Relationship between Threads and println() statements

Question

I was trying to create a few scenarios to demonstrate visibility issues while sharing variable across threads. And I noticed that in almost all the cases I tested, if inside

Answer 1

This behaviour is implementation specific. In OpenJDK, println's body is synchronized altough the API does not state that it is.

Answer 2

PrintWriter is synchronized

public void println(String x) {
    synchronized(this) {
        this.print(x);
        this.newLine();
    }
}

Two sequential calls of System.out.println() in main thread and in second thread create a synchronization order between two threads. That means that all actions (in your case it is variable update), that happened in main thread before releasing a monitor (exiting synchronized method) will be seen by the code, executed in second thread after it acquires a monitor (enter synchronized method).

In simple words, yes, calling System.out.println() makes this synchronization.