Java五種方式實現(xiàn)多線程循環(huán)打印問題

更新時間：2021年12月14日 10:57:37 作者：北城酒卿

本文主要介紹了Java五種方式實現(xiàn)多線程循環(huán)打印問題，文中通過示例代碼介紹的非常詳細，具有一定的參考價值，感興趣的小伙伴們可以參考一下

wait-notify

循環(huán)打印問題可以通過設置目標值，每個線程想打印目標值，如果拿到鎖后這次輪到的數(shù)不是它想要的就進入wait

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

    private void print_ABC(int target) {
        synchronized (Lock) {
            //循環(huán)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問題可以理解為有個全局計數(shù)器記錄當前打印到了哪個數(shù)，其它就和循環(huán)打印ABC問題相同。

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

一個線程內調用另一個線程的join()方法可以讓另一個線程插隊執(zhí)行，比如Main方法里調用了A.join()，那么此時cpu會去執(zhí)行A線程中的任務，執(zhí)行完后再看Main是否能搶到運行權。所以對于ABC，我們可以對B說讓A插隊，對C說讓B插隊

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都是我們常用的加鎖方式，不過后者可以中斷，可以實現(xiàn)公平鎖，可以使用condition…但是需要我們手動釋放鎖。jdk8后二者性能差不多，畢竟synchronized有鎖升級的過程嘛。

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

以上方式如果線程搶到鎖后發(fā)現(xiàn)自己無法執(zhí)行任務，那么就釋放，然后別的線程再搶占再看是不是自己的…這種方式比較耗時，如果我們能實現(xiàn)精準喚醒鎖呢，即A完成任務后喚醒它的下一個即B，這就用到我們的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();  //阻塞當前線程
                }
                num++;
                i++;
                System.out.print(Thread.currentThread().getName());
                nextThread.signal();    //喚醒下一個線程
            } 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

小伙伴們有沒有想到過，在生產(chǎn)者消費者模型中我們有哪幾種實現(xiàn)方式呢？wait\notify，ReentrantLock，Semaphone，阻塞隊列，管道輸入輸出流。
對的就是Semaphone。
Semaphore有acquire方法和release方法。當調用acquire方法時線程就會被阻塞，直到獲得許可證為止。當調用release方法時將向Semaphore中添加一個許可證。如果沒有獲取許可證的線程， Semaphore只是記錄許可證的可用數(shù)量。
使用Semaphore也可以實現(xiàn)精準喚醒。

class SemaphoreABC {

    private static Semaphore s1 = new Semaphore(1); //因為先執(zhí)行線程A，所以這里設s1的計數(shù)器為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();       //阻塞當前線程，即信號量的計數(shù)器減1為0
                System.out.print(Thread.currentThread().getName());
                nextThread.release();          //喚醒下一個線程，即信號量的計數(shù)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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