SpringBoot Redis分布式锁的正确实现方式(5)

SpringBoot

加锁 tryLock、lock

tryLock以阻塞等待waitTime时间的方式来尝试获取锁。获取成功则返回 true，反之 false。tryAcquire方法相当于执行了 Redis 的SET resourceName uuid:threadID NX PX {leaseTime}指令。

与tryLock不同的是，lock一直尝试自旋阻塞等待获取分布式锁，直到获取成功为止。而tryLock只会阻塞等待waitTime时间。

此外，为了让程序更加健壮，不念实现了阻塞等待获取分布式锁，让你用的更加开心，面试不慌加薪不难。如果你不需要自旋阻塞等待获取锁，那把 while 代码块删除即可。

@Override
public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {
    long time = unit.toMillis(waitTime);
    long current = System.currentTimeMillis();
    long threadId = Thread.currentThread().getId();
    // 获取锁
    Boolean isAcquire = tryAcquire(leaseTime, unit, threadId);
    // lock acquired
    if (Boolean.TRUE.equals(isAcquire)) {
        return true;
    }
    time -= System.currentTimeMillis() - current;
    // 等待时间用完，获取锁失败
    if (time <= 0) {
        return false;
    }
    // 自旋获取锁
    while (true) {
        long currentTime = System.currentTimeMillis();
        isAcquire = tryAcquire(leaseTime, unit, threadId);
        // lock acquired
        if (Boolean.TRUE.equals(isAcquire)) {
            return true;
        }
        time -= System.currentTimeMillis() - currentTime;
        if (time <= 0) {
            return false;
        }
    }
}
@Override
public void lock(long leaseTime, TimeUnit unit) {
    long threadId = Thread.currentThread().getId();
    Boolean acquired;
    do {
        acquired = tryAcquire(leaseTime, unit, threadId);
    } while (Boolean.TRUE.equals(acquired));
}
private Boolean tryAcquire(long leaseTime, TimeUnit unit, long threadId) {
    return redisTemplate.opsForValue().setIfAbsent(resourceName, getRequestId(threadId), leaseTime, unit);
}

解锁unlock

解锁的逻辑是通过执行 lua 脚本实现。

@Override
public void unlock() {
    long threadId = Thread.currentThread().getId();
    // 执行 lua 脚本
    DefaultRedisScript<Long> redisScript = new DefaultRedisScript<>(LuaScript.unlockScript(), Long.class);
    Long opStatus = redisTemplate.execute(redisScript, keys, getRequestId(threadId));
    if (opStatus == null) {
        throw new IllegalMonitorStateException("attempt to unlock lock, not locked by current thread by node id: "
                + id + " thread-id: " + threadId);
    }
}

LuaScript

其实这个脚本就是在讲解青铜板分布式锁原理的那段代码，具体逻辑已经解释过，这里就不再重复分析。

public class LuaScript {
    private LuaScript() {
    }
    /**
     * 分布式锁解锁脚本
     *
     * @return 当且仅当返回 `1`才表示加锁成功.
     */
    public static String unlockScript() {
        return "if (redis.call('exists', KEYS[1]) == 0) then " +
                "return nil;" +
                "end; "+
                "if redis.call('get',KEYS[1]) == ARGV[1] then" +
                "   return redis.call('del',KEYS[1]);" +
                "else" +
                "   return 0;" +
                "end;";
    }
}