redis 读写锁实现
一
先搞清楚读写锁要做什么。
基本就是
读读不互斥，读写互斥，写写互斥。可重入。
关于redis读写锁，我写了一次之后，总觉得很怪，然后就上网看到大神的redisson了，果断借鉴一番。
二
读行为
当写锁未获取，加上读锁（通知其他请求数据在读状态），读数据
当写锁被获取，等待，直到写锁未获取，加读锁，读数据
写行为
当写锁未获取，等待获取写锁
当写锁被获取，加写锁。读锁未获取，等待获取读锁
当写锁被获取，读锁被获取，写数据
可以看出读锁可重入一定意义都没有，写锁才有意义
三 初版
先说下总结
1.重入也只是本机重入，不能实现锁在其他服务器的重入。
2.读写锁获取锁的时候，是两个redis操作，原子性不行，所以要用redis的eval命令或者直接使用lua脚本。
3.用switch来判断读写模式太蠢了，代码可读性低，早期想的简单，但是逻辑一复杂就很麻烦了。
ps.
spring自带的redisTemplate则没有提供eval的接口，只提供使用lua脚本，相应的读写锁代码要自己写。
netty自带的redisson则是用了eval命令，则已经写好了代码，只需要傻瓜式调用就好了。
代码
--存放读写锁的信息
public enum LockModel { READ("%s:READ"), WRITE("%s:WRITE"),; String lockFormat; LockModel(String lockFormat) { this.lockFormat = lockFormat; } public String getLockModelName() { return super.name(); } public String getLockFormat() { return lockFormat; } public static void main(String[] args) { LockModel read = LockModel.READ; System.out.println(read.getLockFormat()); System.out.println(read.getLockModelName()); } }
--实现java自带的读写锁接口
public class ReadWriteLock implements java.util.concurrent.locks.ReadWriteLock {
/**
* 应该是唯一标识组成的key，可以使线程id，可以使用户id，可以使服务器id
*/
String name;
/**
* 毫秒
* */
Long timeInterval;
public ReadWriteLock(String name, Long timeInterval) {
this.name = name;
this.timeInterval = timeInterval;
}
@Override
public Lock readLock() {
return new ReentrantLock(this, LockModel.READ);
}
@Override
public Lock writeLock() {
return new ReentrantLock(this, LockModel.WRITE);
}
}
--重入锁
public class ReentrantLock implements Lock {
@Autowired
RedisTemplate redisTemplate;
ReadWriteLock rwLock;
LockModel lockModel;
String lockName;
Long deadTime = 0L;
boolean localWriteLocked = false;
public ReentrantLock(ReadWriteLock rwLock, LockModel lockModel) {
this.rwLock = rwLock;
this.lockModel = lockModel;
setLockName(lockModel);
}
@Override
public void lock() {
try {
lockInterruptibly();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
@Override
public void lockInterruptibly() throws InterruptedException {
switch (getLockModel()) {
case WRITE:
if (!isLocalWriteLocked()) {
setLockModel(LockModel.READ);
while (!tryLock()) {
Thread.sleep(500);
}
redisTemplate.opsForValue().set(getLockName(), getDeadTime(), getRwLock().getTimeInterval());
setLockModel(LockModel.WRITE);
while (!tryLock()) {
Thread.sleep(500);
}
setLocalWriteLocked(true);
} else {
/**
* 本机持有写锁，重入,但要等待之前的写操作完成
* */
while (!isLocalWriteLocked()) {
Thread.sleep(500);
}
/**
* 更新写锁的过期时间
* */
redisTemplate.opsForValue().set(getLockName(), getDeadTime(), getRwLock().getTimeInterval());
setLocalWriteLocked(true);
}
break;
case READ:
while (!tryLock()) {
Thread.sleep(500);
}
setDeadTime();
redisTemplate.opsForValue().set(getLockName(), getDeadTime(), getRwLock().getTimeInterval());
break;
}
}
@Override
public boolean tryLock() {
return null != redisTemplate.opsForValue().get(getOpposeLockName());
}
@Override
public void unlock() {
switch (getLockModel()) {
case WRITE:
if (isLocalWriteLocked()) {
setLocalWriteLocked(false);
}
redisTemplate.delete(getLockName());
break;
case READ:
redisTemplate.delete(getLockName());
break;
}
}
public Long getTimeInterval() {
return rwLock.getTimeInterval();
}
public void setDeadTime() {
this.deadTime = System.currentTimeMillis() + getTimeInterval();
}
private String getOpposeLockName() {
String opposeLockName = "";
switch (getLockModel()) {
case READ:
opposeLockName = String.format(LockModel.WRITE.getLockFormat(), getRwLock().getName());
break;
case WRITE:
opposeLockName = String.format(LockModel.READ.getLockFormat(), getRwLock().getName());
break;
default:
break;
}
return opposeLockName;
}
}
四 redisson分析
还是先总结
1.用hashmap存读写锁的信息。读锁写锁的本质则是model的不同。读锁写锁只是不同的mapfield。而读锁还有过期时间为属性。
2.用频道记录线程的操作。具体为什么用频道就要看LockPubSub和PublishSubscribe，这里因为不涉及我就不细说了。
RedissonReadLock
// 判断有没有锁
