1114. 按序打印

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题目

我们提供了一个类：

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public class Foo {
  public void one() { print("one"); }
  public void two() { print("two"); }
  public void three() { print("three"); }
}

三个不同的线程将会共用一个 Foo 实例。

线程 A 将会调用 one() 方法
线程 B 将会调用 two() 方法
线程 C 将会调用 three() 方法

请设计修改程序，以确保 two() 方法在 one() 方法之后被执行，three() 方法在 two() 方法之后被执行。

示例1:

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输入: [1,2,3]
输出: "onetwothree"
解释: 
有三个线程会被异步启动。
输入 [1,2,3] 表示线程 A 将会调用 one() 方法，线程 B 将会调用 two() 方法，线程 C 将会调用 three() 方法。
正确的输出是 "onetwothree"。

示例2:

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输入: [1,3,2]
输出: "onetwothree"
解释: 
输入 [1,3,2] 表示线程 A 将会调用 one() 方法，线程 B 将会调用 three() 方法，线程 C 将会调用 two() 方法。
正确的输出是 "onetwothree"。

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

解法一：

while循环死等

JAVA

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class Foo {
		private transient int flag = 1;

		public Foo() {

		}

		public void first(Runnable printFirst) throws InterruptedException {
			// printFirst.run() outputs "first". Do not change or remove this
			// line.
			printFirst.run();
			flag = 2;
		}

		public void second(Runnable printSecond) throws InterruptedException {
			// printSecond.run() outputs "second". Do not change or remove
			// this line.
			while (true) {
				if (2 == flag) {
					break;
				}
			}
			printSecond.run();
			flag = 3;

		}

		public void third(Runnable printThird) throws InterruptedException {
			while (true) {
				if (3 == flag) {
					break;
				}
			}
			// printThird.run() outputs "third". Do not change or remove
			// this line.
			printThird.run();

		}
	}

解法二：

使用Condition做精准唤醒

Java

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class Foo {
		private int flag = 1;
		 ReentrantLock lock = new ReentrantLock();
		 Condition condition1 = lock.newCondition();
		 Condition condition2 = lock.newCondition();
		
		public Foo() {
	        
	    }

	    public void first(Runnable printFirst) throws InterruptedException {
	        // printFirst.run() outputs "first". Do not change or remove this line.
	    	try {
	    		lock.lock();
	    		printFirst.run();
	    		flag = 2;
		        condition1.signal();
	    	} finally {
	    		lock.unlock();
	    	}
	    }

	    public void second(Runnable printSecond) throws InterruptedException {
	    	try {
	    		lock.lock();
	    		if (2 != flag) {
	    			condition1.await();
	    		}
	    		
	    		// printSecond.run() outputs "second". Do not change or remove this line.
		        printSecond.run();
		        flag = 3;
		        condition2.signal();
	    	} finally {
	    		lock.unlock();
	    	}
	        
	    }

	    public void third(Runnable printThird) throws InterruptedException {
	    	try {
	    		lock.lock();
	    		if (3 != flag) {
	    			condition2.await();
	    		}
	    		// printThird.run() outputs "third". Do not change or remove this line.
		        printThird.run();
	    	} finally {
	    		lock.unlock();
	    	}
	    }
	}

解法三：

使用wait和notify

Java

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class Foo {
		private int flag = 1;

		private Object lock = new Object();
		
		public Foo() {

		}

		public void first(Runnable printFirst) throws InterruptedException {
			// printFirst.run() outputs "first". Do not change or remove this
			// line.
			synchronized(lock) {
				while (1 != flag) {
					lock.wait();
				}
			printFirst.run();
			flag = 2;
			lock.notifyAll();
			}
			
		}

		public void second(Runnable printSecond) throws InterruptedException {
			// printSecond.run() outputs "second". Do not change or remove
			// this line.
			synchronized(lock) {
				while (2 != flag) {
					lock.wait();
				}
				printSecond.run();
				flag = 3;
				lock.notifyAll();
			}
		}

		public void third(Runnable printThird) throws InterruptedException {
			synchronized(lock) {
				while (3 != flag) {
					lock.wait();
				}
				// printThird.run() outputs "third". Do not change or remove
				// this line.
				printThird.run();
				flag = 3;
			}
		}
	}

解法四：

使用CountDownLatch

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class Foo {
		private CountDownLatch c1 = new CountDownLatch(1);
		private CountDownLatch c2 = new CountDownLatch(1);
		
		public Foo() {
		}

		public void first(Runnable printFirst) throws InterruptedException {
			// printFirst.run() outputs "first". Do not change or remove this
			// line.
			printFirst.run();
			c1.countDown();
		}

		public void second(Runnable printSecond) throws InterruptedException {
			c1.await();
			// printSecond.run() outputs "second". Do not change or remove
			// this line.
			printSecond.run();
			c2.countDown();
		}

		public void third(Runnable printThird) throws InterruptedException {
			c2.await();
			// printThird.run() outputs "third". Do not change or remove
			// this line.
			printThird.run();
		}
	}

解法五：

使用Semaphore

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class Foo {
		private Semaphore s1 = new Semaphore(0);
		private Semaphore s2 = new Semaphore(0);
		
		public Foo() {
		}

		public void first(Runnable printFirst) throws InterruptedException {
			// printFirst.run() outputs "first". Do not change or remove this
			// line.
			printFirst.run();
			s1.release();;
		}

		public void second(Runnable printSecond) throws InterruptedException {
			s1.acquire();;
			// printSecond.run() outputs "second". Do not change or remove
			// this line.
			printSecond.run();
			s2.release();;
		}

		public void third(Runnable printThird) throws InterruptedException {
			s2.acquire();
			// printThird.run() outputs "third". Do not change or remove
			// this line.
			printThird.run();
			s2.release();
		}
	}
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