Redisson分布式闭锁RCountDownLatch的使用详细讲解

来源：jb51　　时间：2023/2/13 8:43:55　　对本文有异议

一、RCountDownLatch的使用

RCountDownLatch的功能跟CountDownLatch，用于实现某个线程需要等待其他线程都完成之后，我再去执行，这种场景就可以使用CountDownLatch。

@Test
public void testRCountDownLatch() {
    Config config = new Config();
    config.useSingleServer().setAddress("redis://127.0.0.1:6379");
    RedissonClient redissonClient = Redisson.create(config);
    RCountDownLatch rCountDownLatch = redissonClient.getCountDownLatch("anyCountDownLatch");
    rCountDownLatch.trySetCount(5);
    for (int i = 1; i <= 5; i++) {
        new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + "离开教师...");
            rCountDownLatch.countDown();
        }, "A" + i).start();
    }
    try {
        rCountDownLatch.await();
    } catch (InterruptedException e) {
        throw new RuntimeException(e);
    }
    System.out.println("班长锁门...");
}

A1离开教师...
A2离开教师...
A4离开教师...
A3离开教师...
A5离开教师...
班长锁门...

二、trySetCount()设置计数器

/**
 * 仅当先前的计数已达到零或根本未设置时才设置新的计数值。
 */
boolean trySetCount(long count);

public RFuture<Boolean> trySetCountAsync(long count) {
    return commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
            // 往redis中写入一个String类型的数据    anyCountDownLatch:5
            "if redis.call('exists', KEYS[1]) == 0 then "
                + "redis.call('set', KEYS[1], ARGV[2]); "
                + "redis.call('publish', KEYS[2], ARGV[1]); "
                + "return 1 "
            + "else "
                + "return 0 "
            + "end",
            Arrays.asList(getRawName(), getChannelName()), CountDownLatchPubSub.NEW_COUNT_MESSAGE, count);
}

同样，在redis中写入了一个{key}:{计数器总数}的String类型的数据。

三、countDown()源码

减少锁存器的计数器。当计数达到零时通知所有等待线程。

public RFuture<Void> countDownAsync() {
    return commandExecutor.evalWriteNoRetryAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
                    // 减少redis中计数器的值
                    "local v = redis.call('decr', KEYS[1]);" +
                    // 计数器减为0后，删除对应的key        
                    "if v <= 0 then redis.call('del', KEYS[1]) end;" +
                    "if v == 0 then redis.call('publish', KEYS[2], ARGV[1]) end;",
                Arrays.<Object>asList(getRawName(), getChannelName()), CountDownLatchPubSub.ZERO_COUNT_MESSAGE);
}

四、await()源码

等到计数器达到零。

public void await() throws InterruptedException {
    // 如果计数器为0，直接返回
    if (getCount() == 0) {
        return;
    }
    // 订阅redisson_countdownlatch__channel__{anyCountDownLatch}的消息
    CompletableFuture<RedissonCountDownLatchEntry> future = subscribe();
    RedissonCountDownLatchEntry entry = commandExecutor.getInterrupted(future);
    try {
        // 不断循环判断计数器的值是否大于0，大于0说明还有线程没执行完成，在这里阻塞：LockSupport.park(this)
        while (getCount() > 0) {
            // waiting for open state
            entry.getLatch().await();
        }
    } finally {
        unsubscribe(entry);
    }
}