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java多線程實(shí)現(xiàn)有序輸出ABC

更新時(shí)間：2019年08月29日 15:41:41 作者：MeAndJack

這篇文章主要為大家詳細(xì)介紹了java多線程實(shí)現(xiàn)有序輸出ABC，具有一定的參考價(jià)值，感興趣的小伙伴們可以參考一下

3個(gè)線程，線程1輸出A，線程2輸出B，線程3輸出C，讓這個(gè)3個(gè)線程循環(huán)有序地輸出ABCABC…

看到這個(gè)題目，感覺(jué)很有意思，問(wèn)題的本質(zhì)是在多線程執(zhí)行環(huán)境，控制線程的執(zhí)行順序，實(shí)現(xiàn)的方式有非常多種，本質(zhì)上需要解決Java多線程環(huán)境下的線程執(zhí)行的同步和利用鎖機(jī)制來(lái)控制線程的執(zhí)行順序。

方式1：利用synchronized

這種方式也就是使用java內(nèi)置的monitor機(jī)制，配合wait和notifyAll，代碼如下：

（1）利用volatile做線程間資源的同步訪問(wèn)，同時(shí)作為線程調(diào)度的標(biāo)志；
（2）利用notifyAll來(lái)喚醒其他等待當(dāng)前的monitor資源的線程；

public class ThreadOrderWithSync {

  private volatile int flag = 'A';
  private final static Object LOCK = new Object();

  Runnable a = () -> {
    while (true) {
      synchronized (LOCK) {
        if (flag == 'A' ) {
          System.out.println("A");

          flag = 'B';
          // let other thread race to get the monitor
          LOCK.notifyAll();
        } else {
          try {
            LOCK.wait();
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
        }
      }
    }
  };

  Runnable b = () -> {
    while (true) {
      synchronized (LOCK) {
        if (flag == 'B' ) {
          System.out.println("B");

          flag = 'C';
          // let other thread race to get the monitor
          LOCK.notifyAll();
        } else {
          try {
            LOCK.wait();
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
        }
      }
    }
  };

  Runnable c = () -> {
    while (true) {
      synchronized (LOCK) {
        if (flag == 'C' ) {
          System.out.println("C");

          flag = 'A';
          // let other thread race to get the monitor
          LOCK.notifyAll();
        } else {
          try {
            LOCK.wait();
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
        }
      }
    }
  };

  public void runTest() {
    Thread ta = new Thread(a);
    Thread tb = new Thread(b);
    Thread tc = new Thread(c);

    ta.start();
    tb.start();
    tc.start();
  }

  public static void main(String[] args) {
    ThreadOrderWithSync sync = new ThreadOrderWithSync();
    sync.runTest();
  }
}

方式2：利用并發(fā)包ReentrantLock和Condition的鎖機(jī)制

上面方式1的synchronized機(jī)制，因?yàn)楫?dāng)前的所有線程都爭(zhēng)用同一個(gè)monitor資源，因此只能通過(guò)notifyAll來(lái)通知其他線程來(lái)加鎖，因此每次都會(huì)出現(xiàn)race condition，但是，通過(guò)ReentrantLock的Condition，我們可以精確控制，下一個(gè)該喚醒signal的線程是哪一個(gè)(因?yàn)槲覀冎缊?zhí)行的順序是A->B->C的循環(huán))，相比synchronized的機(jī)制，Condition機(jī)制可以更精細(xì)化線程的調(diào)度設(shè)計(jì)，代碼示例如下：

/**
 * @author xijin.zeng created on 2018/8/31
 * Thrads runing order: A->B->C
 */
public class ThreadOrderWithCondition {

  private static final ReentrantLock LOCK = new ReentrantLock();
  private static final Condition C_A = LOCK.newCondition();
  private static final Condition C_B = LOCK.newCondition();
  private static final Condition C_C = LOCK.newCondition();

  /**
   * init for A to run first
   */
  private volatile int flag = 'A';

  Runnable a = () -> {
    while (true) {
      LOCK.lock();

      if (flag == 'A') {
        System.out.println("A");
        flag = 'B';
        // signal B to run
        C_B.signal();
      } else {
        try {
          // block and wait signal to invoke
          C_A.await();
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }

      LOCK.unlock();
    }
  };

  Runnable b = () -> {
    while (true) {
      LOCK.lock();

      if (flag == 'B') {
        System.out.println("B");
        flag = 'C';
        // signal C to run
        C_C.signal();
      } else {
        try {
          // block and wait signal to invoke
          C_B.await();
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }

      LOCK.unlock();
    }
  };

  Runnable c = () -> {
    while (true) {
      LOCK.lock();

      if (flag == 'C') {
        System.out.println("C");
        flag = 'A';
        // signal A to run
        C_A.signal();
      } else {
        try {
          // block and wait signal to invoke
          C_C.await();
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }

      LOCK.unlock();
    }
  };

  public void runTest() {
    Thread threadA = new Thread(a);
    Thread threadB = new Thread(b);
    Thread threadC = new Thread(c);

    threadA.start();
    threadB.start();
    threadC.start();
  }

  public static void main(String[] args) {
    ThreadOrderWithCondition o = new ThreadOrderWithCondition();
    o.runTest();
  }
}

以上就是本文的全部?jī)?nèi)容，希望對(duì)大家的學(xué)習(xí)有所幫助，也希望大家多多支持腳本之家。

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