Why wait and notify kept in object class only
why wait and notify kept in object class only. is it maintain locks while doing wait and notify . please explain internals,
What do you mean in Object class "only"? If they're in Object, then they're in ALL classes.
They're in Object because any Object can serve as a lock.
For details of how to use them, see a Java threading tutorial such as http://java.sun.com/docs/books/tutorial/essential/threads/
Similar Messages
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Why notify() and notifyall() is in Object class
pls help me
why notify() and notifyall() methods which are related to thread are define in Object class,instead of defining in Thread classshouldn't be called on thread objects ever
at all (one of my pet peeves with Java)Why not? It would make a nice entry in the IOJJJ (International
Obfuscated Java Juggling Jamboree) if it existed ;-)First of all, sorry for my bad english. It's early here :)Nah, it's just a late a rainy Sunday afternoon here, so no problem ;-)
Second of all, the way that Thread is implemented, at least on
windows, it that calling Thread.join does a wait() on that thread object.
Thus, if you call notify() on a thread, you could actually be waking up
joined threads, which wouldn't be good. And if you called wait, you
could get woken up if the thread finishes which violates the contract of
wait and notify to some extent and also relies on an undocumented
feature.I take back my previous remark: it could not just be a nice little entry in
the IOJJJ, it would be a great entry in that contest! ;-)
kind regards,
Jos
ps. your example is the first example that clearly gives a reason for
spurious wakeups I've ever read; thanks; joined threads, I've got to
remember that example. <scribble, scribble, scribble/> done. ;-) -
Hi,
Who can give me a short java code segment to show the method wait and notify of the class Object.
I've already tryed as forlow:
try {
A a = new A();
a.wait() ;
catch (InterruptedException ex) {
ex.printStackTrace() ;
But a thread is always be thrown:
java.lang.IllegalMonitorStateException: current thread not owner
at java.lang.Object.wait(Native Method)
at java.lang.Object.wait(Object.java:420)
at A.main(A.java:34)
Who can tell which thread should be the owner thread? I did realize noly one thread there.
Thanks a million.The classic example is two threads, on adding items to a queue and one taking them off and processing them:
// consumer
Object item = null;
do {
synchronized(queue) {
if(queue.isEmpty())
queue.wait();
item = queue.remove(0);
// process item
} while(!Thread.currentThread.isInterrupted());
// producer
synchronized(queue) {
if(queue.isEmpty())
queue.notify();
queue.add(item);
}The use of synchronized blocks or methods is essential. They guard against conflicts and race conditions between different sections that change the same data. -
A bug in wait and notify? -- help
Now I am engaged in a simulation project. The threads are all controlled by the controller program, that is, they are often blocked and unblocked by using wait and notify methods. However, when I was debugging my program, there were always some errors occurring and halting the simulation. And I found that the cause was that the threads sometimes might halt after having executed the wait or notify methods!!
Example,
in thread 1, it would be blocked by a wait method like:
synchronized(lock){
lock.wait();
System.out.println("Thread 1 go on.");
In thread 2, it would unblock thread 1 with the notify method:
synchrnoized(lock){
System.out.println("Thread 2 would notify the threads blocked on lock.");
lock.notifyAll();
System.out.println("Thread 2 go on.");
The simulation program controls the time to execute the thread 2. However, in debugging the program I often encountered the problem that thread 2 did notify thread 1 and thread would go on executing, but thread 2 would not go further. That is, the output is like:
Thread 2 would notify the threads blocked on lock.
Thread 1 go on.
But the String, "Thread 2 go on." would not be printed out.
It was so strange, and I repeated for tens of times, it always occurred here and there.
