Java五种方式实现多线程循环打印问题

目录

• wait-notify
• join方式
• ReentrantLock
• ReentrantLock+Condition
• Semaphore

三个线程T1、T2、T3轮流打印ABC，打印n次，如ABCABCABCABC…
N个线程循环打印1-100…

wait-notify

循环打印问题可以通过设置目标值，每个线程想打印目标值，如果拿到锁后这次轮到的数不是它想要的就进入wait

class Wait_Notify_ABC {
    private int num;
    private static final Object Lock = new Object();

    private void print_ABC(int target) {
        synchronized (Lock) {
            //循环打印
            for (int i = 0; i < 10; i++) {
                while (num % 3 != target) {
                    try {
                        Lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                num++;
                System.out.print(Thread.currentThread().getName());
                Lock.notifyAll();
            }
        }
    }

    public static void main(String[] args) {
        Wait_Notify_ABC wait_notify_abc = new Wait_Notify_ABC();
        new Thread(() -> {
            wait_notify_abc.print_ABC(0);
        }, "A").start();
        new Thread(() -> {
            wait_notify_abc.print_ABC(1);
        }, "B").start();
        new Thread(() -> {
            wait_notify_abc.print_ABC(2);
        }, "C").start();
    }
}

打印1-100问题可以理解为有个全局计数器记录当前打印到了哪个数，其它就和循环打印ABC问题相同。

class Wait_Notify_100 {

    private int num;
    private static final Object LOCK = new Object();
    private int maxnum = 100;

    private void printABC(int targetNum) {
        while (true) {
            synchronized (LOCK) {
                while (num % 3 != targetNum) {
                    if (num >= maxnum) {
                        break;
                    }
                    try {
                        LOCK.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                if (num >= maxnum) {
                    break;
                }
                num++;
                System.out.println(Thread.currentThread().getName() + ": " + num);
                LOCK.notifyAll();
            }
        }

    }

    public static void main(String[] args) {
        Wait_Notify_100 wait_notify_100 = new Wait_Notify_100();
        new Thread(() -> {
            wait_notify_100.printABC(0);
        }, "thread1").start();
        new Thread(() -> {
            wait_notify_100.printABC(1);
        }, "thread2").start();
        new Thread(() -> {
            wait_notify_100.printABC(2);
        }, "thread3").start();
    }
}

join方式

一个线程内调用另一个线程的join()方法可以让另一个线程插队执行，比如Main方法里调用了A.join()，那么此时cpu会去执行A线程中的任务，执行完后再看Main是否能抢到运行权。所以对于ABC，我们可以对B说让A插队，对C说让B插队

class Join_ABC {
    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();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.print(Thread.currentThread().getName());
        }
    }

    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();
            Thread.sleep(100);
        }
    }
}

ReentrantLock

同理，synchronized和reentrantlock都是我们常用的加锁方式，不过后者可以中断，可以实现公平锁，可以使用condition…但是需要我们手动释放锁。jdk8后二者性能差不多，毕竟synchronized有锁升级的过程嘛。

class ReentrantLock_ABC {

    private int num;
    private Lock lock = new ReentrantLock();

    private void printABC(int targetNum) {
        for (int i = 0; i < 100; ) {
            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();
    }
}

ReentrantLock+Condition

以上方式如果线程抢到锁后发现自己无法执行任务，那么就释放，然后别的线程再抢占再看是不是自己的…这种方式比较耗时，如果我们能实现精准唤醒锁呢，即A完成任务后唤醒它的下一个即B，这就用到我们的Condition啦

class ReentrantLock_Condition_ABC {

    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 < 100; ) {
            lock.lock();
            try {
                while (num % 3 != targetNum) {
                    currentThread.await();  //阻塞当前线程
                }
                num++;
                i++;
                System.out.print(Thread.currentThread().getName());
                nextThread.signal();    //唤醒下一个线程
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    }

    public static void main(String[] args) {
        ReentrantLock_Condition_ABC reentrantLockConditionAbc = new ReentrantLock_Condition_ABC();
        new Thread(() -> {
            reentrantLockConditionAbc.printABC(0, c1, c2);
        }, "A").start();
        new Thread(() -> {
            reentrantLockConditionAbc.printABC(1, c2, c3);
        }, "B").start();
        new Thread(() -> {
            reentrantLockConditionAbc.printABC(2, c3, c1);
        }, "C").start();
    }
}

Semaphore

小伙伴们有没有想到过，在生产者消费者模型中我们有哪几种实现方式呢？wait\notify，ReentrantLock，Semaphone，阻塞队列，管道输入输出流。
对的就是Semaphone。
Semaphore有acquire方法和release方法。 当调用acquire方法时线程就会被阻塞，直到获得许可证为止。 当调用release方法时将向Semaphore中添加一个许可证。如果没有获取许可证的线程， Semaphore只是记录许可证的可用数量。
使用Semaphore也可以实现精准唤醒。

class SemaphoreABC {

    private static Semaphore s1 = new Semaphore(1); //因为先执行线程A，所以这里设s1的计数器为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 {
                currentThread.acquire();       //阻塞当前线程，即信号量的计数器减1为0
                System.out.print(Thread.currentThread().getName());
                nextThread.release();          //唤醒下一个线程，即信号量的计数器加1
            } 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(100);
        new Thread(() -> {
            printer.printABC(s2, s3);
        }, "B").start();
        Thread.sleep(100);
        new Thread(() -> {
            printer.printABC(s3, s1);
        }, "C").start();
    }
}

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