Java多线程通信:交替打印ABAB实例

使用wait()和notify()实现Java多线程通信：两个线程交替打印A和B，如ABABAB

public class Test {
  public static void main(String[] args) {
    final PrintAB print = new PrintAB();
    new Thread(new Runnable() {
      public void run(){
        for(int i=0;i<5;i++) {
          print.printA();
          }
          }
    }).start();
    new Thread(new Runnable() {
      public void run() {
        for(int i=0;i<5;i++) {
          print.printB(); }
              }
     }).start();
    }
 }
 class PrintAB{
  private boolean flag = true;
  public synchronized void printA () {
      while(!flag) {
        try {
          this.wait();
        } catch (InterruptedException e) {
          e.printStackTrace();
               } }
        System.out.print("A");
         flag = false;
         this.notify();
      }
  public synchronized void printB () {
      while(flag) {
        try {
          this.wait();
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
      System.out.print("B");
      flag = true;
      this.notify(); }
 }

补充知识：Java多个线程顺序打印数字

要求

启动N个线程, 这N个线程要不间断按顺序打印数字1-N. 将问题简化为3个线程无限循环打印1到3

方法一: 使用synchronized

三个线程无序竞争同步锁, 如果遇上的是自己的数字, 就打印. 这种方式会浪费大量的循环

public class TestSequential1 {
  private volatile int pos = 1;
  private volatile int count = 0;

  public void one(int i) {
    synchronized (this) {
      if (pos == i) {
        System.out.println("T-" + i + " " + count);
        pos = i % 3 + 1;
        count = 0;
      } else {
        count++;
      }
    }
  }

  public static void main(String[] args) {
    TestSequential1 demo = new TestSequential1();
    for (int i = 1; i <=3; i++) {
      int j = i;
      new Thread(()->{
        while(true) {
          demo.one(j);
        }
      }).start();
    }
  }
}

输出

T-1 0
T-2 5793
T-3 5285
T-1 2616
T-2 33
T-3 28
T-1 22
T-2 44
T-3 6
T-1 881
T-2 118358
T-3 247380
T-1 30803
T-2 29627
T-3 52044
...

方法二: 使用synchronized配合wait()和notifyAll()

竞争同步锁时使用wait()和notifyAll(), 可以避免浪费循环

public class TestSequential4 {
  private volatile int pos = 1;
  private volatile int count = 0;
  private final Object obj = new Object();

  public void one(int i) {
    System.out.println(i + " try");
    synchronized (obj) {
      System.out.println(i + " in");
      try {
        while (pos != i) {
          count++;
          System.out.println(i + " wait");
          obj.wait();
        }
        System.out.println("T-" + i + " " + count);
        pos = i % 3 + 1;
        count = 0;
        obj.notifyAll();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }

  public static void main(String[] args) {
    TestSequential4 demo = new TestSequential4();
    for (int i = 3; i >=1; i--) {
      int j = i;
      new Thread(()->{
        while(true) {
          demo.one(j);
        }
      }).start();
    }
  }
}

输出

3 try
3 in
3 wait
2 try
2 in
2 wait
1 try
1 in
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
1 try
1 in
1 wait
T-2 1
2 try
2 in
2 wait
T-3 1
3 try
3 in
3 wait
2 wait
T-1 2
...

方法三: 使用可重入锁

用Lock做, 非公平锁, 三个线程竞争, 如果遇上的是自己的数字, 就打印. 这种方式也会浪费大量的循环

public class TestSequential2 {
  private final Lock lock = new ReentrantLock();
  private volatile int pos = 1;
  private volatile int count = 0;

  public void one(int i) {
    lock.lock();
    if (pos == i) {
      System.out.println("T-" + i + " " + count);
      pos = i % 3 + 1;
      count = 0;
    } else {
      count++;
    }
    lock.unlock();
  }

  public static void main(String[] args) {
    TestSequential2 demo = new TestSequential2();
    for (int i = 1; i <=3; i++) {
      int j = i;
      new Thread(()->{
        while(true) {
          demo.one(j);
        }
      }).start();
    }
  }
}

输出

T-1 0
T-2 0
T-3 323
T-1 54
T-2 68964
T-3 97642
T-1 6504
T-2 100603
T-3 6989
T-1 1313
T-2 0
T-3 183741
T-1 233
T-2 5081
T-3 164367
..

方法四: 使用可重入锁, 启用公平锁

和3一样, 但是使用公平锁, 这种情况下基本上可以做到顺序执行, 偶尔会产生多一次循环

private final Lock lock = new ReentrantLock(true);

输出

T-1 0
T-2 0
T-3 0
T-1 0
T-2 0
T-3 0
T-1 0
T-2 0
T-3 0
T-1 0
T-2 0
T-3 1
T-1 1
T-2 1
T-3 1
...

方法五: 使用Condition

每个线程如果看到不是自己的计数, 就await(), 如果是自己的计数, 就完成打印动作, 再signalAll()所有其他线程去继续运行, 自己在下一个循环后, 即使又继续执行, 也会因为计数已经变了而await.

如果ReentrantLock构造参数使用true, 可以基本消除 ~await 这一步的输出.

public class ReentrantLockCondition2 {
  private static Lock lock = new ReentrantLock();
  private static Condition condition = lock.newCondition();
  private volatile int state = 1;

  private void handle(int state) {
    lock.lock();
    try {
      while(true) {
        while(this.state != state) {
          System.out.println(state + " ~await");
          condition.await();
        }
        System.out.println(state);
        this.state = state % 3 + 1;
        condition.signalAll();
        System.out.println(state + " await");
        condition.await();
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    } finally {
      lock.unlock();
    }
  }

  public static void main(String[] args) {
    ReentrantLockCondition2 rlc = new ReentrantLockCondition2();
    new Thread(()->rlc.handle(1)).start();
    new Thread(()->rlc.handle(2)).start();
    new Thread(()->rlc.handle(3)).start();
  }
}

方法六: 使用多个Condition

给每个线程不同的condition. 这个和4的区别是, 可以用condition.signal()精确地通知对应的线程继续执行(在对应的condition上await的线程, 可能是多个). 这种情况下是可以多个线程都不unlock锁的情况下进行协作的. 注意下面的while(true)循环是在lock.lock()方法内部的.

public class ReentrantLockCondition {
  private static Lock lock = new ReentrantLock();
  private static Condition[] conditions = {lock.newCondition(), lock.newCondition(), lock.newCondition()};
  private volatile int state = 1;

  private void handle(int state) {
    lock.lock();
    try {
      while(true) {
        while(this.state != state) {
          conditions[state - 1].await();
        }
        System.out.println(state);
        this.state = state % 3 + 1;
        conditions[this.state - 1].signal();
        conditions[state - 1].await();
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    } finally {
      lock.unlock();
    }
  }

  public static void main(String[] args) {
    ReentrantLockCondition rlc = new ReentrantLockCondition();
    new Thread(()->rlc.handle(1)).start();
    new Thread(()->rlc.handle(2)).start();
    new Thread(()->rlc.handle(3)).start();
  }
}

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