@Override public boolean isLocked() { RFuture<String> future = commandExecutor.writeAsync(getName(), StringCodec.INSTANCE, RedisCommands.HGET, getName(), "mode"); String res = get(future); return "read".equals(res); }
可以看出尝试获取锁的状态的代码都写的很简单，但是redisson用了hashmap来存放。
@Override <T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) { internalLockLeaseTime = unit.toMillis(leaseTime); return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command, "local mode = redis.call('hget', KEYS[1], 'mode'); " +
//锁出错 "if (mode == false) then " + "redis.call('hset', KEYS[1], 'mode', 'read'); " + "redis.call('hset', KEYS[1], ARGV[2], 1); " + "redis.call('set', KEYS[2] .. ':1', 1); " + "redis.call('pexpire', KEYS[2] .. ':1', ARGV[1]); " + "redis.call('pexpire', KEYS[1], ARGV[1]); " + "return nil; " + "end; " +
//在读模式或者本线程获取写锁的时候进行读 "if (mode == 'read') or (mode == 'write' and redis.call('hexists', KEYS[1], ARGV[3]) == 1) then " + "local ind = redis.call('hincrby', KEYS[1], ARGV[2], 1); " + "local key = KEYS[2] .. ':' .. ind;" + "redis.call('set', key, 1); " + "redis.call('pexpire', key, ARGV[1]); " + "redis.call('pexpire', KEYS[1], ARGV[1]); " + "return nil; " + "end;" + "return redis.call('pttl', KEYS[1]);", Arrays.<Object>asList(getName(), getReadWriteTimeoutNamePrefix(threadId)), internalLockLeaseTime, getLockName(threadId), getWriteLockName(threadId)); }
注意里面的getWriteLockName(threadId)
protected RFuture<Boolean> unlockInnerAsync(long threadId) { String timeoutPrefix = getReadWriteTimeoutNamePrefix(threadId); String keyPrefix = getKeyPrefix(threadId, timeoutPrefix); return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN, "local mode = redis.call('hget', KEYS[1], 'mode'); " + "if (mode == false) then " + "redis.call('publish', KEYS[2], ARGV[1]); " + "return 1; " + "end; " +
//锁不存在 "local lockExists = redis.call('hexists', KEYS[1], ARGV[2]); " + "if (lockExists == 0) then " + "return nil;" + "end; " + //给读锁的值-1，返回结果值 "local counter = redis.call('hincrby', KEYS[1], ARGV[2], -1); " +
// 结果值为0，删除读锁 "if (counter == 0) then " + "redis.call('hdel', KEYS[1], ARGV[2]); " + "end;" +
// 把自己的超时标记删除 "redis.call('del', KEYS[3] .. ':' .. (counter+1)); " + //还有其他读 "if (redis.call('hlen', KEYS[1]) > 1) then " + "local maxRemainTime = -3; " + "local keys = redis.call('hkeys', KEYS[1]); " + "for n, key in ipairs(keys) do " + "counter = tonumber(redis.call('hget', KEYS[1], key)); " + "if type(counter) == 'number' then " + "for i=counter, 1, -1 do " + "local remainTime = redis.call('pttl', KEYS[4] .. ':' .. key .. ':rwlock_timeout:' .. i); " + "maxRemainTime = math.max(remainTime, maxRemainTime);" + "end; " + "end; " + "end; " + "if maxRemainTime > 0 then " + "redis.call('pexpire', KEYS[1], maxRemainTime); " + "return 0; " + "end;" + //有写锁直接返回 "if mode == 'write' then " + "return 0;" + "end; " + "end; " + "redis.call('del', KEYS[1]); " + "redis.call('publish', KEYS[2], ARGV[1]); " + "return 1; ", Arrays.<Object>asList(getName(), getChannelName(), timeoutPrefix, keyPrefix), LockPubSub.unlockMessage, getLockName(threadId)); }
解锁还给其他锁续命，，，最大存活时间maxRemainTime很有意思，存在就给他加上等量的剩余存活时间，而不是固定加多少。那是不是无限续然后过期不了？但是这里是读写锁的存活时间而不是读锁的时间。
并且publish到相应的频道，更新状态。
protected RFuture<Boolean> renewExpirationAsync(long threadId) { String timeoutPrefix = getReadWriteTimeoutNamePrefix(threadId); String keyPrefix = getKeyPrefix(threadId, timeoutPrefix); return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN, "local counter = redis.call('hget', KEYS[1], ARGV[2]); " +
//不是false "if (counter ~= false) then " + "redis.call('pexpire', KEYS[1], ARGV[1]); " + "if (redis.call('hlen', KEYS[1]) > 1) then " + "local keys = redis.call('hkeys', KEYS[1]); " + "for n, key in ipairs(keys) do " + "counter = tonumber(redis.call('hget', KEYS[1], key)); " + "if type(counter) == 'number' then " + "for i=counter, 1, -1 do " + "redis.call('pexpire', KEYS[2] .. ':' .. key .. ':rwlock_timeout:' .. i, ARGV[1]); " + "end; " + "end; " + "end; " + "end; " + "return 1; " + "end; " + "return 0;", Arrays.<Object>asList(getName(), keyPrefix), internalLockLeaseTime, getLockName(threadId)); }