Is it a bug of java?There seems to be nothing wrong with this to me:
You should use better debugging messages. I couldn't read it the way you had it.
public class BugThread extends Thread
Object lock;
Notifier notifier;
public static void main(String[] arg)
for(int i = 0; i < 5; i ++)
Object lock = "lock " + i;
Notifier noti = new Notifier(lock);
BugThread bug = new BugThread(lock, noti);
bug.start();
noti.start();
public BugThread(Object lock, Notifier notifier)
this.lock = lock;
this.notifier = notifier;
public void run()
int i = 0;
while( i++ < 100)
synchronized(lock)
System.out.println("Bug thread: "
+ lock + " would block at Object: "
+ lock);
try
lock.wait();
catch(InterruptedException ex)
ex.printStackTrace();
System.out.println("Bug thread: "
+ lock + " unblocked at Object: "
+ lock);
notifier.stop();
class Notifier extends Thread
Object lock;
public Notifier(Object lock)
this.lock = lock;
public void run()
int i = 0;
while(true)
synchronized(lock)
System.out.println("Notifier: " + lock
+ " would notify Bugthread blocking at object: "
+ lock.toString() + ". time no." + i);
lock.notifyAll();
System.out.println("Notifier: " + lock
+ " notified bugthread. time no." + i);
System.out.println("Notifier: " + lock
+ " go on working. time no." + i);
i ++;
} -
Wait() and notify() or Semaphores with single permits?
Hi,
What's the difference between using wait(), notify() and single permitted Semaphores? Does using the latter affect the performance in massively multi-threaded environments?I wrote a very simple program to simulate a Queue like structure, and a Producer with a Consumer which both are running in separate threads. I changed the Queue class to hold only one value (It's not a queue any more, but the name didn't change). Then I used a boolean value, wait() and notify() methods of the Object class to ensure that every put() by the producer is followed by a get() by the consumer. Here's the Queue class code:
class Queue {
private int num;
private boolean valueSet = false;
synchronized void put(int n) {
while(valueSet) {
try {
wait();
} catch(InterruptedException err) {
System.out.println("put() interrupted.");
num = n;
valueSet = true;
notify();
System.out.println("Put: "+n);
synchronized int get() {
while(!valueSet) {
try {
wait();
} catch(InterruptedException err) {
System.out.println("get() interrupted.");
valueSet = false;
notify();
System.out.println("Got: "+num);
return num;
}Producer and Consumer classes simply repeat calling put() and get() methods respectively for a number of times. I also implemented all this by using Semaphores with single permits:
import java.util.concurrent.*;
class Queue {
private int num;
private final static Semaphore semCon = new Semaphore(0);
private final static Semaphore semPro = new Semaphore(1);
void put(int n) {
try {
semPro.acquire();
} catch(InterruptedException err) {
System.out.println("put() interrupted.");
num = n;
semCon.release();
System.out.println("Put: "+n);
int get() {
try {
semCon.acquire();
} catch(InterruptedException err) {
System.out.println("get() interrupted.");
semPro.release();
System.out.println("Got: "+num);
return num;
}Which one is better? Better in every aspect, such as performance, being maintainable, etc. I don't know whether it's a good idea to use Semaphores in these cases.
Thanks for your reply. -
i am having a servlet , which is implementing wait and notify of java.lang.Object.
when serving it for single request , its waiting and working fine...
but for multiple requests its not serving properly. works one by one means the second request is waiting for finishing the first request just like SingleThreadModel (after finishing the first request only second request entering inside servlet)
please help me to solve this?
or is there any other way to call wait method without synchronized block??Shynihan wrote:
or is there any other way to call wait method without synchronized block??No, and that's partly because if wait or notify is the only thing in the synchronized block then you probably aren't using them right. The problem is blocking the server thread. Servlet engines use a thread pool to execute transactions, when a new transaction comes in a thread is taken from the pool to run it, and there's usually a limited number of threads in the pool. If there's no threads left then the transaction will wait for another to finish.
If your servlets freeze up, then that's a thread gone from the pool.
And remeber the unlocking transaction may never arive. The client could go down at any time, leaving the server thread permanently hung.
You need to move this waiting to the client side. If the transaction cannot proceed then return a "come back later" response immediately. The client can then retry after a decent interval. -
I have a main that calls a drawing window. The user draws in the window and clicks a button when finished drawing. Then main then gets this image. I don't know how to call the getImage in the main ONLY after the user has clicked the button in the draw window, however. I've heard about wait and notify commands, but I don't know how to use them. Can anyone help me? Thanks.
Oh my... I'm not a java superstar. I'll post my code and maybe someone can help me. I tried running wait, but it says not thread owner or something weird.
