交替打印

给你一个类：

class FooBar {
  public void foo() {
    for (int i = 0; i < n; i++) {
      print("foo");
    }
  }

  public void bar() {
    for (int i = 0; i < n; i++) {
      print("bar");
    }
  }
}

两个不同的线程将会共用一个 FooBar 实例：

• 线程 A 将会调用 foo() 方法，而
• 线程 B 将会调用 bar() 方法

请设计修改程序，以确保 "foobar" 被输出 n 次。

示例 1：

输入：n = 1
输出："foobar"
解释：这里有两个线程被异步启动。其中一个调用 foo() 方法, 另一个调用 bar() 方法，"foobar" 将被输出一次。

示例 2：

输入：n = 2
输出："foobarfoobar"
解释："foobar" 将被输出两次。

提示：

• 1 <= n <= 1000

Blocking Queue

public class FooBar {
    private int n;
    private BlockingQueue<Integer> bar = new LinkedBlockingQueue<>(1);
    private BlockingQueue<Integer> foo = new LinkedBlockingQueue<>(1);
    public FooBar(int n) {
        this.n = n;
    }
    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            foo.put(i);
            printFoo.run();
            bar.put(i);
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            bar.take();
            printBar.run();
            foo.take();
        }
    }
}

ReentrantLock+Condition

class FooBar {
    private int n;

    public FooBar(int n) {
        this.n = n;
    }
    Lock lock = new ReentrantLock(true);
    private final Condition foo = lock.newCondition();
    volatile boolean flag = true;
    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            lock.lock();
            try {
                while(!flag) {
                    foo.await();
                }
                printFoo.run();
                flag = false;
                foo.signal();
            }finally {
                lock.unlock();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n;i++) {
            lock.lock();
            try {
                while(flag) {
                    foo.await();
                }
                printBar.run();
                flag = true;
                foo.signal();
            }finally {
                lock.unlock();
            }
        }
    }
}

synchronized + 标志位 + 唤醒

class FooBar {
    private int n;
    // 标志位，控制执行顺序，true执行printFoo，false执行printBar
    private volatile boolean type = true;
    private final Object foo=  new Object(); // 锁标志

    public FooBar(int n) {
        this.n = n;
    }
    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            synchronized (foo) {
                while(!type){
                    foo.wait();
                }
                printFoo.run();
                type = false;
                foo.notifyAll();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            synchronized (foo) {
                while(type){
                    foo.wait();
                }
                printBar.run();
                type = true;
                foo.notifyAll();
            }
        }
    }
}

Semaphore

class FooBar {
    private int n;
    private Semaphore foo = new Semaphore(1);
    private Semaphore bar = new Semaphore(0);
    public FooBar(int n) {
        this.n = n;
    }

    public void foo(Runnable printFoo) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            foo.acquire();
            printFoo.run();
            bar.release();
        }
    }
    public void bar(Runnable printBar) throws InterruptedException {
        for (int i = 0; i < n; i++) {
            bar.acquire();
            printBar.run();
            foo.release();
        }
    }
}

自旋 + 让出CPU

class FooBar {
    private int n;

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

    volatile boolean permitFoo = true;

    public void foo(Runnable printFoo) throws InterruptedException {     
        for (int i = 0; i < n; ) {
            if (permitFoo) {
                printFoo.run();
                i++;
                permitFoo = false;
            } else {
                Thread.yield();
            }
        }
    }

    public void bar(Runnable printBar) throws InterruptedException {       
        for (int i = 0; i < n; ) {
            if(!permitFoo) {
            　　printBar.run();
            　　i++;
            　　permitFoo = true;
            } else {
               Thread.yield();
            }
        }
    }
}