java - 我可以通过参数同步方法吗

Question

我可以通过参数同步方法吗？

例如 - 我让 person 使用某种方法，我想为 person 做一些操作，但是如果很少有线程为同一个人调用这个方法，我想一个一个地做。

private void dosomething(Long id, Person person) {
    dosomethingelse(id, person);
}

如何只对同一个id逐一调用dosomethingelse(id, person)？但我希望这段针对不同 id-s 的代码可以被多线程调用

我写了这段代码，但也许这里有问题或者可以做得更好。

public static class LatchByValue <T> {
    public void latch(T value, ConsumerWithException<T> consummer) throws Exception {
        CountDownLatch latch = new CountDownLatch(1);
        try {
            CountDownLatch previousLatch = null;
            // we are checking if another thread is already calling this method with the same id
            // if sync has CountDownLatch so another thread is already calling this method 
            // or we put our latch and go on
            while ((previousLatch = sync.putIfAbsent(value, latch)) != null) {
                try {
                    // we are waiting for another thread, we are waiting for all threads that put their latch before our thread
                    previousLatch.await();
                } catch (InterruptedException e) {
                    return;
                }
            }
            consummer.accept(value);
        } finally {
            latch.countDown();
            sync.remove(value, latch);
        } 
    }
    private ConcurrentHashMap<T, CountDownLatch> sync = new ConcurrentHashMap<>();
}

例子:

LatchByValue<Long> latch = new LatchByValue<>();

private void dosomething(Long id, Person person) {
     latch.latch(
        id,
        currentId -> { dosomethingelse(currentId, person); }
     );
}

Answer 1

使用 CountdownLatch 的问题在于您无法“增加”计数，因此您需要在使用时替换现有的锁存器，这会使代码复杂化。

您可以改为使用 a Semaphore只需一个许可证，您就可以以更简单的方式做同样的事情。

Semaphore s = sync.computeIfAbsent(value, x -> new Semaphore(1, true));
s.acquire(); //this blocks and throws InterruptedException, which you need to handle
try {
  consummer.accept(value);
} finally {
  s.release();
}