Thread-Safe Counter (Java)

Attention.md

Thread-Safe Counter (Java)

Counter.java

import java.util.concurrent.locks.*;

class Counter {
  private final ReentrantLock lock = new ReentrantLock(true);
  private volatile long count = 0;

  public void increment() {
    System.out.println(String.format("Acquiring Lock…%s", 
                                              Thread.currentThread().getName()));
    lock.lock();
    System.out.println(String.format("Acquired Lock %s", 
                                              Thread.currentThread().getName()));
    try {
      count++;
    } finally {
      System.out.println(String.format("Releasing Lock...%s", 
                                              Thread.currentThread().getName()));
      lock.unlock();
      System.out.println(String.format("Released Lock %s", 
                                              Thread.currentThread().getName()));
    }
  }
  public long value() {
    return count;
  }
  
  public static void main(String[] args) throws Exception {
    final Counter counter = new Counter();
    final Runnable incrementor = () -> counter.increment();

    final Thread thread1 = new Thread(incrementor, "Thread#1");
    final Thread thread2 = new Thread(incrementor, "Thread#2");

    thread1.start();
    thread2.start();

    System.out.println("counter.value() = " + counter.value());
    Thread.sleep(100);
    System.out.println("counter.value() = " + counter.value());
  }
}

But this will not solve the memory visibility issue, right? You need to make the counter value volatile?

that's correct