condition的使用

condition 的作用：条件锁

需求：

　　按需执行三个线程。

用wait，notify的方式：　　

/**
 * 有序线程 wait,notify版
 */
public class OrderThreadW_NVersion {
  private int flag=0;

  public synchronized void A(){
    while (flag != 0){
      try {
        wait();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    System.out.println("A");
    flag=1;
    notifyAll();
  }
  public synchronized void B(){
    while (flag != 1){
      try {
        wait();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    System.out.println("B");
    flag=2;
    notifyAll();
  }
  public synchronized void C(){
    while (flag != 2){
      try {
        wait();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    System.out.println("C");
    flag=0;
    notifyAll();
  }

  public static void main(String[] args) {
    OrderThreadW_NVersion ot = new OrderThreadW_NVersion();
    new Thread(()->{
      while (true){
        ot.A();
        try {
          Thread.sleep(500);
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
    }).start();
    new Thread(()->{
      while (true){
        ot.B();
        try {
          Thread.sleep(500);
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
    }).start();
    new Thread(()->{
      while (true){
        ot.C();
        try {
          Thread.sleep(500);
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
    }).start();
  }
}

缺点：

　　notifyAll，会唤醒所有等待的线程，不能指定特定线程的唤醒。此时就引入了condition

用condition的方式来实现：

/**
 * 有序线程，conditin版
 */
public class OrderThreadCondition {

  private int flag = 0;
  Lock lock = new ReentrantLock();
  Condition a = lock.newCondition();
  Condition b = lock.newCondition();
  Condition c = lock.newCondition();

  public void A() {
    lock.lock();
    while (flag != 0) {
      try {
        a.await();//跟object的wait一样
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    System.out.println("A");
    flag = 1;
    b.signal();//唤醒指定线程
    lock.unlock();
  }

  public void B() {
    lock.lock();
    while (flag != 1) {
      try {
        b.await();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    System.out.println("B");
    flag = 2;
    c.signal();
    lock.unlock();
  }

  public void C() {
    lock.lock();
    while (flag != 2) {
      try {
        c.await();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    System.out.println("C");
    flag = 0;
    a.signal();
    lock.unlock();
  }

  public static void option(OrderThreadCondition ot,String methodName){
    while (true){
      try {
        Method method = ot.getClass().getMethod(methodName, null);
        method.invoke(ot,null);
        Thread.sleep(1000);
      } catch (Exception e) {
        e.printStackTrace();
        System.exit(-1);
      }
    }
  }

  public static void main(String[] args) {
    OrderThreadCondition ot = new OrderThreadCondition();
    new Thread(() -> option(ot,"A")).start();
    new Thread(() -> option(ot,"B")).start();
    new Thread(() -> option(ot,"C")).start();
  }
}