Java如何实现多线程循环打印

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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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