题目
给你一个类：
public class Foo {
public void first() { print("first"); }
public void second() { print("second"); }
public void third() { print("third"); }
}
三个不同的线程 A、B、C 将会共用一个
Foo实例。
- 线程 A 将会调用
first()方法- 线程 B 将会调用
second()方法- 线程 C 将会调用
third()方法请设计修改程序，以确保
second()方法在first()方法之后被执行，third()方法在second()方法之后被执行。提示：
- 尽管输入中的数字似乎暗示了顺序，但是我们并不保证线程在操作系统中的调度顺序。
- 你看到的输入格式主要是为了确保测试的全面性。
解法一
先声明两个AtomicInteger变量并初始化为0，使用他来控制first()和second(),second() 和third()之间的顺序关系。
在进入second和third方法时，提前进入循环去检验变量是否被增加为1，如果是，则进入方法，否则空转。
1. class Foo { 2. 3. private AtomicInteger firstJobDone = new AtomicInteger(0); 4. private AtomicInteger secondJobDone = new AtomicInteger(0); 5. 6. public Foo() {} 7. 8. public void first(Runnable printFirst) throws InterruptedException { 9. // printFirst.run() outputs "first". 10. printFirst.run(); 11. // mark the first job as done, by increasing its count. 12. firstJobDone.incrementAndGet(); 13. } 14. 15. public void second(Runnable printSecond) throws InterruptedException { 16. while (firstJobDone.get() != 1) { 17. // waiting for the first job to be done. 18. } 19. // printSecond.run() outputs "second". 20. printSecond.run(); 21. // mark the second as done, by increasing its count. 22. secondJobDone.incrementAndGet(); 23. } 24. 25. public void third(Runnable printThird) throws InterruptedException { 26. while (secondJobDone.get() != 1) { 27. // waiting for the second job to be done. 28. } 29. // printThird.run() outputs "third". 30. printThird.run(); 31. } 32. } 33.
方法二
CountDownLatch(int count); //构造方法，创建一个值为count 的计数器。
//阻塞当前线程，将当前线程加入阻塞队列。
//当计数器的值变为0时，在CountDownLatch上await()的线程就会被唤醒
await();
await(long timeout, TimeUnit unit);//在timeout的时间之内阻塞当前线程,时间一过则当前线程可以执行，
countDown();//对计数器进行递减1操作，当计数器递减至0时，当前线程会去唤醒阻塞队列里的所有线程。
在构造函数中使用 CountDownLatch（）来说设置两个值为1的计数器，然后在second和third函数中加入awati（）函数进行等待,当计数器为0时被唤醒
代码：
1. class Foo { 2. 3. private final CountDownLatch firstDone; 4. private final CountDownLatch secondDone; 5. 6. public Foo() { 7. firstDone = new CountDownLatch(1); 8. secondDone = new CountDownLatch(1); 9. } 10. 11. public void first(Runnable printFirst) throws InterruptedException { 12. // printFirst.run() outputs "first". Do not change or remove this line. 13. printFirst.run(); 14. firstDone.countDown(); 15. } 16. 17. public void second(Runnable printSecond) throws InterruptedException { 18. firstDone.await(); 19. // printSecond.run() outputs "second". Do not change or remove this line. 20. printSecond.run(); 21. secondDone.countDown(); 22. } 23. 24. public void third(Runnable printThird) throws InterruptedException { 25. secondDone.await(); 26. // printThird.run() outputs "third". Do not change or remove this line. 27. printThird.run(); 28. } 29. }
方法三
使用信号量Semaphore ，先初始化信号量，不允许second和third函数运行，在first函数末尾调用release函数使得计数值加一，second函数可以运行；同理，在second函数结束后调用release函数增肌计数值，使得third()函数可以运行。
1. class Foo { 2. 3. private Semaphore two = new Semaphore(0); 4. private Semaphore three = new Semaphore(0); 5. 6. public Foo() { 7. 8. } 9. 10. public void first(Runnable printFirst) throws InterruptedException { 11. 12. // printFirst.run() outputs "first". Do not change or remove this line. 13. printFirst.run(); 14. two.release(); 15. } 16. 17. public void second(Runnable printSecond) throws InterruptedException { 18. two.acquire(); 19. // printSecond.run() outputs "second". Do not change or remove this line. 20. printSecond.run(); 21. three.release(); 22. } 23. 24. public void third(Runnable printThird) throws InterruptedException { 25. three.acquire(); 26. // printThird.run() outputs "third". Do not change or remove this line. 27. printThird.run(); 28. } 29. }
方法四
使用object和synchronized进行原子操作
1. public class Foo { 2. private int flag = 1; 3. private final Object object = new Object(); 4. 5. public Foo() { 6. 7. } 8. 9. public void first(Runnable printFirst) throws InterruptedException { 10. synchronized (object) { 11. while (flag != 1) object.wait(); 12. printFirst.run(); 13. flag = 2; 14. object.notifyAll(); 15. } 16. } 17. 18. public void second(Runnable printSecond) throws InterruptedException { 19. synchronized (object) { 20. while (flag != 2) object.wait(); 21. printSecond.run(); 22. flag = 3; 23. object.notifyAll(); 24. } 25. } 26. 27. public void third(Runnable printThird) throws InterruptedException { 28. synchronized (object) { 29. while (flag != 3) object.wait(); 30. printThird.run(); 31. } 32. 33. } 34. }
