全局变量仅会瞬时更改，这是因为同时进行读/写吗？

Question

I have a global variable and multiple threads which are consistently reading the variable every few seconds.

Now I have a method in one thread which writes to the variable, and when I print out the global variable within the function where I am writing to it, I see the write has taken place and the variable value is changed as desired.

But when I read from outside the scope of the function immediately after it executes, the global variable has in fact not changed, and the write has not been successful.

Does it sound like this is due to simultaneous read/write? I'm thinking there is something going on I don't understand, though intuitively I can't see how multithreading would change anything as the global variable is being changed at least momentarily.

No writes are happening other than the one mentioned.

I haven't read up on multithreading too much so I just want a high-level "yes this is possible and makes sense with multithreading and global variables" and I will read up as necessary.

Answer 1

Without seeing the actual code I can smell the problem, just use volatile in front of your global static variable and see the magic. Problem solved.

More information can be found at Volatile vs static

Answer 2

Are you sure you don't have two variables with the same name?

I print out the global variable within the function where I am writing to it, I see the write has taken place...But when I read from outside the scope ... the global variable has in fact not changed.

That's what I would expect if your function declared a local variable with a name that shadows the global.

Answer 3

You should declare your variable as volatile as well as synchronize access to it. The following code is an example to prevent collisions between threads to access myVariable.

public class ChangeVariable {
    static volatile int myVariable;
    static final Object lock = new Object();

    public static void setVariable(int value) {
        synchronized (lock) {
            myVariable = value;
        }
    }

    public static int getVariable() {
        synchronized (lock) {
            return myVariable;
        }
    }
}