Re: Speaking of thread safety?

Ian Shef <invalid@avoiding.spam>
Wed, 10 Mar 2010 20:57:38 GMT
Knute Johnson <> wrote in news:MkRln.9227

Does anybody know if the Observable/Observer calls to update() are on
the EDT?


In java 1.6.0_17, NO, unless notifyObservers(...) is called on the EDT.
However, there is no guarantee, and the Javadoc specifically states that
subclasses of Observable may even deliver notifications on separate

The following sample program prints


for me, demonstrating that the update() calls are on the thread used for
notifyObservers(...). Your Java may be different.

Here is the sample program:

package testpack1;
import java.util.*;
import javax.swing.SwingUtilities;
public class OberverTest {
  static void showThread(Observable1 o1) {
  public static void main(String[] args) {
    // Create an Observer that prints the Thread used for update(...)
    Observer observer = new Observer() {
      public void update(Observable arg0, Object arg1) {
        System.out.println(Thread.currentThread()) ;
    } ;
    final Observable1 observable = new Observable1();
    showThread(observable); // Notify from the main thread.
    SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        showThread(observable); // Notify from the EDT.
// Define a minimally useful Observable
class Observable1 extends Observable {
  public void change() {setChanged();}

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