前言
我在上一篇文章聊了Redisson的可重入锁,这次继续来聊聊Redisson的公平锁。下面是官方原话:
它保证了当多个Redisson客户端线程同时请求加锁时,优先分配给先发出请求的线程。所有请求线程会在一个队列中排队,当某个线程出现宕机时,Redisson会等待5秒后继续下一个线程,也就是说如果前面有5个线程都处于等待状态,那么后面的线程会等待至少25秒。
源码版本:3.17.7
这是我 fork 的分支,添加了自己理解的中文注释:https://github.com/xiaoguyu/redisson
公平锁
先上官方例子:
RLock fairLock = redisson.getFairLock("anyLock");
// 尝试加锁,最多等待100秒,上锁以后10秒自动解锁
boolean res = fairLock.tryLock(100, 10, TimeUnit.SECONDS);
...
fairLock.unlock();
因为在Redisson中,公平锁和普通可重入锁的逻辑大体上一样,我在上一篇文章都介绍了,这里就不再赘述。下面开始介绍合理逻辑。
加锁
加锁的 lua 脚本在 RedissonFairLock#tryLockInnerAsync
方法中
<T> RFuture<T> tryLockInnerAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {
long wait = threadWaitTime;
if (waitTime > 0) {
wait = unit.toMillis(waitTime);
}
long currentTime = System.currentTimeMillis();
if (command == RedisCommands.eva l_NULL_BOOLEAN) {
......
}
if (command == RedisCommands.eva l_LONG) {
return eva lWriteAsync(getRawName(), LongCodec.INSTANCE, command,
// remove stale threads
"while true do " + // list为空,证明没有人排队,退出循环
"local firstThreadId2 = redis.call('lindex', KEYS[2], 0);" +
"if firstThreadId2 == false then " +
"break;" +
"end;" +
// 能到这里,证明有人排队,拿出在排队的第一个人的超时时间,如果超时了,则移除相应数据
"local timeout = tonumber(redis.call('zscore', KEYS[3], firstThreadId2));" +
"if timeout <= tonumber(ARGV[4]) then " +
// remove the item from the queue and timeout set
// NOTE we do not alter any other timeout
"redis.call('zrem', KEYS[3], firstThreadId2);" +
"redis.call('lpop', KEYS[2]);" +
"else " +
"break;" +
"end;" +
"end;" +
// check if the lock can be acquired now
// 检查是否可以获取锁。如果hash和list都不存在,或者线程队列的第一个是当前线程,则可以获取锁
"if (redis.call('exists', KEYS[1]) == 0) " +
"and ((redis.call('exists', KEYS[2]) == 0) " +
"or (redis.call('lindex', KEYS[2], 0) == ARGV[2])) then " +
// remove this thread from the queue and timeout set
// 都获取锁了,当然要从线程队列和时间队列中移除
"redis.call('lpop', KEYS[2]);" +
"redis.call('zrem', KEYS[3], ARGV[2]);" +
// decrease timeouts for all waiting in the queue
// 刷新时间集合中的时间
"local keys = redis.call('zrange', KEYS[3], 0, -1);" +
"for i = 1, #keys, 1 do " +
"redis.call('zincrby', KEYS[3], -tonumber(ARGV[3]), keys[i]);" +
"end;" +
// acquire the lock and set the TTL for the lease
// 和公平锁的设置一样,值加1并且设置过期时间
"redis.call('hset', KEYS[1], ARGV[2], 1);" +
"redis.call('pexpire', KEYS[1], ARGV[1]);" +
"return nil;" +
"end;" +
// check if the lock is already held, and this is a re-entry
// 能到这里,证明前面拿不到锁,但是也要做可重入锁的处理
"if redis.call('hexists', KEYS[1], ARGV[2]) == 1 then " +
"redis.call('hincrby', KEYS[1], ARGV[2],1);" +
"redis.call('pexpire', KEYS[1], ARGV[1]);" +
"return nil;" +
"end;" +
// the lock cannot be acquired
// check if the thread is already in the queue
// 时间集合中有值,证明线程已经在队列中,不需要往后执行逻辑了
"local timeout = redis.call('zscore', KEYS[3], ARGV[2]);" +
"if timeout ~= false then " +
// the real timeout is the timeout of the prior thread
// in the queue, but this is approximate