Main:
public static void main (String [] args) throws IOException, InterruptedException, AWTException
Image image = null;
PaintArea pa = new PaintArea ();
pa.loadArea (pa);
//After screen capture
image = pa.getImage ();}PaintArea:
import java.awt.*;
import java.awt.event.*;
import java.io.*;
class PaintArea extends Frame
static int x, y;
static Image image;
Canvas canvas = null;
PaintArea ()
setSize (110, 160);
setVisible (true);
addWindowListener (new WindowAdapter ()
public void windowClosing (WindowEvent e)
hide ();
public void loadArea (PaintArea pa) throws InterruptedException
Button button = new Button ("Save");
Canvas drawArea = new Canvas ();
drawArea.setSize (100, 100);
canvas = drawArea;
pa.setLayout (new BorderLayout ());
pa.add (drawArea, BorderLayout.CENTER);
pa.add (button, BorderLayout.SOUTH);
pa.show ();
Graphics g = drawArea.getGraphics ();
final Graphics g2 = g;
final PaintArea pa2 = pa;
button.addActionListener (new ActionListener ()
public void actionPerformed (ActionEvent ae)
try
image = saveScreen (pa2);
catch (AWTException e)
System.err.println (e);
System.exit (1);
catch (InterruptedException ie)
System.err.println (ie);
System.exit (1);
drawArea.addMouseListener (new MouseAdapter ()
public void mousePressed (MouseEvent me)
x = me.getX ();
y = me.getY ();
g2.fillOval (x - 5, y - 5, 10, 10);
drawArea.addMouseMotionListener (new MouseMotionAdapter ()
public void mouseDragged (MouseEvent me)
x = me.getX ();
y = me.getY ();
g2.fillOval (x - 5, y - 5, 10, 10);
public Image saveScreen (PaintArea pa) throws AWTException
Image image = null;
try
image = createImage (new Robot ().createScreenCapture (pa.getDrawArea ().getBounds ()).getSource ());
catch (NullPointerException npe)
System.err.println (npe);
System.exit (1);
return image;
public Picture getImage ()
return image;
public Canvas getDrawArea ()
return canvas;
}Could someone please run that and try to make the pa.getImage() in the main happen only after the button in Paint Area is pressed? Thanks! -
IllegalMonitorStateException? wait() and notify()
Can someone tell me what's the deal with wait() and notify() methods? Like how to use them properly? Right now I have two threads, one being the main program and the other should run occationally when I need it... I try to get the second to wait() and second.notify() when I want to to run but IllegalMonitorStateException... so if you know a tutorial or you can explain it me it'll be great!
Thanks!This is a bit hard to answer without a code example. Have you gone through the tutorial available on this topic?
http://java.sun.com/docs/books/tutorial/essential/threads/index.html
If you have already looked at these, post your sample code (inside code tags, please - use the code button above) so others can read it and run it. -
Stuck with wait() and notify()
Hi,
I have a problem understanding synchronous access.
I have an object that should provide a list of data. The list is used later in my code, but it would increase performance to load it in a separate thread at the beginning. However it is possible that the list is not ready when the first other object tries to access the list.
I thought that this code should solve the problem
public class SingleObject implements Runnable {
ArrayList list;
public SingleObject(){
list = new ArrayList();
public synchronized void run() {
try {
makeALongProc(100);
} catch (InterruptedException e) {
e.printStackTrace();
notify();
private synchronized void makeALongProc(int size) throws InterruptedException{
System.out.println("...doing something ...");
Thread.sleep(5000);
for (int i = 0 ; i < size ; i++ ){
list.add(""+i);
System.out.println("finished!");
notifyAll();
public synchronized String getValueAt(int pos) throws InterruptedException{
wait();
return list.get(pos).toString();
public synchronized List getList() throws InterruptedException{
wait();
return list;
} The object is invoked by the following code.
public class ThreadProject {
* @param args
public static void main(String[] args) {
System.out.println("...starting thread...");
SingleObject so = new SingleObject();
Thread th = new Thread(so);
th.start();
try {
System.out.println("waiting ...");
List list = so.getList();
System.out.println("got list"+list.size());
String sso = so.getValueAt(5);
System.out.println(sso);
} catch (InterruptedException e) {
e.printStackTrace();
}Unfortunateley this is not doing what I have expected. Instead of calling the methodsso.getList() and so.getValueAt() it enters so.getList() and waits and waits....
