三个线程交替打印 §

使用 synchronized §

public class Main {
    public static void main(String[] args) 
            throws InterruptedException {
        PrintABC printABC = new PrintABC();
        new Thread(() -> {printABC.printABC(0);}).start();
        new Thread(() -> {printABC.printABC(1);}).start();
        new Thread(() -> {printABC.printABC(2);}).start();         
    }  
}
 
class PrintABC {
    private static final Object LOCK = new Object();
    private int num = 0;
    
    public void printABC(int targetNum) {
        for (int i = 0; i < 10;) {
            synchronized (LOCK) {
                while (num % 3 != targetNum) {
                    try {
                        LOCK.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println(Thread.currentThread().getName() 
                        + ": " + num);
                num++;
                i++;
                LOCK.notifyAll();
            }
        }
    }   
}

使用 thread.join §

class Join_ABC {
 
    public static void main(String[] args) 
            throws InterruptedException {
        for (int i = 0; i < 10; i++) {
            Thread t1 = new Thread(new printABC(null), "A");
            Thread t2 = new Thread(new printABC(t1), "B");
            Thread t3 = new Thread(new printABC(t2), "C");
            t1.start();
            t2.start();
            t3.start();
            //这里是要保证只有t1、t2、t3为一组，进行执行才能保证t1->t2->t3的执行顺序。
            Thread.sleep(10); 
        }
 
    }
 
    static class printABC implements Runnable{
        private Thread beforeThread;
        public printABC(Thread beforeThread) {
            this.beforeThread = beforeThread;
        }
        @Override
        public void run() {
            if (beforeThread != null) {
                try {
                    beforeThread.join(); 
                    System.out.print(
                            Thread.currentThread().getName());
                } catch (Exception e) {
                    e.printStackTrace();
                }
            } else {
                System.out.print(Thread.currentThread().getName());
            }
        }
    }
}

使用 Lock §

 class Lock_ABC {
 
    private int num; 
    private Lock lock = new ReentrantLock();
 
    private void printABC(int targetNum) {
        for (int i = 0; i < 10; ) {
            lock.lock();
            if (num % 3 == targetNum) {
                num++;
                i++;
                System.out.print(Thread.currentThread().getName());
            }
            lock.unlock();
        }
    }
 
    public static void main(String[] args) {
        Lock_ABC lockABC = new Lock_ABC();
 
        new Thread(() -> {
            lockABC.printABC(0);
        }, "A").start();
 
        new Thread(() -> {
            lockABC.printABC(1);
        }, "B").start();
 
        new Thread(() -> {
            lockABC.printABC(2);
        }, "C").start();
    }
}

使用 Lock & Condition §

class LockConditionABC {
 
    private int num;
    private static Lock lock = new ReentrantLock();
    private static Condition c1 = lock.newCondition();
    private static Condition c2 = lock.newCondition();
    private static Condition c3 = lock.newCondition();
 
    private void printABC(int targetNum, Condition currentThread
            , Condition nextThread) {
        for (int i = 0; i < 10; ) {
            lock.lock();
            try {
                while (num % 3 != targetNum) {
                    //阻塞当前线程
                    currentThread.await();  
                }
                num++;
                i++;
                System.out.print(Thread.currentThread().getName());
                // 唤醒下一个线程，而不是唤醒所有线程
                nextThread.signal();    
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    }
 
    public static void main(String[] args) {
        LockConditionABC print = new LockConditionABC();
        new Thread(() -> {
            print.printABC(0, c1, c2);
        }, "A").start();
        new Thread(() -> {
            print.printABC(1, c2, c3);
        }, "B").start();
        new Thread(() -> {
            print.printABC(2, c3, c1);
        }, "C").start();
    }
}

使用 Semaphore §

class SemaphoreABC {
    //因为先执行线程A，所以这里设s1的计数器为1
    private static Semaphore s1 = new Semaphore(1); 
    private static Semaphore s2 = new Semaphore(0);
    private static Semaphore s3 = new Semaphore(0);
 
    private void printABC(Semaphore currentThread
            , Semaphore nextThread) {
        for (int i = 0; i < 10; i++) {
            try {
                //阻塞当前线程，即信号量的计数器减1为0
                currentThread.acquire();       
                System.out.println(Thread.currentThread().getName());
                //唤醒下一个线程，即信号量的计数器加1
                nextThread.release();          
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
 
    public static void main(String[] args) 
            throws InterruptedException {
        SemaphoreABC printer = new SemaphoreABC();
        
        new Thread(() -> {
            printer.printABC(s1, s2);
        }, "A").start();
        Thread.sleep(10);
        
        new Thread(() -> {
            printer.printABC(s2, s3);
        }, "B").start();
        Thread.sleep(10);
        
        new Thread(() -> {
            printer.printABC(s3, s1);
        }, "C").start();
    }
}