Volatile is volatile or not ??

Question

Using "volatile" with two private variables accessed concurrently from two o more threads it seems don't work, at least for me.
Using "synchronized' with the methods that access the variables, yes
I.m using JDeveloper 3.2, with JDK 1.3 under WNT.4.0
What's the problem ?
Is "The Java Programming Language Spec" - 8.3.1.4 wrong ?

Answer 1

Hi !
Here follow two examples that probe, in my experience, that volatile is different form synchronize.
My problems is that "The Java Lang. Spec. 8.3.1.4" say that they are equal. I don;t understand what it means.
// ----------------------- Using_sync.java --------------------------------------------------
// My examples Using_sync and Using_vol are variations of
// the example [ sharing2.java ] of chapter 14
// from "Thinking in Java, second edition" by Bruce Eckel : www.bruceEckel.com
// To start, push the two buttons, in any order.
// Problem :
// The Java Language Specification, second edition, section "8.3.1.4 Volatile fields"
// says that "synchronized" and "volatile" work in the same way
// These examples probe that they work in different ways.
// The Watcher threads watch the TwoCounter threads.
// When count1 <> count2, the right label of the applet change
// I use JDeveloper 3.2, with JDK 1.3, under WNT 4.0,
// In "Using_sync", with "synchronized", the labels don't change, so, count1 == count2, for ever !
// In "Using_vol", With "volatile", the labels, after a seconds, change all of them.
// The Java Language Specification, second edition, section "8.3.1.4 Volatile fields"
// says the "synchronized" and "volatile" work in the same way, and it give these
// snippets of code :
// <<<<<<<<<<<<<
// class Test {
//      static int i = 0, j = 0;
// static synchronized void one() { i++; j++; }
// static synchronized void two() {
// System.out.println("i=" + i + " j=" + j);
// This prevents method one and method two from being executed concurrently,
// and furthermore guarantees that the shared values of i and j are both updated before
// method one returns.
// Therefore method two never observes a value for j greater than that for i; indeed,
// it always observes the same value for i and j.
// Another approach would be to declare i and j to be volatile : ( !!!??? )
// class Test {
//      static volatile int i = 0, j = 0;
// static void one() { i++; j++; }
// static void two() {
// System.out.println("i=" + i + " j=" + j);
// This allows method one and method two to be executed concurrently, . . . .
// >>>>>>>>>>>
// Note : the JTextFields may show different values even is the label say count1 == count2
// It's just a problem of applet update.
// The important thing is that if the synchTest() method detects any difference
// the label change
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Using_sync extends JApplet {
private static int accessCount = 0;
private static JTextField aCount = new JTextField( "0", 14 );
// to probe that the Watcher thread really runs.
TwoCounter[] s;
private JButton start = new JButton( "Start" );
private JButton watcher = new JButton( "Watch" );
private int numCounters = 12;
private int numWatchers = 15;
class TwoCounter extends Thread {
public TwoCounter() {
JPanel p = new JPanel();
p.add( t1 );
p.add( t2 );
p.add( l );
getContentPane().add( p );
private boolean started = false;
private JTextField t1 = new JTextField( 5 );
private JTextField t2 = new JTextField( 5 );
private JLabel l = new JLabel( "count1 == count2" );
private int count1 = 0; // volatile vs synchronized ?
private int count2 = 0; // volatile vs synchronized ?
public void start() {
if ( !started ) {
started = true;
super.start();
public void run() {
while ( true ) {
synchronized( this ) {      // just synch this block so that synchTest() can execute
t1.setText( Integer.toString( count1++ ) );
t2.setText( Integer.toString( count2++ ) );
try {
sleep( 500 );
} catch( InterruptedException e ) {
System.err.println( "Interrupted" );
public synchronized void synchTest() {
if ( count1 != count2 )
l.setText( "Unsynched" );
accessCount++;
aCount.setText( Integer.toString( accessCount ) );
class Watcher extends Thread {
