Java Concurrent Exception

Question

Hi I have a paint method that is drawing an image and I have another method that is constantly modifying the image to be drawn however I experience concurrency exceptions now and again. What is the most efficient way to resolve this please? I know I could use synchronized blocks on the buffered image but then it throws up warnings on synchronizing a none final variable.

private BufferedImage img;

public void modImage(BufferedImage image) { 
     img = image;
}

public void paintComponent(Graphics g) {
    if (img != null) {
        g.drawImage(img, 0, 0, this);
    }
}

Answer 1

you could sync on instance,

private BufferedImage img;

public void modImage(BufferedImage image) { 
     synchronized(this){
         img = image;
     }
}

public void paintComponent(Graphics g) {
    synchronized(this){
    if (img != null) {
        g.drawImage(img, 0, 0, this);
       }
    }
}

Answer 2

You have a race condition because the value of img can changed in another thread between the if conditional and the use of img in drawImage

if (img != null) {                     // <-- img can be non-null here
    g.drawImage(img, 0, 0, this);      // <-- img can now be null
}

You can assign img to a local variable, then use the local variable in-place of img in the above code. The local variable will remain constant even if img is changed to a different object in another thread

final BufferedImage localImg = img;      // <-- localImg won't change locally
if (localImg != null) {
    g.drawImage(localImg, 0, 0, this);
}

In addition to this, img should be declared as volatile so its value is not cached thread-locally; changes in one thread will be seen by others.

private volatile BufferedImage img;

Bear in mind that declaring a variable as volatile will cause synchronization to occur whenever that variable is accessed; so synchronization is still happening. However, synchronization occurs on the img reference itself, not the BufferedImage object to which it refers, so there are no issues here if img is null.