1195. 交替打印字符串

题目描述

编写一个可以从 1 到 n 输出代表这个数字的字符串的程序，但是：

• 如果这个数字可以被 3 整除，输出 "fizz"。
• 如果这个数字可以被 5 整除，输出 "buzz"。
• 如果这个数字可以同时被 3 和 5 整除，输出 "fizzbuzz"。

例如，当 n = 15，输出： 1, 2, fizz, 4, buzz, fizz, 7, 8, fizz, buzz, 11, fizz, 13, 14, fizzbuzz。

假设有这么一个类：

class FizzBuzz {
  public FizzBuzz(int n) { ... }               // constructor
  public void fizz(printFizz) { ... }          // only output "fizz"
  public void buzz(printBuzz) { ... }          // only output "buzz"
  public void fizzbuzz(printFizzBuzz) { ... }  // only output "fizzbuzz"
  public void number(printNumber) { ... }      // only output the numbers
}

请你实现一个有四个线程的多线程版  FizzBuzz， 同一个 FizzBuzz 实例会被如下四个线程使用：

1. 线程A将调用 fizz() 来判断是否能被 3 整除，如果可以，则输出 fizz。
2. 线程B将调用 buzz() 来判断是否能被 5 整除，如果可以，则输出 buzz。
3. 线程C将调用 fizzbuzz() 来判断是否同时能被 3 和 5 整除，如果可以，则输出 fizzbuzz。
4. 线程D将调用 number() 来实现输出既不能被 3 整除也不能被 5 整除的数字。

提示：

• 本题已经提供了打印字符串的相关方法，如 printFizz() 等，具体方法名请参考答题模板中的注释部分。

解法

Java

class FizzBuzz {
    private int n;
    private int current = 1;

    public FizzBuzz(int n) {
        this.n = n;
    }

    // printFizz.run() outputs "fizz".
    public void fizz(Runnable printFizz) throws InterruptedException {
        while (current <= n) {
            synchronized (this) {
                if (current % 3 == 0 && current % 5 != 0) {
                    if (current <= n) {
                        printFizz.run();
                        current++;
                        notifyAll();
                    }
                } else {
                    wait();
                }
            }
        }
    }

    // printBuzz.run() outputs "buzz".
    public void buzz(Runnable printBuzz) throws InterruptedException {
        while (current <= n) {
            synchronized (this) {
                if (current % 5 == 0 && current % 3 != 0) {
                    if (current <= n) {
                        printBuzz.run();
                        current++;
                        notifyAll();
                    }
                } else {
                    wait();
                }
            }
        }
    }

    // printFizzBuzz.run() outputs "fizzbuzz".
    public void fizzbuzz(Runnable printFizzBuzz) throws InterruptedException {
        while (current <= n) {
            synchronized (this) {
                if (current % 15 == 0) {
                    if (current <= n) {
                        printFizzBuzz.run();
                        current++;
                        notifyAll();
                    }
                } else {
                    wait();
                }
            }
        }
    }

    // printNumber.accept(x) outputs "x", where x is an integer.
    public void number(java.util.function.IntConsumer printNumber) throws InterruptedException {
        while (current <= n) {
            synchronized (this) {
                if (current % 3 != 0 && current % 5 != 0) {
                    if (current <= n) {
                        printNumber.accept(current);
                        current++;
                        notifyAll();
                    }
                } else {
                    wait();
                }
            }
        }
    }
}

Java

关于"唤醒所有线程以便退出"这个操作，分析如下：

是否必须取决于具体实现

• 不是绝对必须的：当 current > n 时，理论上各线程应该已经在各自的 while(current <= n) 循环中自然退出

• 建议保留的原因：

  1. 确保及时退出：避免某些线程长时间处于 await() 状态
  2. 防止内存泄漏：确保所有线程都能正常结束，释放资源
  3. 提高健壮性：即使主流程出现异常，也能保证线程正常终止

替代方案

如果不使用 signalAll()，也可以采用以下方式：

private void signalNext() {
    if (current > n) {
        return; // 直接返回，让各线程自然退出
    } else if (current % 15 == 0) {
        fizzbuzzCondition.signal();
    } else if (current % 3 == 0) {
        fizzCondition.signal();
    } else if (current % 5 == 0) {
        buzzCondition.signal();
    } else {
        numberCondition.signal();
    }
}

结论

虽然不是严格必需，但添加 signalAll() 是一种良好的编程实践，能确保程序更可靠地退出。

class FizzBuzz {
    private int n;
    private int current = 1;
    
    private final Lock lock = new ReentrantLock();
    private final Condition fizzCondition = lock.newCondition();
    private final Condition buzzCondition = lock.newCondition();
    private final Condition fizzbuzzCondition = lock.newCondition();
    private final Condition numberCondition = lock.newCondition();

    public FizzBuzz(int n) {
        this.n = n;
    }

    public void fizz(Runnable printFizz) throws InterruptedException {
        lock.lock();
        try {
            while (current <= n) {
                if (current % 3 == 0 && current % 5 != 0) {
                    printFizz.run();
                    current++;
                    signalNext();
                } else {
                    fizzCondition.await();
                }
            }
        } finally {
            lock.unlock();
        }
    }

    public void buzz(Runnable printBuzz) throws InterruptedException {
        lock.lock();
        try {
            while (current <= n) {
                if (current % 5 == 0 && current % 3 != 0) {
                    printBuzz.run();
                    current++;
                    signalNext();
                } else {
                    buzzCondition.await();
                }
            }
        } finally {
            lock.unlock();
        }
    }

    public void fizzbuzz(Runnable printFizzBuzz) throws InterruptedException {
        lock.lock();
        try {
            while (current <= n) {
                if (current % 15 == 0) {
                    printFizzBuzz.run();
                    current++;
                    signalNext();
                } else {
                    fizzbuzzCondition.await();
                }
            }
        } finally {
            lock.unlock();
        }
    }

    public void number(IntConsumer printNumber) throws InterruptedException {
        lock.lock();
        try {
            while (current <= n) {
                if (current % 3 != 0 && current % 5 != 0) {
                    printNumber.accept(current);
                    current++;
                    signalNext();
                } else {
                    numberCondition.await();
                }
            }
        } finally {
            lock.unlock();
        }
    }
    
    private void signalNext() {
        if (current > n) {
            // 唤醒所有线程以便退出
            fizzCondition.signalAll();
            buzzCondition.signalAll();
            fizzbuzzCondition.signalAll();
            numberCondition.signalAll();
        } else if (current % 15 == 0) {
            fizzbuzzCondition.signal();
        } else if (current % 3 == 0) {
            fizzCondition.signal();
        } else if (current % 5 == 0) {
            buzzCondition.signal();
        } else {
            numberCondition.signal();
        }
    }
}