multithreading - Java IO's implements on thread variable sharing -

this question has answer here:

• loop doesn't see changed value without print statement 1 answer

i confused code snippet this:

public class learntest {  private static boolean tag = true; private static int = 0;  public static void main(string[] args) {      new thread(new runnable() {         @override         public void run() {             while (tag) {                 //system.out.println("ok");                 i++;             }         }     }).start();      try {         thread.sleep(1000);     }catch (exception e) {         e.printstacktrace();     }      tag = false;     system.out.println(i);   } } 

in code, have new-thread , main-thread.the result code random value of i , new-thread not exit.because new-thread not new tag value.

if change define of tag decorated volatile, print value of i , new-thread exit. because volatile keep visibility thread.

but when cannel comment commentted code line,and thetag not decorated witch volatile,it print "ok" , exit.

why?

what suppose java's io synchronized, force tag side value of new-thread refresh main shared memory.

this's corret?

without volatile thread not guaranteed see changes value. may see them.

in case adding system.out.println() results (apparently) in value being removed cache, it's fetched main memory @ point. without println() value remains in cache, , infinite (or @ least long) loop.

that's 1 difficulty concurrent programming, code may appear work correctly, there's no way always works correctly.