Java Wait/Notify not working

Question

So I have been working on a simple wait/notify example in Java and for some reason I have not been able to get it to run properly. If anyone is able to see what might be the issue It would be very appreciated!

public class ThreadDemonstration
{
private String str = null;

Thread stringCreator = new Thread(new Runnable()
{
    public void run()
    {           
        synchronized(this)
        {               
            str = "I have text";
            notify();            
        }
    }
});

private Thread stringUser = new Thread(new Runnable()
{
    public void run()
    {
        synchronized(this)
        {
            if(str == null)
            {                   
                try {
                    System.out.println("str is null, I need help from stringCreator");
                    wait();
                    System.out.println(str);
                }
                catch (InterruptedException e) 
                {
                    e.printStackTrace();
                }
            }

        }
    }
});

public static void main (String [] args)
{
    ThreadDemonstration td = new ThreadDemonstration();
    td.stringUser.start();
    td.stringCreator.start();
}

}

My current output is: str is null, I need help from stringCreator

So for some reason the thread stringCreator does not wake up the stringUser or am I missing something else entirely?

Thank you!

Answer 1

Your blocks are synchronized over different objects. They should be synchronized over a common object, for example the monitor object below:

public class ThreadDemonstration
{
private String str = null;
    private final Object monitor = new Object();

Thread stringCreator = new Thread(new Runnable()
{
    public void run()
    {           
        synchronized(monitor)
        {               
            str = "I have text";
            monitor.notify();            
        }
    }
});

private Thread stringUser = new Thread(new Runnable()
{
    public void run()
    {
        synchronized(monitor)
        {
            while(str == null) //changed from if to while. This allows you to wait again if the thread gets woken up by something other than the appropriate notify.
            {                   
                try {
                    System.out.println("str is null, I need help from stringCreator");
                    monitor.wait();
                    //removed print statement from here
                }
                catch (InterruptedException e) 
                {
                    e.printStackTrace();
                }
            }
            System.out.println(str); //added print statement here. str is guaranteed to not be null here.
        }
    }
});

In order to avoid creating a separate object for synchronization, you can use synchronized(ThreadDemonstration.this) or synchronized(ThreadDemonstration.class) for example.