CyclicBarrier in Java is a synchronization tool that allows multiple threads to wait at a barrier until all threads reach the barrier before execution can continue. CyclicBarrier can be used to coordinate the execution of multiple threads so that they can execute simultaneously at a certain point.

  CyclicBarrier is a synchronization tool in Java that allows multiple threads to wait at a barrier point until all threads reach that point before execution can continue. CyclicBarrier can be used to coordinate the execution of multiple threads so that they can execute simultaneously at a certain point.

Usage

The basic usage of CyclicBarrier is as follows:

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

public class CyclicBarrierExample {

    public static void main(String[] args) {
        int n = 3;
        CyclicBarrier barrier = new CyclicBarrier(n, new Runnable() {
            public void run() {
                System.out.println("All threads have reached the barrier");
            }
        });

        Thread t1 = new Thread(new MyRunnable(barrier), "Thread 1");
        Thread t2 = new Thread(new MyRunnable(barrier), "Thread 2");
        Thread t3 = new Thread(new MyRunnable(barrier), "Thread 3");

        t1.start();
        t2.start();
        t3.start();
    }

    static class MyRunnable implements Runnable {
        private final CyclicBarrier barrier;

        public MyRunnable(CyclicBarrier barrier) {
            this.barrier = barrier;
        }

        public void run() {
            try {
                System.out.println(Thread.currentThread().getName() + " is waiting at the barrier...");
                barrier.await();
                System.out.println(Thread.currentThread().getName() + " has crossed the barrier");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            }
        }
    }
}

  In this example, we create a CyclicBarrier object that needs to wait for 3 threads to reach the barrier point. When all threads reach the barrier point, a callback function will be triggered and a message will be printed.

  We create 3 threads and pass them a custom Runnable object. In the run method of each thread, we first print a message indicating that the thread is waiting for the barrier point. Then call the barrier.await() method to add the thread to the waiting queue. Execution will not continue until all threads reach the barrier point. At the end, we print a message indicating that the thread has crossed the barrier point.

The results of running the above code are as follows:

Thread 1 is waiting at the barrier...
Thread 3 is waiting at the barrier...
Thread 2 is waiting at the barrier...
All threads have reached the barrier
Thread 2 has crossed the barrier
Thread 1 has crossed the barrier
Thread 3 has crossed the barrier

   As can be seen from the above code, CyclicBarrier also supports an optional callback function. After all threads reach the barrier point, the specified callback function will be called. In the above example, when all threads reach the barrier point, the specified callback function will be called. Execute the callback function to indicate that the barrier point has been reached.

  CyclicBarrier also supports a more advanced usage, which is to perform some additional operations while waiting for the thread to reach the barrier point. This can be achieved through the return value of the await method, as shown below:

int  index  = barrier.await();
 if (index == 0 ) {
     // perform additional operations 
}

  In this example, the return value of the await method indicates the thread’s position in the waiting queue. If the return value is 0, it means that the current thread is the last thread to reach the barrier point and can perform some additional operations, such as doing some data. Finishing touches such as cleaning.

Precautions

When using CyclicBarrier in Java, you need to pay attention to the following points:

1. The counter of CyclicBarrier is reusable, that is, when all threads reach the barrier point, the counter will be reset to the initial value and can be used again. If an exception occurs while waiting, the counter will be reset and all waiting threads will throw a BrokenBarrierException exception.
2. If the number of waiting threads exceeds the initial value of the counter when using CyclicBarrier, all threads will wait forever. Therefore, when using CyclicBarrier, you need to ensure that the number of waiting threads does not exceed the initial value of the counter.
3. The callback function of CyclicBarrier is executed when the last thread reaches the barrier point. Therefore, the operations performed in the callback function should be thread-safe , otherwise it may lead to unpredictable results.
4. CyclicBarrier can be used to coordinate the execution of multiple threads so that they can execute simultaneously at a certain point. However, if the order of execution between threads is important to the correctness of your program, CyclicBarrier may not be the best choice. In this case, you may need to use other synchronization tools, such as CountDownLatch or Semaphore.
5. The performance of CyclicBarrier may be affected by the number of waiting threads and the initial value of the counter. If the number of waiting threads is large, or the initial value of the counter is large, performance degradation may occur. Therefore, when using CyclicBarrier, it needs to be adjusted according to the actual situation.

  In summary, when using CyclicBarrier in Java, various scenarios need to be carefully considered to ensure program correctness and performance.

Summarize

  CyclicBarrier is a very useful synchronization tool that allows multiple threads to wait at a barrier point until all threads have reached that point before execution can continue. CyclicBarrier can be used to coordinate the execution of multiple threads so that they can execute simultaneously at a certain point. CyclicBarrier also supports advanced usage such as reusability, callback functions, and additional operations to meet various synchronization needs.