DEL

加锁时&＃xff0c;使用SETNX设置锁名和锁的到期时间&＃xff0c;若设置成功则获取锁&＃xff1b;否则再检查锁是否已过期&＃xff0c;是则使用GETSET设置新的到期时间&＃xff0c;设置成功则获取到锁&＃xff0c;获取到锁后记一下状态&＃xff1b;解锁时&＃xff0c;若锁已过期则直接解锁&＃xff0c;否则根据状态判断是否由自己执有锁&＃xff0c;是则解锁。

2.      SETNX、EXPIRE、GET、DEL

加锁时&＃xff0c;使用SETNX设置锁名和锁的执有者&＃xff0c;使用EXPIRE设置锁的过期时间&＃xff1b;解锁时&＃xff0c;先查询锁是否还由锁执有者执有&＃xff0c;是则直接DEL解锁。

这两种实现方式在实际代码中都会各种各样的问题&＃xff0c;上代码来分析&＃xff1a;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import redis.clients.jedis.Jedis;public class WrongARedisDLock implements DLock {private static final Logger LOG &＃61; LogManager.getLogger(WrongARedisDLock.class);private final String lockName;private final int lockDuration;private final String lockSubject;private final Jedis jedis;public WrongARedisDLock(String lockName, int lockDuration, String lockSubject, Jedis jedis) {this.lockName &＃61; lockName;this.lockDuration &＃61; lockDuration;this.lockSubject &＃61; lockSubject;this.jedis &＃61; jedis;}public void lock() {tryLock(Long.MAX_VALUE);}public boolean tryLock() {return tryLock(0);}public boolean tryLock(long waitTimeout) {long startTime &＃61; System.currentTimeMillis();while (true) {try {// 设置锁名和锁申请主体的唯一标识if (jedis.setnx(lockName, lockSubject) !&＃61; null) {// 若此时程序故障退出&＃xff0c;锁将无法释放jedis.expire(lockName, lockDuration);return true;}String lockOwner &＃61; jedis.get(lockName);if (lockSubject.equals(lockOwner)) {return true;}if (lockOwner &＃61;&＃61; null) {continue;}if (System.currentTimeMillis() - startTime }


import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import redis.clients.jedis.Jedis;public class WrongBRedisDLock implements DLock {private static final Logger LOG &＃61; LogManager.getLogger(WrongBRedisDLock.class);private final String lockName;private final long lockDuration;private final String lockSubject;private final Jedis jedis;private boolean isLocked &＃61; false;public WrongBRedisDLock(String lockName, long lockDuration, String lockSubject, Jedis jedis) {this.lockName &＃61; lockName;this.lockDuration &＃61; lockDuration;this.lockSubject &＃61; lockSubject;this.jedis &＃61; jedis;}public void lock() {tryLock(Long.MAX_VALUE);}public boolean tryLock() {return tryLock(0);}public boolean tryLock(long waitTimeout) {long loopStartTime &＃61; System.currentTimeMillis();while (true) {try {long startTime &＃61; System.currentTimeMillis();String expireTimeStr &＃61; String.valueOf(startTime &＃43; lockDuration &＃43; 1);// 设置锁名和锁的到期时间&＃xff0c;此处就存在多个不同进程或服务器当前时间可能会不一致的问题if (jedis.setnx(lockName, expireTimeStr) !&＃61; null) {isLocked &＃61; true;return true;}// 获取锁的过期时间String oldExpireTimeStr &＃61; jedis.get(lockName);if (oldExpireTimeStr &＃61;&＃61; null) {continue;}long oldExpireTime &＃61; Long.parseLong(oldExpireTimeStr);if (startTime > oldExpireTime) {// 在并发量较大时&＃xff0c;可能会有多个锁申请主体同时进入到这里&＃xff0c;并且都会修改锁的过期时间&＃xff0c;// 这样会造成锁的实际过期时间比锁执有者设置的时间靠后oldExpireTimeStr &＃61; jedis.getSet(lockName, expireTimeStr);if (oldExpireTimeStr !&＃61; null) {// 以下代码可以保证第一个申请成功的主体获取到锁oldExpireTime &＃61; Long.parseLong(oldExpireTimeStr);if (startTime > oldExpireTime) {isLocked &＃61; true;return true;}}}if (System.currentTimeMillis() - loopStartTime }


正确的实现方式如下&＃xff1a;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import redis.clients.jedis.Jedis;
import redis.clients.jedis.Transaction;public class RedisDLock implements DLock {private static final Logger LOG &＃61; LogManager.getLogger(RedisDLock.class);private final String lockName;private final long lockDuration;private final String lockSubject;private final Jedis jedis;public RedisDLock(String lockName, long lockDuration, String lockSubject, Jedis jedis) {this.lockName &＃61; lockName;this.lockDuration &＃61; lockDuration;this.lockSubject &＃61; lockSubject;this.jedis &＃61; jedis;}public void lock() {tryLock(Long.MAX_VALUE);}public boolean tryLock() {return tryLock(0);}public boolean tryLock(long waitTimeout) {long startTime &＃61; System.currentTimeMillis();while (true) {try {// 尝试加锁。等同于 SETNX &＃43; EXPIRE&＃xff0c;通过设置一个属性值完成加锁过程if (jedis.set(lockName, // 锁名lockSubject, // 锁申请人"NX", // 锁名不存在时插入"PX", // 锁的有效时长的时间单位为millisecondlockDuration // 锁的有效时长) !&＃61; null) {// 加锁成功return true;}// 若加锁不成功&＃xff0c;获取锁的拥有者。加锁不成功有可能是申请者已执有此锁&＃xff0c;也可能是其他人执有此锁String lockOwner &＃61; jedis.get(lockName);// 若锁执有者与锁申请者相同&＃xff0c;则返回申请者已占用此锁if (lockSubject.equals(lockOwner)) {return true;}// 若锁未被占用&＃xff0c;则再次申请锁if (lockOwner &＃61;&＃61; null) {continue;}if (System.currentTimeMillis() - startTime }