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Advantages and disadvantages using wait(1) over Thread.sleep(1) [duplicate]

What is the difference between a wait() and sleep() in Threads?
Is my understanding that a wait()-ing Thread is still in running mode and uses CPU cycles but a sleep()-ing does not consume any CPU cycles correct?
Why do we have both wait() and sleep()?
How does their implementation vary at a lower level?

A wait can be "woken up" by another thread calling notify on the monitor which is being waited on whereas a sleep cannot. Also a wait (and notify) must happen in a block synchronized on the monitor object whereas sleep does not:
Object mon = ...;
synchronized (mon) {
mon.wait();
}
At this point the currently executing thread waits and releases the monitor. Another thread may do
synchronized (mon) { mon.notify(); }
(on the same mon object) and the first thread (assuming it is the only thread waiting on the monitor) will wake up.
You can also call notifyAll if more than one thread is waiting on the monitor – this will wake all of them up. However, only one of the threads will be able to grab the monitor (remember that the wait is in a synchronized block) and carry on – the others will then be blocked until they can acquire the monitor's lock.
Another point is that you call wait on Object itself (i.e. you wait on an object's monitor) whereas you call sleep on Thread.
Yet another point is that you can get spurious wakeups from wait (i.e. the thread which is waiting resumes for no apparent reason). You should always wait whilst spinning on some condition as follows:
synchronized {
while (!condition) { mon.wait(); }
}

One key difference not yet mentioned is that:
sleep() does not release the lock it holds on the Thread,
synchronized(LOCK) {
Thread.sleep(1000); // LOCK is held
}
wait() releases the lock it holds on the object.
synchronized(LOCK) {
LOCK.wait(); // LOCK is not held
}

I found this post helpful. It puts the difference between Thread.sleep(), Thread.yield(), and Object.wait() in human terms. To quote:
It all eventually makes its way down to the OS’s scheduler, which
hands out timeslices to processes and threads.
sleep(n) says “I’m done with my timeslice, and please don’t give me
another one for at least n milliseconds.” The OS doesn’t even try to
schedule the sleeping thread until requested time has passed.
yield() says “I’m done with my timeslice, but I still have work to
do.” The OS is free to immediately give the thread another timeslice,
or to give some other thread or process the CPU the yielding thread
just gave up.
wait() says “I’m done with my timeslice. Don’t give me another
timeslice until someone calls notify().” As with sleep(), the OS won’t
even try to schedule your task unless someone calls notify() (or one of
a few other wakeup scenarios occurs).
Threads also lose the remainder of their timeslice when they perform
blocking IO and under a few other circumstances. If a thread works
through the entire timeslice, the OS forcibly takes control roughly as
if yield() had been called, so that other processes can run.
You rarely need yield(), but if you have a compute-heavy app with
logical task boundaries, inserting a yield() might improve system
responsiveness (at the expense of time — context switches, even just
to the OS and back, aren’t free). Measure and test against goals you
care about, as always.

There are a lot of answers here but I couldn't find the semantic distinction mentioned on any.
It's not about the thread itself; both methods are required as they support very different use-cases.
sleep() sends the Thread to sleep as it was before, it just packs the context and stops executing for a predefined time. So in order to wake it up before the due time, you need to know the Thread reference. This is not a common situation in a multi-threaded environment. It's mostly used for time-synchronization (e.g. wake in exactly 3.5 seconds) and/or hard-coded fairness (just sleep for a while and let others threads work).
wait(), on the contrary, is a thread (or message) synchronization mechanism that allows you to notify a Thread of which you have no stored reference (nor care). You can think of it as a publish-subscribe pattern (wait == subscribe and notify() == publish). Basically using notify() you are sending a message (that might even not be received at all and normally you don't care).
To sum up, you normally use sleep() for time-syncronization and wait() for multi-thread-synchronization.
They could be implemented in the same manner in the underlying OS, or not at all (as previous versions of Java had no real multithreading; probably some small VMs doesn't do that either). Don't forget Java runs on a VM, so your code will be transformed in something different according to the VM/OS/HW it runs on.

