java - Multithreading "unpredictable" behavior -

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• relationship between threads , println() statements 2 answers

i going through use cases volatile , have encountered below scenario:

private static boolean stop = false; public static void main(string[] args) throws interruptedexception {     thread runner = new thread(new runnable() {          @override         public void run() {             int = 0;             long start = system.currenttimemillis();             while(!stop){                 i++;             }             system.out.println("i m done after "+(system.currenttimemillis() - start));         }     });      runner.start();     thread.sleep(2000);     stop = true; } 

the above code runs indefinitely. fine due hoisting done compiler in attempt optimize code. however, if replace, i++ other statement e.g. system.out.println("hello"). stops after 2 seconds.

why behavior this? using jdk1.7.0_51.

note: know should declare stop volatile. however, know reason behavior in above case.

the standard output shared resource across threads. java's printstream class made thread-safe using synchronized blocks, , synchronized blocks memory barriers in java, that's why changes on stop field become visible inside loop. however, shouldn't rely on this.

for example, take @ printstream.println implementation:

public void println(string paramstring) {   synchronized (this)   {     print(paramstring);     newline();   } }