检测和解决 Java 函数中的线程安全问题

// 线程不安全的计数器
class UnsafeCounter {
    private int count = 0;

    public void increment() {
        count++;
    }
}

// 线程安全的计数器
class SafeCounter {
    private AtomicInteger count = new AtomicInteger(0);

    public void increment() {
        count.getAndIncrement();
    }
}

public class ThreadSafety {
    public static void main(String[] args) {
        UnsafeCounter uCounter = new UnsafeCounter();
        for (int i = 0; i < 1000; i++) {
            Thread t = new Thread(() -> {
                for (int j = 0; j < 1000; j++) {
                    uCounter.increment();
                }
            });
            t.start();
        }

        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Unsafe counter: " + uCounter.count); // 可能不是 1000000

        SafeCounter sCounter = new SafeCounter();
        for (int i = 0; i < 1000; i++) {
            Thread t = new Thread(() -> {
                for (int j = 0; j < 1000; j++) {
                    sCounter.increment();
                }
            });
            t.start();
        }

        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Safe counter: " + sCounter.count); // 总是为 1000000
    }
}

Unsafe counter: 998998
Safe counter: 1000000