A question of wait() and notifyAll()

[L. Wei]

Hi, I have a question of wait() and notifyAll(). I understand all the threads need in the loop need to be in a synchronized block to lock the object. Now I have a small code using singleton to be a lock, but it doesn't work properly when invoke notifyAll().

class Lock {
  static Lock myAccount;
  static double amt;
 
  private Lock () {
    amt=1000.00;
  }
 
  public static void withdrawMoney(double amount) {
    amt-=amount;
  }
 
  public static synchronized Lock getMyAccount() {
    if(myAccount==null) {
      myAccount=new Lock();
    }
    return myAccount;
  }
}
 
class WaitingThread extends Thread {
  Lock acct;
  WaitingThread() {
    acct=Lock.getMyAccount();
  }
 
  public void run() {
    synchronized(acct) {
      try {
        acct.withdrawMoney(200);
        System.out.println("Name: "+Thread.currentThread().getName());
        acct.wait();
      } catch (InterruptedException ie) {
        ie.printStackTrace();
      }
      System.out.println(Thread.currentThread().getName());
    }
  }
}
 
public class PlayThread2 {
  public static void main(String...args) {
    Thread a=new WaitingThread();
    Thread b=new WaitingThread();
    Thread c=new WaitingThread();
    Thread d=new WaitingThread();
 
    a.setName("a");
    b.setName("b");
    c.setName("c");
    d.setName("d");
 
    a.start();
    b.start();
    c.start();
    d.start();
 
    try {
      Thread.sleep(1000);
    } catch (Exception e) {}
 
    Lock acct=Lock.getMyAccount();
    System.out.println(acct.amt);
    synchronized(acct) {
      while(true) {
        try {
          Thread.sleep(2000);
        } catch(InterruptedException ie) {
          ie.printStackTrace();
        }
        acct.notifyAll();
      }
    }
  }
}

While running the code, the waiting threads are not able to be wake up. What's the problem? Please help.

Thanks

[Henry Wong]

While running the code, the waiting threads are not able to be wake up. What's the problem? Please help.

Your thread that sends the notifications, is *not* releasing the lock. The threads do wakeup. They just can't get the synchronization lock, hence, remain blocked.

Henry

[L. Wei]

You are right! It actually release the lock but quickly regain the lock again. When I change the class PlayThread2 to

public class PlayThread2 {
  public static void main(String...args) {
    Thread a=new WaitingThread();
    Thread b=new WaitingThread();
    Thread c=new WaitingThread();
    Thread d=new WaitingThread();
 
    a.setName("a");
    b.setName("b");
    c.setName("c");
    d.setName("d");
 
    a.start();
    b.start();
    c.start();
    d.start();
 
    Lock acct=Lock.getMyAccount();
    System.out.println(acct.amt);
    while(true) {
      try {
        Thread.sleep(100);
      } catch (Exception e) {}
 
      synchronized(acct) {
        try {
          Thread.sleep(1000);
          System.out.println("here i am");
          acct.notify();
        } catch(InterruptedException ie) {
          ie.printStackTrace();
        }
      }
    }
  }
}

It works properly

Originally posted by Henry Wong:


Your thread that sends the notifications, is *not* releasing the lock. The threads do wakeup. They just can't get the synchronization lock, hence, remain blocked.

Henry