Can anybody give my a clue or an explanation what I am doing wrong?
Regards
Sas
Message was edited by:
Poncho1975I have tried a little bit further.
I changed the following part:
public class SingleObject {
ArrayList list;
public SingleObject(){
list = new ArrayList();
public synchronized void makeALongProc(int size) throws InterruptedException{
System.out.println("...doing something ...");
Thread.sleep(5000);
for (int i = 0 ; i < size ; i++ ){
list.add(""+i);
System.out.println("finished!");
notify();
public synchronized String getValueAt(int pos) throws InterruptedException{
if (list==null || list.size()<=pos){
wait();
return list.get(pos).toString();
public synchronized List getList() throws InterruptedException{
if (list==null){
wait();
return list;
}and ...
public class ThreadProject {
* @param args
public static void main(String[] args) {
System.out.println("...starting thread...");
final SingleObject so = new SingleObject();
Thread th = new Thread(new Runnable(){
public synchronized void run() {
try {
so.makeALongProc(100);
} catch (InterruptedException e) {
e.printStackTrace();
th.start();
try {
System.out.println("waiting ...");
List list = so.getList();
System.out.println("got list"+list.size());
String sso = so.getValueAt(5);
System.out.println(sso);
} catch (InterruptedException e) {
e.printStackTrace();
}Now I am satisfied. Every call on a method from this object would lead to the situation, that either makeALongProc is called first or all other methods must wait until makeALongProc has been called once to fill the array.
However I suppose that accessing elements of a list with a synchronized object is not a good decision. I would rather take only the whole List and do all other access outside of the object.
Regards
Poncho -
The program tries to find the first socket connection. Theirs a wait and the notify call in the inner class seams to have to effect. Attach is the code to try out
import java.io.*;
import java.util.*;
import java.net.*;
import java.nio.channels.*;
* SocketSearch class to scan for an open socket from an array of sockets passed. <br>
* Inner class DetectSocketConnection tries to open the socket and assign hostname
* a value if successful. <br>
* The SocketSearch thread class is waiting to find the hostname or active DetectSocketConnection
* threads to be zero - i.e No hosts found
public class SocketSearch extends Thread {
private Object syncObject = new Object();
private String hostname = null;
private ThreadGroup threadGroup = new ThreadGroup("SOCKETS_CONNECT");
private ArrayList sockets = null;
* Inner class DetectSocketConnection to find an open socket. <br>
* It will assign the hostname a value if successful
private class DetectSocketConnection extends Thread {
private SocketChannel sc = null;
private String host = null;
private int port = -1;
public DetectSocketConnection(String host, int port) {
super(threadGroup, host);
this.host = host;
this.port = port;
System.out.println("DetectSocketConnection::syncObject = " + syncObject);
public void run() {
System.out.println("Scanning " + host + " at port number " + port);
synchronized(syncObject) {
try {
InetSocketAddress isa = new InetSocketAddress(InetAddress.getByName(host), port);
// Connect
sc = SocketChannel.open();
sc.connect(isa);
System.out.println("Found hostname: " + host + "!!!!!!!!!!!!!!!!!!!");
hostname = host;
catch (IOException e) {
System.err.println("DetectSocketConnection: " + e.toString());
finally {
// Make sure we close the channel (and hence the socket)
close();
System.out.println("DetectSocketSonnectio: notify()...");
syncObject.notify();
public void close() {
try {
if (sc != null) sc.close();
catch (IOException e) {
public SocketSearch() {
sockets = new ArrayList();
* Add socket to the sockets ArrayList to prepare to start the Socket Search
* @param host Socket hostname
* @param port Socket port number
public void addSocket(String host, int port) {
DetectSocketConnection detectSocket = new DetectSocketConnection(host, port);
sockets.add(detectSocket);
