Winter Winter - 2 months ago 9
Java Question

How to synchronize or lock upon variables in Java?

Let me use this small and simple sample:

class Sample {
private String msg = null;

public void newmsg(String x){
msg = x;
}

public String getmsg(){
String temp = msg;
msg = null;
return temp;
}
}


Let's assume the function
newmsg()
is called by other threads that I don't have access to.

I want to use the synchonize method to guarantee that the string
msg
is only used by one function per time. In other words, function
newmsg()
cannot run at the same time as
getmsg()
.

Answer

That's pretty easy:

class Sample {
    private String message = null;
    private final Object lock = new Object();

    public void newMessage(String x) {
        synchronized (lock) {
            message = x;
        }
    }

    public String getMessage() {
        synchronized (lock) {
            String temp = message;
            message = null;
            return temp;
        }
    }
}

I fixed your getmsg method by the way - it would always have returned null before.

Note that I didn't either make the methods themselves synchronized or synchronize on this. I firmly believe that it's a good idea to only acquire locks on objects which only your code has access to, unless you're deliberately exposing the lock. It makes it a lot easier to reassure yourself that nothing else is going to acquire locks in a different order to your code, etc.

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