redis实现分布式锁
1.主要命令:
- setnx
- setex
2.主要问题:
使用redis实现分布式锁,利用上面两个命令的特性。但是最重要的是锁要有过期时间,不然万一服务器宕机或者redis宕机,redis锁将永远得不到释放,出现死锁,其他线程一直获取不到资源。为了避免这种情况发生就必须保证这两个命令setnx与setex(设置过期时间)执行的原子性。
还有一个问题就是如果设置了过期时间,但是线程a在过期时间内没有执行完,这时可能会出现:
B执行到一半,serverA才执行完,这是serverA会释放serverB得锁,会导致无锁。
解决方法保证释放删除得锁与当前锁住得锁是同一个资源,即key,value完全一样,可通过lua脚本,getValue和delKey是一个原子性操作。
3.解决办法:
1.lua脚本实现setnx,setex两个命令的原子性:
读取lua脚本
/**
* 获取lua结果
* @param key
* @param value
* @return
*/
public Boolean luaExpress(String key,String value) {
lockScript = new DefaultRedisScript<Boolean>();
lockScript.setScriptSource(
new ResourceScriptSource(new ClassPathResource("add.lua")));
lockScript.setResultType(Boolean.class);
// 封装参数
List<Object> keyList = new ArrayList<Object>();
keyList.add(key);
keyList.add(value);
Boolean result = (Boolean) redisTemplate.execute(lockScript, keyList);
Long expire = redisTemplate.getExpire(key);
System.out.println("超时时间:"+expire);
return result;
}
lua脚本:
local lockKey = KEYS[1]
local lockValue = KEYS[2]
-- setnx info
local result_1 = redis.call('SETNX', lockKey, lockValue)
if result_1 == 1
then
local result_2= redis.call('SETEX', lockKey,3600, lockValue)
return result_1
else
return result_1
end
2. lua脚本保证释放的是当前锁:
/**
* 释放锁操作
* @param key
* @param value
* @return
*/
private boolean releaseLock(String key, String value) {
lockScript = new DefaultRedisScript<Boolean>();
lockScript.setScriptSource(
new ResourceScriptSource(new ClassPathResource("unlock.lua")));
lockScript.setResultType(Boolean.class);
// 封装参数
List<Object> keyList = new ArrayList<Object>();
keyList.add(key);
keyList.add(value);
Boolean result = (Boolean) redisTemplate.execute(lockScript, keyList);
return result;
}
unlock.lua
local lockKey = KEYS[1]
local lockValue = KEYS[2]
-- get key
local result_1 = redis.call('get', lockKey)
if result_1 == lockValue
then
local result_2= redis.call('del', lockKey)
return result_2
else
return false
end
3. 采用RedisConnection获取JRedisCommand得java客户端:
- https://docs.spring.io/spring-data/redis/docs/2.0.3.RELEASE/api/
- 搜索redisconnection
- 点击RedisStringCommands
- 搜索set方法,即可查找到
public boolean setLock(String key, long expire) {
try {
Boolean result = redisTemplate.execute(new RedisCallback<Boolean>() {
@Override
public Boolean doInRedis(RedisConnection connection) throws DataAccessException {
return connection.set(key.getBytes(), getHostIp().getBytes(), Expiration.seconds(expire) ,RedisStringCommands.SetOption.ifAbsent());
}
});
return result;
} catch (Exception e) {
logger.error("set redis occured an exception", e);
}
return false;
}
public String get(String key) {
try {
RedisCallback<String> callback = (connection) -> {
JedisCommands commands = (JedisCommands) connection.getNativeConnection();
return commands.get(key);
};
String result = redisTemplate.execute(callback);
return result;
} catch (Exception e) {
logger.error("get redis occured an exception", e);
}
return "";
}