How does volatile keyword ensure an object`s fields are visible to other threads?

问题

public class ObjectPropertiesVolatileTest {

    static Cup cup = new Cup();

    public static void changeColor() {
        cup.setColor("black"); // change color of cup to black
    }

    public static void main(String[] args) {
        cup.setColor("red"); // initialize color of cup as red

        for(int i = 0; i < 3; i++) {
            new Thread(){
                public void run() {
                    for(int i = 0; i < 10; i++) 
                        System.out.println(Thread.currentThread().getName()
                                + " " + i + " is " + cup.getColor());

                    if(Thread.currentThread().getName().equals("Thread-1"))
                        changeColor(); // Thread-1 changes the color of cup

                    for(int i = 0; i < 10; i++) 
                        System.out.println(Thread.currentThread().getName()
                                + " " +i + " is " + cup.getColor());

                }
            }.start();
        }

    }

}

class Cup {

    private volatile String color;

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }
}

My question is: how to ensure the field color of the object cup is visible to other threads only using volatile keyword instead of using synchronization or lock?

回答1:

I believe it is working fine. The problem is that the printing, call to print statement, merging of strings in System.out.println(Thread.currentThread().getName() + " " +i + " is " + cup.getColor());, etc. are not synchronised.

I added print statements to your getters/setters to log the time of calls:-

public String getColor() {
    System.out.println("-------------------------get " + Thread.currentThread().getName() + this.color + " " + System.nanoTime());
    return color;
}

public void setColor(String color) {
    System.out.println("-------------------------set " + Thread.currentThread().getName() + color + this.color + " " + System.nanoTime());
    this.color = color;
}

A snippet of what it printed:-

.
.
.
Thread-0 4 is red
-------------------------get Thread-1red 1356826035808750
Thread-1 8 is red
-------------------------get Thread-2red 1356826035425706
Thread-2 2 is red
-------------------------get Thread-1red 1356826035963697
Thread-1 9 is red
-------------------------get Thread-0red 1356826035894581
-------------------------set Thread-1blackred 1356826036165653
-------------------------get Thread-2red 1356826036036858
Thread-2 3 is black
-------------------------get Thread-1black 1356826036219728
Thread-1 0 is black
Thread-0 5 is red
-------------------------get Thread-1black 1356826036402925
Thread-1 1 is black
-------------------------get Thread-2black 1356826036305139
Thread-2 4 is black
-------------------------get Thread-1black 1356826036535236
Thread-1 2 is black
.
.
.

The problem you are concerned about is this line:- Thread-0 5 is red It should have printed black, right?

Well, this thread entered the getter before the other thread changed the value. This is evident from these lines:-

-------------------------get Thread-0red 1356826035894581
-------------------------set Thread-1blackred 1356826036165653

So it read the value correctly but the processing done after that took a long time as compared to the time taken by other threads in doing what they were doing.

Happy for someone to correct me if I am wrong. :)

来源：https://stackoverflow.com/questions/52470335/how-does-volatile-keyword-ensure-an-objects-fields-are-visible-to-other-threads

标签

java

concurrency

volatile