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); } }
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