While loop blocking other threads

Question

I am waiting for a boolean to change with the while loop, but the while loop never ends? I'm probably somehow stupid about this

Code that does work. but the while loop never ends and my code when the boolean has changed doesn't get executed:

while(!myCoolBoolean);
//my code when the boolean has changed to true

It's a bit weird, because it works when I do:

while(!myCoolBoolean) {
    System.out.print();
}
//my code again

Any help? EDIT: The while loop is actually in a thread, and myCoolBoolean is being changed in an another thread

Where does the `myCoolBoolean` supposedly change? Please [edit] your question and add a [mcve], because what you've posted now **should** never end... — Zephyr, Jul 04 '19 at 17:24
You are blocking the entire process with your while loop. `System.out.print();` gives the processor some time to execute other threads (that will then apparently update the variable). — theblackips, Jul 04 '19 at 17:25
also related - https://stackoverflow.com/questions/9459657/is-multi-thread-output-from-system-out-println-interleaved - it shows that while this is not guaranteed, implementations of `System.out.print` are usually synchronized internally. It *can* therefore cause changes from other threads to become visible. — Hulk, Jul 04 '19 at 17:28

score 0 · Accepted Answer · answered Jul 04 '19 at 17:38

0

Try to declare:

volatile boolean myCoolBoolean;

answered Jul 04 '19 at 17:38

Barat Sahdzijeu

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Thank you, never knew what "volatile" is. – Jul 04 '19 at 17:40
It basically ensures that thread which runs your `while` loop does not have stale value of the `myCoolBoolean` variable – Barat Sahdzijeu Jul 04 '19 at 17:44

score 0 · Answer 2 · answered Jul 04 '19 at 17:50

I assume you are trying to wait for the value of myCoolBoolean to be changed from another thread. If it's your code that's updating myCoolBoolean, you should use wait() and notify() instead.

Initialize an object somewhere so that both threads can access it.
```
public Object myCoolObject = new Object();
```

Instead of your while loop, write

synchronized(myCoolObject) {
    myCoolObject.wait();
}

In the place where you would have updated myCoolBoolean, do

synchronized(myCoolObject) {
    myCoolObject.notify();
}

wait(); will pause the first thread until the second thread calls notify();.

If you cannot change the code that updates myCoolBoolean, you can change your loop to

while (!myCoolBoolean) {
    wait(1);
}

This way your code will pause one millisecond per iteration to give the other thread time to update the flag. Without the wait(1); it will use up all your processing time just executing the loop.

While loop blocking other threads

2 Answers2