Question

I’m new to Java so I have a simple question that I don’t know where to start from –
I need to write a function that accepts an Action, at a multi-threads program , and only the first thread that enter the function do the action, and all the other threads wait for him to finish, and then return from the function without doing anything.

As I said – I don’t know where to begin because,
first – there isn’t a static var at the function (static like as in c / c++ ) so how do I make it that only the first thread would start the action, and the others do nothing ?
second – for the threads to wait, should I use

public synchronized void lala(Action doThis)
{....}

or should i write something like that inside the function

synchronized (this)
{
...
notify();
}

Thanks !

Answer 1

If you want all threads arriving at a method to wait for the first, then they must synchronize on a common object. It could be the same instance (this) on which the methods are invoked, or it could be any other object (an explicit lock object).

If you want to ensure that the first thread is the only one that will perform the action, then you must store this fact somewhere, for all other threads to read, for they will execute the same instructions.

Going by the previous two points, one could lock on this ‘fact’ variable to achieve the desired outcome

static final AtomicBoolean flag = new AtomicBoolean(false); // synchronize on this, and also store the fact. It is static so that if this is in a Runnable instance will not appear to reset the fact. Don't use the Boolean wrapper, for the value of the flag might be different in certain cases.

public void lala(Action doThis)
{
    synchronized (flag) // synchronize on the flag so that other threads arriving here, will be forced to wait
    {
        if(!flag.get()) // This condition is true only for the first thread.
        {
           doX();
           flag.set(true); //set the flag so that other threads will not invoke doX.
        }
    }
    ...
    doCommonWork();
    ...
}