* SocketSearch start method to fire up the sockets threads to search for
public void start() {
super.start();
if (sockets != null) {
DetectSocketConnection[] arrSockets = (DetectSocketConnection[]) sockets.toArray(new DetectSocketConnection[0]);
for (int i = 0 ; i < arrSockets.length ; i++) {
arrSockets.start();
* Main code to do the socket search
public void run() {
try {
boolean loop = (sockets.size() > 0);
while (loop) {
synchronized(syncObject) {
System.out.println("SocketSearch.wait() => syncObject = " + syncObject);
syncObject.wait();
System.out.println("SocketSearch.wait() => syncObject = " + syncObject + ", finished...");
if (hostname != null) {
// use the hostname
// you could interrupt the threads now - its your choice
loop = false;
else {
System.out.println("Invalid hostname...");
ThreadGroup currentGroup = Thread.currentThread().getThreadGroup();
int numThreads = currentGroup.activeCount();
Thread[] listOfThreads = new Thread[numThreads];
currentGroup.enumerate(listOfThreads);
int activeThreads = 0;
for (int i = 0 ; i < numThreads ; i++) {
if (listOfThreads[i] instanceof DetectSocketConnection) {
activeThreads++;
System.out.println("activeThreads: " + activeThreads);
if (activeThreads == 0) {
// host name is NULL and active threads are finished
throw new UnknownHostException("Host not found.");
if (hostname != null) {
System.out.println("Found hostname: " + hostname);
// Do something here, use callback to maybe dispose ProgressDialog etc...
catch (UnknownHostException e) {
// Do something here, use callback to maybe dispose ProgressDialog etc...
System.err.println(e.toString());
catch (InterruptedException e) {
// Do something here, use callback to maybe dispose ProgressDialog etc...
System.err.println(e.toString());
* Test to find a socket from an array of test socket hosts
public static void main(String args[]) {
String[] yahooPOP3PossibleSettings = new String[] {
"yahoo.com",
"mail.yahoo.com",
"pop.yahoo.com",
"pop.mail.yahoo.com",
"pop3.yahoo.com"
int PORT_NUMBER = 110;
SocketSearch socketSearch = new SocketSearch();
for (int i = 0 ; i < yahooPOP3PossibleSettings.length ; i++) {
socketSearch.addSocket(yahooPOP3PossibleSettings[i], PORT_NUMBER);
socketSearch.start();Forget it. The Inner class was doing a wait and so was the main thread.
Not meant to put a wait in the main thread... -
Java compiler should check if wait() and notify()
wait(), notify(), and notifyAll() methods should be called within a critical section, but the Java compiler does not check for this. It is usually during running that an Exception is thrown when the methods was not called within a critical section.
To me, it seems that the compiler should check and enforce that wait(), notify() and notifyAll() be called within a critical section.I think I mis-understood you.
Yes, you are right to say that because callNotify() was called within a critical section at some time, and may be called from other non-critical sections, so the compiler does not have means to keep trace of the calling stack.
How about this:
Because critical sections are always marked by the synchronized keyword, the compiler can actually keep count of the synchronized word and its corresponding braces, then when a wait(), or notify(), or notifyAll() call is made at certain point, the compiler just check for the nested synchronized cunt number, if it is non-zero, it will be syntaxically correct.
Take the example of your code, I see that the proper point for
public class Test {
Object lock = new Object();
public void lockObject() {
synchronized (lock) {
callNotify(); // here the synchronized brace count is 1, so it is OK!
private void callNotify() {
lock.notify(); // here the synchronized brace count is 0,
// but because it is a stand alone function, the compiler
// can deliver warning information.
public void dumbMethod() {
callNotify(); // should deliver warning
public static void main(String[] args) {
Test t = new Test();
t.lockObject();
t.callNotify(); // but here, the synchronized brace count is 0, so failure!
