Question

Why does the following code throw CloneNotSupportedException in JDK7 but NOT in JDK6?

public class DemoThread extends Thread implements Cloneable {

    /**
     * @param args
     */
    public static void main(String[] args) {
        DemoThread t = new DemoThread();
        t.cloned();
    }

    public DemoThread cloned()
    {
        try {
            return (DemoThread) super.clone();
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
        return null;
    }

}

Answer 1

Here’s Thread’s implementation of clone() in SE 7

/**
 * Throws CloneNotSupportedException as a Thread can not be meaningfully
 * cloned. Construct a new Thread instead.
 *
 * @throws  CloneNotSupportedException
 *          always
 */
@Override
protected Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
}

Threads were never designed to be cloned. Doing some reading sparked off one of the comments I found this summed it up quite well : “But we either
have to disallow cloning or give it meaningful semantics – and the
latter isn’t going to happen.” — David Holmes