Why won't my threads wake up after the notifyAll();

Question

So I have been trying to sort this out for a couple of hours now and I'm sure its something really simple or just a simple mistake i am missing but i have a three class program, control, account and MyThreads.

Im trying to have multipule threads(cards) modify the single account, i'm trying to use a monitor so only one thread can make changes at a time, this is not what i have archived I have somehow just allowed the one thread to access the account class and no others, they seem to just disappear, i assume they are just all on wait but refuse to wake up... any help before i implode?

account code:

class account{
private static int value = 0;
private static int cards = 0;
private static int count = 0;
private static int lock = 0;


public void setValue(int temp){
value = temp;
}

public int getValue(){

//   while(lock == 1){
//    try{
//      wait();
//    }catch (InterruptedException e){
//    }
//  }
return value;
}

synchronized public void withdraw(int temp, String tempID){
while(lock == 1){
try{
wait();
}catch (InterruptedException e){}
}

lock=1;
value= value - temp;
count++;
System.out.println(count + "(" + tempID +")"+"  "+temp+"  -  "+value);
lock = 0;

this.notifyAll();

}


synchronized public void deposit(int temp, String tempID){
while(lock == 1){
try{
 wait();
}catch (InterruptedException e){}
}

lock=1;
value= value + temp;
count++;
System.out.println(count + "(" + tempID +")"+"  -  "+temp+"  "+value);
lock = 0;
this.notifyAll();
}

public void setCards(int temp){
cards = temp;
}


public int getCards(){
return cards;
}

public int getCount(){
return count;
 }
 }

control code:

class control{

public static void main(String [] args){

account acc = new account();

acc.setValue(1000);

acc.setCards(5);


 //    if(args.length > 0){
 //      try{
 //       int tempCards = Integer.parseInt(args[0]);
 //       
 //      }catch (NumberFormatException e) {
 //        System.err.println("Number of Cards : " + args[0] + " must be an integer.");
//        System.exit(1);
//      }
//      try{
//        int tempVal = 0;
//        tempVal = Integer.parseInt(args[1]);
//        acc.setValue(tempVal);
//      }catch (NumberFormatException e) {
//        System.err.println("Account Value : " + args[1] + " must be an integer.");
//        System.exit(1);
//      }
//    }else{
//      System.err.println("No values found, please start program with the number of Cards and Bank Account Value, both in integer format");
//      System.exit(1);
//    }


System.out.println("Transaction  Withdrawal  Deposit  Balance");
System.out.println("                                  " + acc.getValue());

for(int i=0; i<=((acc.getCards())-1); i++){
new MyThreads(Integer.toString(i+1));

}
}
}

MyThreads code:

class MyThreads implements Runnable{
private String ID;
private Thread t;

account acc = new account();
MyThreads(String tempID){
ID = tempID;
t = new Thread(this, ID);
t.start();
}


public void run(){

try{

for (int i = 0; i < 20; i++){


  if(Math.random()>0.5){

    int tempW = 0;

    tempW = ((int)(Math.random()*100));

    acc.withdraw(tempW, this.ID);

    //System.out.println(acc.getCount() + "(" + this.ID +")"+"  "+tempW+"  -"+acc.getValue());

    }else{

    int tempD = 0;
    tempD = ((int)(Math.random()*100));
    acc.deposit(tempD, this.ID);
    //System.out.println(acc.getCount() + "(" + this.ID +")"+"  "+"  -  "+tempD+"  "+acc.getValue());

    }

    t.sleep(500);
    }
    } catch (InterruptedException e) {
     System.out.println("Thread " +  ID + " interrupted.");
    }
    System.out.println("Thread " +  ID + " exiting.");
    }

    }

I know its a mess, forgive me im lazy.

Answer 1

Have a look at the definition of a Monitor in Java. In your code, you use the keyword synchronized for two methods, which are the same as:

 public void XX(){
    lock.lock(); // lock is a private variable 
    try {
        // code here

    } finally {
        lock.unlock();
    }
 }

In short, It is a shorthand for explicit locking and will prevent multiple threads to access the methods concurrently.

So, just remove the lock part (ie the while(lock==1) block) inside your synchronized methods and it will work. Also, if in other codes you need a real lock, use the Lock class, not an integer.

For more information, there are a lot of good introduction to multithreading on the web, for example this one .

Answer 2

Your question, and thus answer, is a wonderful mixture of static synchronized and wait-notify that's neve being called. Why use static? sounds like a magic word? skip static and make life easier.

Also note that a wait-notify is related to a specific object; if wait-notify are related to different objects they will not communicate. Have a single object that they all synchronize around.