} -
public class test {
public static void main(String[] args) {
new consumer();
new producer();
class producer extends Thread
public static int productcnt=0;
public consumer aconsumer=new consumer();
producer()
super("producer : "+ productcnt++);
start();
public void run()
try
sleep(1000);
synchronized (aconsumer)
System.out.println(this.getName()+" has prepared.");
aconsumer.notify();
catch(InterruptedException e)
throw new RuntimeException(e);
class consumer extends Thread
public static int consumercnt=0;
consumer()
super("consumer : "+ consumercnt++);
start();
public void run()
try
synchronized(this)
System.out.println(getName()+ " is waiting.");
wait();
System.out.println(getName()+ " get product.");
catch(InterruptedException e)
throw new RuntimeException(e);
}We can see the code about producers and consumers above,and the result is:
consumer : 0 is waiting.
consumer : 1 is waiting.
producer : 0 has prepared.
consumer : 1 get product.
---------------------------------------at the end of program ,consumer 0 is waiting however.
consumer 0,started in the main procedure,I can't stop it, who can tell me how tostop it with a notify.
is it real that consumer just can be notified in a procedure which claimed it?The answer is:
public class Test {
public static void main(String[] args)
consumer cs = new consumer(); // ---------
new producer(cs);
class producer extends Thread
public static int productcnt=0;
consumer cs; // ---------
public consumer aconsumer=new consumer();
producer(consumer cs)
super("producer : "+ productcnt++);
this.cs = cs; // ---------
start();
public void run()
try
sleep(1000);
System.out.println(this.getName()+" has prepared.");
synchronized (cs) // ---------
cs.notify(); // ---------
synchronized (aconsumer)
aconsumer.notify();
catch(InterruptedException e)
throw new RuntimeException(e);
class consumer extends Thread
public static int consumercnt=0;
consumer()
super("consumer : "+ consumercnt++);
start();
public void run()
try
synchronized(this)
System.out.println(getName()+ " is waiting.");
wait();
System.out.println(getName()+ " get product.");
catch(InterruptedException e)
throw new RuntimeException(e);
}Well, I simply tried to not to change your code essentially. If you gonna do something like this
but more complicated try to find other solution. -
Hello all,
I have a small problem.Before explaining the problem as such,ill give u a breif description of what i am trying to do
I have two layers.The upper layer is sender and the lower one is the reciever.Now ,there's only one reciever thread and a number of sending threads.So when one sender sends something...the reciever will start processing with the data.
In the mean time ,if another sender thread wants to send something to the reciever thread, then it must wait until the reciever enters wait state after it's current processing.
So the reciever should notify the sender thread which is in wait state...
My probelm is the sender thread if enters in to wait state once...I'm not able to notify it again when the reciever thread is ready for it.
Hope some one among u can help me out
thanking u in advance
regards
aneesyou could make the method that does the processing in the receiver, synchronized. that way, the jvm will take care of your requirement.
if you have to use the wait- notify scheme, then try using notifyAll() instead of notify() -
Hello people,
I am curious to know why the root class OBJECT, empty class, is a class why it was not created as an interface? What difference would make if it were an interface?
I would appreciate and reward points for good reasons.
Thanks & Regards,
Anand Patilmaghia wrote:
wait(), notify(), and notifyAll() methods are always used by Thread class objects.Whatever gave you this idea?
so.why the methods are available in Object class?Because they deal with locks, and every object has a lock.
[http://java.sun.com/docs/books/tutorial/essential/concurrency/] -
Hai Respected Friend ,
I have one doubt regarding Object Class. for what purpose the Object class has some methods like Notify(),Notify All(),wait() though we are not using those methods except with Thread class.
why they are defined these methods in the Object Class ,why not in the Thread Class even though it is only useful with Thread Class.
Please give clear ans.
Thanks
SelvakumarBigDaddyLoveHandles wrote:
malcolmmc wrote:
Actually, though these methods are useful only in a multi-threaded environment, no Thread objects are directly involved. What you need to specify isn't a thread object (the current thread is always the one affected) but the monitor, which can be any kind of Object.I would have been happy if they had defined a Lock class with wait and notify methods, because I prefer doing:
public class C {
private final Object lock = new Lock();
public void method() {
synchronized(lock) {...}
}to
public class C {
public synchronized void method() {
}In the second case, you have to contend with other code incorrectly calling wait/notify on an instance of C.
So I don't see the win in these being java.lang.Object methods, other than allowing you to create one less object -- what an efficiency win ;-)However, there are times when it makes sense to expose the lock. For instance Collections.synchronizedXxx.
Maybe you are looking for
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