Question

If I create a runnable object

Runnable run = new MyRunnable();

And then pass the same exact object to two thread constructors and run them

new Thread(run).start;
new Thread(run).start;

1. Is the possible? What are the implications?
2. If I call Thread.sleep(0); in class MyRunnable, will both threads sleep because they are the same object, or is the thread entity completely separate from the object?
3. Would there ever be a reason to do this, if not please still answer the two questions above, because I don’t think I fully understand the thread mechanism yet?

Answer 1

1. It’s definitely possible and legal. If your Runnable has no state (no fields), then everything will be fine. If your Runnable does have mutable state, then you may need to use one of Java’s many mutual exclusion mechanisms like ReentrantLock or the synchronized keyword. Because both Threads will be mutating the fields of the same Runnable object.

2. No, you created and ran two different Threads. They simply call Runnable.run().

3. It’s not out of the realm of possibility. I wouldn’t even say it’s necessarily bad practice. Specific situations where you might do this left as an exercise to the reader…