三个不同线程顺序打印ABC十种写法，进来跪下吧！

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写法一：使用信号量Semaphore

public class Foo {

    private Semaphore semaphoreB = new Semaphore(0);
    private Semaphore semaphoreC = new Semaphore(0);

    public Foo() {}

    public void first() throws InterruptedException {
        System.out.println("A");
        semaphoreB.release();
    }

    public void second() throws InterruptedException {
        semaphoreB.acquire();
        System.out.println("B");
        semaphoreC.release();
    }

    public void third() throws InterruptedException {
        semaphoreC.acquire();
        System.out.println("C");
    }
}
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写法二：加synchronized锁

public class Foo {

    private Object lock = new Object();
    private boolean a;
    private boolean b;

    public Foo() {}

    public void first() throws InterruptedException {
        synchronized (lock) {
            System.out.println("A");
            a = true;
            lock.notifyAll();
        }
    }

    public void second() throws InterruptedException {
        synchronized (lock) {
            while (!a) {
                lock.wait();
            }
            System.out.println("B");
            b = true;
            lock.notifyAll();
        }
    }

    public void third() throws InterruptedException {
        synchronized (lock) {
            while (!b) {
                lock.wait();
            }
            System.out.println("C");
        }
    }
}

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写法三：使用原子AtomicInteger

public class Foo {

    private AtomicInteger a2 = new AtomicInteger(0);
    private AtomicInteger a3 = new AtomicInteger(0);

    public Foo() {}

    public void first() throws InterruptedException {
        System.out.println("A");
        a2.incrementAndGet();
    }

    public void second() throws InterruptedException {
        while (a2.get() != 1) {}
        System.out.println("B");
        a3.incrementAndGet();
    }

    public void third() throws InterruptedException {
        while (a3.get() != 1) {}
        System.out.println("C");
    }
}
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写法四：使用队列BlockingQueue

public class Foo {

    BlockingQueue<String> queue2 = new ArrayBlockingQueue<>(1);
    BlockingQueue<String> queue3 = new ArrayBlockingQueue<>(1);
    
    public Foo() {}

    public void first() throws InterruptedException {
        System.out.println("A");
        queue2.offer("b");
    }

    public void second() throws InterruptedException {
        while (queue2.size() == 0) {
        }
        System.out.println("B");
        queue3.offer("c");
    }

    public void third() throws InterruptedException {
        while (queue3.size() == 0) {
        }
        System.out.println("C");
    }
}
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写法五：使用Condition

public class Foo {

    private int number = 1;
    private Lock lock = new ReentrantLock();
    private Condition a = lock.newCondition();
    private Condition b = lock.newCondition();
    private Condition c = lock.newCondition();

    public Foo() {}

    public void first() throws InterruptedException {
        lock.lock();
        try {
            while (number != 1) {
                a.await();
            }
            System.out.println("A");
            number = 2;
            b.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void second() throws InterruptedException {
        lock.lock();
        try {
            while (number != 2) {
                b.await();
            }
            System.out.println("B");
            number = 3;
            c.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void third() throws InterruptedException {
        lock.lock();
        try {
            while (number != 3) {
                c.await();
            }
            System.out.println("C");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}
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写法六:使用计数器CountDownLatch

public class Foo {

    private CountDownLatch c2 = new CountDownLatch(1);
    private CountDownLatch c3 = new CountDownLatch(1);

    public Foo() {}

    public void first() throws InterruptedException {
        System.out.println("A");
        c2.countDown();
    }

    public void second() throws InterruptedException {
        c2.await();
        System.out.println("B");
        c3.countDown();
    }

    public void third() throws InterruptedException {
        c3.await();
        System.out.println("C");

    }
 }
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测试代码

public static void main(String[] args) throws InterruptedException {
        Foo foo = new Foo();
        new Thread(() -> {
            try {
                foo.first();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, "A").start();
        new Thread(() -> {
            try {
                foo.second();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, "B").start();

        new Thread(() -> {
            try {
                foo.third();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, "C").start();
    }
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