multithreading - Why does an empty while in Java not break when condition is set by other thread? -

while trying unit test threaded class, decided use active waiting control behavior of tested class. using empty while statements failed intended. question is:

why first code not complete, second does?

there similar question, doesn't have real answer nor mcve , far more specific.

doesn't complete:

public class threadwhiletesting {      private static boolean wait = true;      private static final runnable runnable = () -> {         try {thread.sleep(50);} catch (interruptedexception ignored) {}         wait = false;     };      public static void main(string[] args) {         wait = true;         new thread(runnable).start();         while (wait); // line important     } } 

does complete:

public class threadwhiletesting {      private static boolean wait = true;      private static final runnable runnable = () -> {         try {thread.sleep(50);} catch (interruptedexception ignored) {}         wait = false;     };      public static void main(string[] args) {         wait = true;         new thread(runnable).start();         while (wait) {             system.out.println(wait); // line important         }     } } 

i suspect empty while gets optimized java compiler, not sure. if behavior intended, how can achieve want? (yes, active waiting intented since cannot use locks test.)

wait isn't volatile , loop body empty, thread has no reason believe change. jit'd to

if (wait) while (true); 

which never completes if wait true.

the simple solution make wait volatile, prevents jit making optimization.

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as why second version works: system.out.println internally synchronized; described in jsr133 faq:

before can enter synchronized block, acquire monitor, has effect of invalidating local processor cache variables reloaded main memory.

so wait variable re-read main memory next time around loop.

however, don't guarantee write of wait variable in other thread committed main memory; so, @assylias notes above, might not work in conditions. (making variable volatile fixes also).