java: are global variables visible in threads

Question

if I declare a global variable in the main thread, suppose that from the main thread I run a new thread, can the new thread access the global variable in the main thread?

"msg" string is my variable to acces

/* A simple banner applet.

   This applet creates a thread that scrolls
   the message contained in msg right to left
   across the applet's window.
*/
import java.awt.*;
import java.applet.*;
/*
<applet code="SimpleBanner" width=300 height=50>
</applet>
*/

public class AppletSkel extends Applet implements Runnable {
  String msg = " A Simple Moving Banner.";  //<<-----------------VARIABLE TO ACCESS
  Thread t = null;
  int state;
  boolean stopFlag;

  // Set colors and initialize thread.
  public void init() {
    setBackground(Color.cyan);
    setForeground(Color.red);
  }

  // Start thread
  public void start() {
    t = new Thread(this);
    stopFlag = false;
    t.start();
  }

  // Entry point for the thread that runs the banner.
  public void run() {
    char ch;

    // Display banner 
    for( ; ; ) {
      try {
        repaint();
        Thread.sleep(250);
        ch = msg.charAt(0);
        msg = msg.substring(1, msg.length());
        msg += ch;
        if(stopFlag)
          break;
      } catch(InterruptedException e) {}
    }
  }

  // Pause the banner.
  public void stop() {
    stopFlag = true;
    t = null;
  }

  // Display the banner.
  public void paint(Graphics g) {
    g.drawString(msg, 50, 30);
    g.drawString(msg, 80, 40);
  }
}

What do you mean by global variable? public static? Then yes, so careful when modifying it. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jul 25, 2013 at 17:51
A small code fragment would help to understand what you are asking. — kiheru
– kiheru, Commented Jul 25, 2013 at 17:52
what if the varible in the main thread is just public and not static? — M. Carlo Bramini
– M. Carlo Bramini, Commented Jul 25, 2013 at 17:53
@Luther Then your other thread would have to have a reference to the object that contains the variable. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jul 25, 2013 at 17:55

kiheru · Accepted Answer · 2013-07-25 18:08:59Z

5

Variables that are visible to several threads are generally tricky. Strings, however, are immutable, so that simplifies the situation.

It is visible, but when a modified ordinary value is available to other threads is not guaranteed. You should make it volatile, so that it is not cached thread locally. Use a local variable to build the new string before assigning msg.

If you intend to modify stopFlag from other threads, it should also be volatile.

edited Jul 25, 2013 at 18:08

answered Jul 25, 2013 at 18:02

kiheru

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