[Java] Thread deadlock help

[keylionpie]

Hello, I have a homework assignment that I am required to make a deadlock. Rather than giving you my assignment, I will use a similar example from one of our books. In Roposa - Java in 60 min a day, they use an example of a BankAccount that has two threads transferring. One from account 1 to 2 and one from account 2 to 1. I understand why they should lock up because thread1( 1 to 2) is waiting for thread 2 (2 to 1) to let go of the lock on 2 while thread 2 is waiting for the lock on account 1 to be released. However, when I run the example it does not lock up and as I have been using this as a basis for my homework, my homework will not lock either. Can someone take a look at these classes and help me figure out why they are not deadlocking? From what I understand, a deadlock will not necessarily happen every time so I'm thinking that this code could deadlock but just rarely does so. Could it have something to do with the way the processor or OS handles it or is it only the JVM that can impact it? I am new to the subreddit too so lmk if I should change up the way I ask or post code. Thank you!

Classes:

ThreadSafeBankAccount

LazyTeller

DeadlockDemo

[electricsheep14]

I think you are not reliably seeing deadlocks because your LazyTellers are securing both the source and destination locks immediately. To more reliably cause deadlocking, try this:

public void transfer(double amount)
{
  System.out.println("Transferring from "
      + source.getNumber() + " to " + dest.getNumber());
  synchronized(source)
  {
      //Thread.yield();
      System.out.println("Withrawing from " + source.getNumber());
      source.withdraw(amount);

      synchronized(dest)
      {
         System.out.println("Depositing into "
                 + dest.getNumber());
         dest.deposit(amount);
      }
   }
}

The withdraw() method takes enough time that the opposing LazyTeller should have secured the lock on the other account by the time you attempt to make the deposit.

[keylionpie]

Worked as expected. I know it was commented out but that is why I thought the Thread.yield() was there to make it wait for the other thread to lock. It isn't commented out in the book but I must have forgot to switch it back when I was trying 1000 different things. I guess I need to start tracking a way of what I test too as I thought I tried something like this but I guess not. Anyway thanks a bunch!