目录

一、RCountDownLatch的使用

二、trySetCount()设置计数器

三、countDown()源码

四、await()源码

【本篇文章基于redisson-3.17.6版本源码进行分析】

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);
    }
}