Pre
Redis进阶-细说分布式锁中我们梳理了使用Redis实现分布式锁的演进过程，并提出了目前最完善的解决方案：Redisson 实现分布式锁 。
这里我们来分析下Redisson分布式锁实现原理及源码解析
用法
使用redisson实现分布式锁的操作步骤，三部曲
- 第一步： 获取锁 RLock redissonLock = redisson.getLock(lockKey);
- 第二步： 加锁，实现锁续命功能 redissonLock.lock();
- 第三步：释放锁 redissonLock.unlock();
Redisson分布式锁实现原理
熟悉了基本用法以后，我们来看下Redission实现分布式锁的原理，再理解了原理之后，后续梳理源码实现就更加得心应手了。
Redisson分布式锁源码分析
流程图如下
重点主要是依赖lua脚本的原子性，实现加锁和释放锁的功能
redisson.getLock(lockKey) 的逻辑
代码语言：javascript复制 @Override
public RLock getLock(String name) {
return new RedissonLock(connectionManager.getCommandExecutor(), name);
}实例化RedissonLock，我们看下RedissonLock的构造函数
代码语言：javascript复制 public RedissonLock(CommandAsyncExecutor commandExecutor, String name) {
super(commandExecutor, name);
this.commandExecutor = commandExecutor;
this.id = commandExecutor.getConnectionManager().getId();
this.internalLockLeaseTime = commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout();
}- super(commandExecutor, name); 父类name赋值，后续通过getName()获取
- commandExecutor： 执行lua脚本的executor
- id 是个UUID， 后面被用来当做 和threadId组成 value值，用作判断加锁和释放锁是否是同一个线程的校验。
- internalLockLeaseTime : 取自 Config#lockWatchdogTimeout，默认30秒，这个参数还有另外一个作用，锁续命的执行周期 internalLockLeaseTime/3 = 10秒
redissonLock.lock()的逻辑
主要是实现加锁和锁的续命
代码语言：javascript复制 redissonLock.lock();看看都干了啥
代码语言：javascript复制 @Override
public void lock() {
try {
lockInterruptibly();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}继续看 lockInterruptibly
代码语言：javascript复制 @Override
public void lockInterruptibly() throws InterruptedException {
lockInterruptibly(-1, null);
}继续看 lockInterruptibly(-1, null);
@Override
public void lockInterruptibly(long leaseTime, TimeUnit unit) throws InterruptedException {
// 获取当前线程ID
long threadId = Thread.currentThread().getId();
// 尝试获取锁的剩余时间
Long ttl = tryAcquire(leaseTime, unit, threadId);
// lock acquired ttl为空，说明没有线程持有该锁，直接返回 让当前线程加锁成功
if (ttl == null) {
return;
}
RFuture<RedissonLockEntry> future = subscribe(threadId);
commandExecutor.syncSubscription(future);
// 死循环
try {
while (true) {
// 再此尝试获取锁的剩余时间 ，如果为null, 跳出循环
ttl = tryAcquire(leaseTime, unit, threadId);
// lock acquired
if (ttl == null) {
break;
}
// waiting for message 如果ttl >=0 说明 有其他线程持有该锁
if (ttl >= 0) {
// 获取信号量，尝试加锁，设置最大等待市场为ttl
getEntry(threadId).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
} else {
// 如果ttl小于0 （-1 ，-2 ） 说明已经过期，直接获取
getEntry(threadId).getLatch().acquire();
}
}
} finally {
unsubscribe(future, threadId);
}
// get(lockAsync(leaseTime, unit));
}大流程已经梳理完了，我们看下 Long ttl = tryAcquire(leaseTime, unit, threadId);
private Long tryAcquire(long leaseTime, TimeUnit unit, long threadId) {
return get(tryAcquireAsync(leaseTime, unit, threadId));
}继续看下
代码语言：javascript复制tryAcquireAsync(leaseTime, unit, threadId) private <T> RFuture<Long> tryAcquireAsync(long leaseTime, TimeUnit unit, final long threadId) {
if (leaseTime != -1) {
return tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_LONG);
}
// 刚开始 leaseTime 传入的是 -1 ，所以走这个分支
// 1）尝试加锁 待会细看 先把主要的逻辑梳理完
RFuture<Long> ttlRemainingFuture = tryLockInnerAsync(commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout(), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);
// 2) 注册监听事件
ttlRemainingFuture.addListener(new FutureListener<Long>() {
@Override
public void operationComplete(Future<Long> future) throws Exception {
if (!future.isSuccess()) {
return;
}
Long ttlRemaining = future.getNow();
// lock acquired
if (ttlRemaining == null) {
// 3）获取锁成功的话，给锁延长过期时间
scheduleExpirationRenewal(threadId);
}
}
});
return ttlRemainingFuture;
}继续看
代码语言：javascript复制 // 1）尝试加锁 待会细看 先把主要的逻辑梳理完
RFuture<Long> ttlRemainingFuture = tryLockInnerAsync(commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout(), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);看实现
代码语言：javascript复制 <T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {
internalLockLeaseTime = unit.toMillis(leaseTime);
return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command,
