分布式锁一般有三种实现方式:1. 数据库乐观锁;2. 基于Redis的分布式锁;3. 基于ZooKeeper的分布式锁。本篇博客将介绍第二种方式,基于Redis实现分布式锁。虽然网上已经有各种介绍Redis分布式锁实现的博客,然而他们的实现却有着各种各样的问题,为了避免误人子弟,本篇将介绍如何正确地实现Redis分布式锁。
首先,为了确保分布式锁可用,我们至少要确保锁的实现同时满足以下四个条件:
- 互斥性。在任意时刻,只有一个客户端能持有锁。
- 不会发生死锁。即使有一个客户端在持有锁的期间崩溃而没有主动解锁,也能保证后续其他客户端能加锁。
- 具有容错性。只要大部分的Redis节点正常运行,客户端就可以加锁和解锁。
- 解铃还须系铃人。加锁和解锁必须是同一个客户端,客户端自己不能把别人加的锁给解了。
感谢 myseries博主的教程。。
package com.gdut.redis.lock.test1;
import java.util.Collections;
import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisPool;
public class DistributedLock {
private static final String LOCK_SUCCESS = "OK";
private static final String SET_IF_NOT_EXIST = "NX";
private static final String SET_WITH_EXPIRE_TIME = "PX";
private static final Long RELEASE_SUCCESS = 1L;
private static void validParam(JedisPool jedisPool, String lockKey, String requestId, int expireTime) {
if (null == jedisPool) {
throw new IllegalArgumentException("jedisPool obj is null");
}
if (null == lockKey || "".equals(lockKey)) {
throw new IllegalArgumentException("lock key is blank");
}
if (null == requestId || "".equals(requestId)) {
throw new IllegalArgumentException("requestId is blank");
}
if (expireTime < 0) {
throw new IllegalArgumentException("expireTime is not allowed less zero");
}
}
/**
*
* @param jedis
* @param lockKey
* @param requestId
* @param expireTime
* @return
*/
public boolean tryLock(JedisPool jedisPool, String lockKey, String requestId, int expireTime) {
validParam(jedisPool, lockKey, requestId, expireTime);
Jedis jedis = null;
try {
jedis = jedisPool.getResource();
String result = jedis.set(lockKey, requestId, SET_IF_NOT_EXIST, SET_WITH_EXPIRE_TIME, expireTime);
if (LOCK_SUCCESS.equals(result)) {
return true;
}
} catch (Exception e) {
throw e;
} finally {
if (null != jedis) {
jedis.close();
}
}
return false;
}
/**
*
* @param jedis
* @param lockKey
* @param requestId
* @param expireTime
*/
public void lock(JedisPool jedisPool, String lockKey, String requestId, int expireTime) {
validParam(jedisPool, lockKey, requestId, expireTime);
while (true) {
if (tryLock(jedisPool, lockKey, requestId, expireTime)) {
System.out.println("lock "+ Thread.currentThread().getName()+ " requestId:" + requestId);
return;
}
}
}
/**
*
* @param jedis
* @param lockKey
* @param requestId
* @return
*/
public boolean unLock(JedisPool jedisPool, String lockKey, String requestId) {
validParam(jedisPool, lockKey, requestId, 1);
String script = "if redis.call('get', KEYS[1]) == ARGV[1] then return redis.call('del', KEYS[1]) else return 0 end";
Jedis jedis = null;
try {
jedis = jedisPool.getResource();
Object result = jedis.eval(script, Collections.singletonList(lockKey),
Collections.singletonList(requestId));
if (RELEASE_SUCCESS.equals(result)) {
System.out.println("unlock "+ Thread.currentThread().getName()+ " requestId:" + requestId);
return true;
}
} catch (Exception e) {
throw e;
} finally {
if (null != jedis) {
jedis.close();
}
}
return false;
}
}
package com.gdut.redis.lock.test1;
import java.util.UUID;
import com.gdut.redis.lock.test1.DistributedLock;
import redis.clients.jedis.JedisPool;
import redis.clients.jedis.JedisPoolConfig;
public class Service1 {
private static JedisPool pool = null;
private DistributedLock lock = new DistributedLock();
static {
JedisPoolConfig config = new JedisPoolConfig();
// 设置最大连接数
config.setMaxTotal(500);
// 设置最大空闲数
config.setMaxIdle(100);
// 设置最大等待时间
config.setMaxWaitMillis(1000 * 100);
// 在borrow一个jedis实例时,是否需要验证,若为true,则所有jedis实例均是可用的
config.setTestOnBorrow(true);
pool = new JedisPool(config, "127.0.0.1", 6379, 300000);
}
public void seckill() throws InterruptedException {
String requestId = UUID.randomUUID().toString();
lock.lock(pool, "resource", requestId, 3000);
lock.unLock(pool, "resource", requestId);
}
}
package com.gdut.redis.lock.test1;
import com.gdut.redis.lock.test1.Service1;
public class TaskThread extends Thread {
private Service1 service;
public TaskThread(Service1 service) {
this.service = service;
}
@Override
public void run() {
try {
synchronized (this) {
service.seckill();
}
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Service1 service = new Service1();
for (int i = 0; i < 400; i++) {
TaskThread thread = new TaskThread(service);
thread.start();
}
}
}