java - 同步读取变量

Question

我不知道该怎么做。我有这个限制:

• 最多 5 个线程可以同时读取一个变量，其他线程必须等待直到再次读取该变量。

public void methodA() {
    lockI.lock();
    try {
        while (countIreaders == 5 || modify) {
            conditionI.await();
        }
        countIreaders++;
    } catch (InterruptedException e) {
        e.printStackTrace();
    } finally {
        if (condition) {
            countIreaders = 0;
            lockI.unlock();
            conditionI.notifyAll();
        } else {
            lockI.unlock();
        }
    }
}

通过这段代码，我将其串行化，但这不是我想要实现的目标。如何修改代码？

Answer 1

我不太明白你的代码，但似乎你正在寻找的是 Semaphore 。信号量对于线程同步很有用。您可以使用 5 个 token /许可证创建一个新信号量。像这样:

Semaphore sem = new Semaphore(5); //initialization in your data structure

//...

public void yourThreadFunction() {
    // [...] in your readers threads:
    // each thread will of course have to use the same semaphore
    // A thread must aquire a token from the semaphore before accessing your variable
    sem.aquire(); //this call hangs until a permit is available
    // read your value and do some computation
    // only 5 threads can be inside this part because of the aquire
    sem.release(); // release the token/permits
}