Processing statement error in loop

Question

public static void main(String[] args) throws IOException {

   // new fetchEmailID().fetchIDs();
    new UserInputAndDefaulValues().validateUserInputAndDefaultValues();
    while(!UserInputAndDefaulValues.start_mailing_flag)
    {
        //System.out.println(UserInputAndDefaulValues.start_mailing_flag);

    }
    MainExecutor.main_executor();
}

When I uncomment

//System.out.println(UserInputAndDefaulValues.start_mailing_flag);

the condition in while loop works correctly and the while loop ends on false condition. But as I comment it, the loop goes in infinite loop even when it should escape. Why is that behavior and that too just with one output statement?

total guessing: Isn't the compiler "clever" enough to remove the complete while loop when it is empty because even the while condition does nothing... — WarrenFaith
– WarrenFaith, Commented Jan 1, 2014 at 14:49
@dhamibirendra a jFrame is called from UserInputAndDefaulValues class, and on some button click event in jFrame, this static start_mailing_flag is turned true. — user2368055
– user2368055, Commented Jan 1, 2014 at 14:52
@warrenFaith i tried puting the if(UserInputAndDefaulValues.start_mailing_flag)break; inside while loop but that did no favour — user2368055
– user2368055, Commented Jan 1, 2014 at 14:53

Marko Topolnik · Accepted Answer · 2014-01-01 14:59:39Z

6

First and foremost, your code is not correctly synchronized. In other words, it contains a data race on the start_mailing_flag.

Now, the method PrintStream#println is synchronized so calling it forces the JIT compiler to avoid certain optimizations (namely, hoisting) on the code in question. As soon as you remove that statement, the effect is gone.

To conclude, when you uncomment the println, your code works by pure chance and as a side-effect of calling a synchronized method. It remains as thread-unsafe as before.

edited Jan 1, 2014 at 14:59

answered Jan 1, 2014 at 14:54

Marko Topolnik

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Comments

Avi · Accepted Answer · 2014-01-01 14:52:17Z

5

I'm guessing you change the start_mailing_flag in another thread (otherwise, no wonder the loop goes on forever).

The compiler optimizes such cases and removes the condition check after the first time. It changes the code to something like this:

if (!UserInputAndDefaulValues.start_mailing_flag) {
    while(true)
    {
        //System.out.println(UserInputAndDefaulValues.start_mailing_flag);
    }
}

You need to define start_mailing_flag as volatile:

public class UserInputAndDefaulValues {
   public static volatile boolean start_mailing_flag;
   .....
}

answered Jan 1, 2014 at 14:52

Avi

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3 Comments

Avi Over a year ago

@user2368055 - It'd be nice if you could accept the answer :)

user2368055 Over a year ago

i said it worked, and i even upvoted, any other way to accept the answer?

Avi Over a year ago

@user2368055 - Yes, if you choose the V sign (under the answer's score) you accept the answer which states that you as the question owner accepted this answer as the correct one. You'll get 2 points for doing it.

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