刷新存活时间没啥特殊的
@Override public RFuture<Boolean> forceUnlockAsync() { cancelExpirationRenewal(null); return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN, "if (redis.call('hget', KEYS[1], 'mode') == 'read') then " + "redis.call('del', KEYS[1]); " + "redis.call('publish', KEYS[2], ARGV[1]); " + "return 1; " + "end; " + "return 0; ", Arrays.<Object>asList(getName(), getChannelName()), LockPubSub.unlockMessage); }
没有之前续命的操作了。并且整个删除
RedissonWriteLock
@Override public RFuture<Boolean> forceUnlockAsync() { cancelExpirationRenewal(null); return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN, "if (redis.call('hget', KEYS[1], 'mode') == 'write') then " + "redis.call('del', KEYS[1]); " + "redis.call('publish', KEYS[2], ARGV[1]); " + "return 1; " + "end; " + "return 0; ", Arrays.<Object>asList(getName(), getChannelName()), LockPubSub.readUnlockMessage); } @Override public boolean isLocked() { RFuture<String> future = commandExecutor.writeAsync(getName(), StringCodec.INSTANCE, RedisCommands.HGET, getName(), "mode"); String res = get(future); return "write".equals(res); }
这两方法和读锁类似就不说了，而且增加过期时间写锁不支持这功能
@Override <T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) { internalLockLeaseTime = unit.toMillis(leaseTime); return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command, "local mode = redis.call('hget', KEYS[1], 'mode'); " + "if (mode == false) then " + "redis.call('hset', KEYS[1], 'mode', 'write'); " + "redis.call('hset', KEYS[1], ARGV[2], 1); " + "redis.call('pexpire', KEYS[1], ARGV[1]); " + "return nil; " + "end; " + "if (mode == 'write') then " + "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " + "redis.call('hincrby', KEYS[1], ARGV[2], 1); " + "local currentExpire = redis.call('pttl', KEYS[1]); " + "redis.call('pexpire', KEYS[1], currentExpire + ARGV[1]); " + "return nil; " + "end; " + "end;" + "return redis.call('pttl', KEYS[1]);", Arrays.<Object>asList(getName()), internalLockLeaseTime, getLockName(threadId)); }
显然，如果写锁是这个线程持有的才可以进行写操作。
@Override protected RFuture<Boolean> unlockInnerAsync(long threadId) { String timeoutPrefix = getReadWriteTimeoutNamePrefix(threadId); String keyPrefix = getKeyPrefix(threadId, timeoutPrefix); return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN, "local mode = redis.call('hget', KEYS[1], 'mode'); " + "if (mode == false) then " + "redis.call('publish', KEYS[2], ARGV[1]); " + "return 1; " + "end; " + "local lockExists = redis.call('hexists', KEYS[1], ARGV[2]); " + "if (lockExists == 0) then " + "return nil;" + "end; " + "local counter = redis.call('hincrby', KEYS[1], ARGV[2], -1); " + "if (counter == 0) then " + "redis.call('hdel', KEYS[1], ARGV[2]); " + "end;" + "redis.call('del', KEYS[3] .. ':' .. (counter+1)); " + "if (redis.call('hlen', KEYS[1]) > 1) then " + "local maxRemainTime = -3; " + "local keys = redis.call('hkeys', KEYS[1]); " + "for n, key in ipairs(keys) do " + "counter = tonumber(redis.call('hget', KEYS[1], key)); " + "if type(counter) == 'number' then " + "for i=counter, 1, -1 do " + "local remainTime = redis.call('pttl', KEYS[4] .. ':' .. key .. ':rwlock_timeout:' .. i); " + "maxRemainTime = math.max(remainTime, maxRemainTime);" + "end; " + "end; " + "end; " + "if maxRemainTime > 0 then " + "redis.call('pexpire', KEYS[1], maxRemainTime); " + "return 0; " + "end;" + "if mode == 'write' then " + "return 0;" + "end; " + "end; " + "redis.call('del', KEYS[1]); " + "redis.call('publish', KEYS[2], ARGV[1]); " + "return 1; ", Arrays.<Object>asList(getName(), getChannelName(), timeoutPrefix, keyPrefix), LockPubSub.unlockMessage, getLockName(threadId)); }
这里的也是给读写锁续命，看来就是数据使用次数越多读写锁存活的时间越长，而具体的读锁写锁的存活时间则是hashmap里面的一个属性。
