What is the principle behind Java’s implementation of fences?

In Java, the CyclicBarrier is a synchronization aid that allows a group of threads to wait for each other to reach a common barrier point before continuing execution. The implementation principle of a barrier is as follows:

1. Inside the fence, there is a counter maintained to keep track of the number of threads that have not yet reached the barrier point.
2. When a thread calls the await() method of a barrier, it will be added to the waiting queue and decrement the counter by one.
3. When the counter is reduced to 0, it means that all threads have reached the barrier point, the barrier will wake up all waiting threads, and reset the counter to its initial value.
4. If the counter is still greater than 0, it means that there are threads that have not reached the barrier point yet. The thread calling await() method will be blocked until the counter is 0 or interrupted.
5. After the thread is awakened, it will continue to execute the code after the await() method and then proceed further.

Using a barrier allows for the synchronization of threads, enabling a group of threads to wait at a specific point before proceeding to execute subsequent operations together. Barriers are often used in tasks that require multiple threads to work together, such as data sharding in distributed computing.