"if (redis.call('exists', KEYS[1]) == 0) then "
"redis.call('hset', KEYS[1], ARGV[2], 1); "
"redis.call('pexpire', KEYS[1], ARGV[1]); "
"return nil; "
"end; "
"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; "
"return redis.call('pttl', KEYS[1]);",
Collections.<Object>singletonList(getName()), internalLockLeaseTime, getLockName(threadId));
}lua 脚本
KEYS1 ---------> getName()
ARGV1 ---------> internalLockLeaseTime
ARGV2 ---------> getLockName(threadId) 实现如下
代码语言：javascript复制 String getLockName(long threadId) {
return id ":" threadId;
}这个id就是自开始实例化RedissonLock的id ,是个UUID
我们来解释下这段lua脚本
代码语言：javascript复制 // 如果 lockKey不存在 ，设置 使用hset设置 lockKey ，field为 uuid:threadId ,value为1 ,并设置过期时间
//就是这个命令
//127.0.0.1:6379> hset lockkey uuid:threadId 1
//(integer) 1
//127.0.0.1:6379> PEXPIRE lockkey internalLockLeaseTime
"if (redis.call('exists', KEYS[1]) == 0) then "
"redis.call('hset', KEYS[1], ARGV[2], 1); "
"redis.call('pexpire', KEYS[1], ARGV[1]); "
"return nil; "
"end; "
// 如果 lockKey 存在和 filed 和 当前线程的uuid:threadId相同 key 加1 ，执行多少次 就加多次 设置过期时间 其实就是如下命令
//127.0.0.1:6379> HEXISTS lockkey uuid:threadId
//(integer) 1
//127.0.0.1:6379> PEXPIRE lockkey internalLockLeaseTime
"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; "
// 最后返回 lockkey的 pttl
"return redis.call('pttl', KEYS[1]);"那继续监听时间中的 scheduleExpirationRenewal(threadId); 逻辑
private void scheduleExpirationRenewal(final long threadId) {
if (expirationRenewalMap.containsKey(getEntryName())) {
return;
}
Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {
// 重点是run方法
@Override
public void run(Timeout timeout) throws Exception {
// 又是lua脚本 判断是否存在，存在就调用pexpire
RFuture<Boolean> future = commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
"if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then "
"redis.call('pexpire', KEYS[1], ARGV[1]); "
"return 1; "
"end; "
"return 0;",
Collections.<Object>singletonList(getName()), internalLockLeaseTime, getLockName(threadId));
// 监听事件中又 调用了自己 scheduleExpirationRenewal
future.addListener(new FutureListener<Boolean>() {
@Override
public void operationComplete(Future<Boolean> future) throws Exception {
expirationRenewalMap.remove(getEntryName());
if (!future.isSuccess()) {
log.error("Can't update lock " getName() " expiration", future.cause());
return;
}
if (future.getNow()) {
// reschedule itself
scheduleExpirationRenewal(threadId);
}
}
});
}
}, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);
if (expirationRenewalMap.putIfAbsent(getEntryName(), task) != null) {
task.cancel();
}
}redissonLock.unlock();逻辑
代码语言：javascript复制 @Override
public void unlock() {
Boolean opStatus = get(unlockInnerAsync(Thread.currentThread().getId()));
if (opStatus == null) {
throw new IllegalMonitorStateException("attempt to unlock lock, not locked by current thread by node id: "
id " thread-id: " Thread.currentThread().getId());
}
if (opStatus) {
cancelExpirationRenewal();
}
}重点看 unlockInnerAsync(Thread.currentThread().getId())
protected RFuture<Boolean> unlockInnerAsync(long threadId) {
return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
"if (redis.call('exists', KEYS[1]) == 0) then "
"redis.call('publish', KEYS[2], ARGV[1]); "
"return 1; "
"end;"
"if (redis.call('hexists', KEYS[1], ARGV[3]) == 0) then "
"return nil;"
"end; "
"local counter = redis.call('hincrby', KEYS[1], ARGV[3], -1); "
"if (counter > 0) then "
"redis.call('pexpire', KEYS[1], ARGV[2]); "
"return 0; "
"else "
"redis.call('del', KEYS[1]); "
"redis.call('publish', KEYS[2], ARGV[1]); "
"return 1; "
"end; "
"return nil;",
Arrays.<Object>asList(getName(), getChannelName()), LockPubSub.unlockMessage, internalLockLeaseTime, getLockName(threadId));
}又是lua脚本，核心就是 把value减到为0 ，删除key
KEYS1 ---------> getName()
KEYS2 ---------> getChannelName()
ARGV1 ---------> LockPubSub.unlockMessage
ARGV2 ---------> internalLockLeaseTime
ARGV2 ---------> getLockName(threadId)
总结
至此，原理和源码我们粗略的梳理完了 ，梳理了主要的核心流程，主要是依靠lua脚本，代码写的还是非常优秀的，向开源学习！！！