public Watcher() {
start();
public void run() {
while ( true ) {
for ( int i = 0 ; i < s.length ; i++ )
s[ i ].synchTest();
public void init() {
s = new TwoCounter[  numCounters  ];
Container cp = getContentPane();
cp.setLayout( new FlowLayout() );
for ( int i = 0 ; i < s.length ; i++ )
s[ i ] = new TwoCounter();
JPanel p = new JPanel();
// button start launch TwoCounter threads
start.addActionListener( new ActionListener() {
public void actionPerformed( ActionEvent e ) {
for ( int i = 0 ; i < s.length ; i++ )
s[ i ].start();
p.add( start );
// button watcher launch Watcher threads
watcher.addActionListener( new ActionListener() {
public void actionPerformed( ActionEvent e ) {
for ( int i = 0 ; i < numWatchers ; i++ )
new Watcher();
p.add( watcher );
p.add( new Label( "Access Count" ) );
p.add( aCount );
cp.add( p );
public static void main( String[] args ) {
Using_sync applet = new Using_sync();
applet.numCounters = 2; // number of TwoCounter threads
applet.numWatchers = 2; // number of Watcher threads : watch TwoCounter
// bigger so that spy more
          JFrame frame = new JFrame( "synchronize Vs volatile" );
          frame.setDefaultCloseOperation( javax.swing.JFrame.EXIT_ON_CLOSE );
          frame.getContentPane().add( applet );
          frame.setSize( 450, applet.numCounters * 50 + 50 );
          applet.init();
          applet.start();
frame.setVisible( true );
// ----------------------- Using_vol.java --------------------------------------------------
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Using_vol extends JApplet {
private static int accessCount = 0;
private static JTextField aCount = new JTextField( "0", 14 );
TwoCounter[] s;
private JButton start = new JButton( "Start" );
private JButton watcher = new JButton( "Watch" );
private int numCounters = 12;
private int numWatchers = 15;
class TwoCounter extends Thread {
public TwoCounter() {
JPanel p = new JPanel();
p.add( t1 );
p.add( t2 );
p.add( l );
getContentPane().add( p );
private boolean started = false;
private JTextField t1 = new JTextField( 5 );
private JTextField t2 = new JTextField( 5 );
private JLabel l = new JLabel( "count1 == count2" );
private volatile int count1 = 0; // volatile vs synchronized ?
private volatile int count2 = 0; // volatile vs synchronized ?
public void start() {
if ( !started ) {
started = true;
super.start();
public void run() {
while ( true ) {
t1.setText( Integer.toString( count1++ ) );
t2.setText( Integer.toString( count2++ ) );
try {
sleep( 500 );
} catch( InterruptedException e ) {
System.err.println( "Interrupted" );
public void synchTest() {
if ( count1 != count2 )
l.setText( "Unsynched" );
accessCount++;
aCount.setText( Integer.toString( accessCount ) );
class Watcher extends Thread {
public Watcher() {
start();
public void run() {
while ( true ) {
for ( int i = 0 ; i < s.length ; i++ )
s[ i ].synchTest();
public void init() {
s = new TwoCounter[  numCounters  ];
Container cp = getContentPane();
cp.setLayout( new FlowLayout() );
for ( int i = 0 ; i < s.length ; i++ )
s[ i ] = new TwoCounter();
JPanel p = new JPanel();
// button start launch TwoCounter threads
start.addActionListener( new ActionListener() {
public void actionPerformed( ActionEvent e ) {
for ( int i = 0 ; i < s.length ; i++ )
s[ i ].start();
p.add( start );
// button watcher launch Watcher threads
watcher.addActionListener( new ActionListener() {
public void actionPerformed( ActionEvent e ) {
for ( int i = 0 ; i < numWatchers ; i++ )
new Watcher();
p.add( watcher );
p.add( new Label( "Access Count" ) );
p.add( aCount );
cp.add( p );
public static void main( String[] args ) {
Using_vol applet = new Using_vol();
applet.numCounters = 2; // number of TwoCounter threads
applet.numWatchers = 2; // number of Watcher threads : watch TwoCounter
// bigger so that spy more
          JFrame frame = new JFrame( "synchronize Vs volatile" );
          frame.setDefaultCloseOperation( javax.swing.JFrame.EXIT_ON_CLOSE );
          frame.getContentPane().add( applet );
          frame.setSize( 450, applet.numCounters * 50 + 50 );
          applet.init();
          applet.start();
frame.setVisible( true );