Here, I have listed few important differences between wait() and sleep() methods.
PS: Also click on the links to see library code (internal working, just play around a bit for better understanding).
wait()
wait() method releases the lock.
wait() is the method of Object class.
wait() is the non-static method - public final void wait() throws InterruptedException { //...}
wait() should be notified by notify() or notifyAll() methods.
wait() method needs to be called from a loop in order to deal with false alarm.
wait() method must be called from synchronized context (i.e. synchronized method or block), otherwise it will throw IllegalMonitorStateException
sleep()
sleep() method doesn't release the lock.
sleep() is the method of java.lang.Thread class.
sleep() is the static method - public static void sleep(long millis, int nanos) throws InterruptedException { //... }
after the specified amount of time, sleep() is completed.
sleep() better not to call from loop(i.e. see code below).
sleep() may be called from anywhere. there is no specific requirement.
Ref: Difference between Wait and Sleep
Code snippet for calling wait and sleep method
synchronized(monitor){
while(condition == true){
monitor.wait() //releases monitor lock
}
Thread.sleep(100); //puts current thread on Sleep
}

Difference between wait() and sleep()
The fundamental difference is that wait() is non static method of Object and sleep() is a static method of Thread.
The major difference is that wait() releases the lock while sleep() doesn’t release any lock while waiting.
wait() is used for inter-thread communication while sleep() is used to introduce a pause on execution, generally.
wait() should be called from inside synchronise or else we get an IllegalMonitorStateException, while sleep() can be called anywhere.
To start a thread again from wait(), you have to call notify() or notifyAll() indefinitely. As for sleep(), the thread gets started definitely after a specified time interval.
Similarities
Both make the current thread go into the Not Runnable state.
Both are native methods.

There are some difference key notes i conclude after working on wait and sleep, first take a look on sample using wait() and sleep():
Example1: using wait() and sleep():
synchronized(HandObject) {
while(isHandFree() == false) {
/* Hand is still busy on happy coding or something else, please wait */
HandObject.wait();
}
}
/* Get lock ^^, It is my turn, take a cup beer now */
while (beerIsAvailable() == false) {
/* Beer is still coming, not available, Hand still hold glass to get beer,
don't release hand to perform other task */
Thread.sleep(5000);
}
/* Enjoy my beer now ^^ */
drinkBeers();
/* I have drink enough, now hand can continue with other task: continue coding */
setHandFreeState(true);
synchronized(HandObject) {
HandObject.notifyAll();
}
Let clarity some key notes:
Call on:
wait(): Call on current thread that hold HandObject Object
sleep(): Call on Thread execute task get beer (is class method so affect on current running thread)
Synchronized:
wait(): when synchronized multi thread access same Object (HandObject) (When need communication between more than one thread (thread execute coding, thread execute get beer) access on same object HandObject )
sleep(): when waiting condition to continue execute (Waiting beer available)
Hold lock:
wait(): release the lock for other object have chance to execute (HandObject is free, you can do other job)
sleep(): keep lock for at least t times (or until interrupt) (My job still not finish, i'm continue hold lock and waiting some condition to continue)
Wake-up condition:
wait(): until call notify(), notifyAll() from object
sleep(): until at least time expire or call interrupt
And the last point is use when as estani indicate:
you normally use sleep() for time-syncronization and wait() for
multi-thread-synchronization.
Please correct me if i'm wrong.

This is a very simple question, because both these methods have a totally different use.
The major difference is to wait to release the lock or monitor while sleep doesn't release any lock or monitor while waiting. Wait is used for inter-thread communication while sleep is used to introduce pause on execution.
This was just a clear and basic explanation, if you want more than that then continue reading.
In case of wait() method thread goes in waiting state and it won't come back automatically until we call the notify() method (or notifyAll() if you have more then one thread in waiting state and you want to wake all of those thread). And you need synchronized or object lock or class lock to access the wait() or notify() or notifyAll() methods. And one more thing, the wait() method is used for inter-thread communication because if a thread goes in waiting state you'll need another thread to wake that thread.
But in case of sleep() this is a method which is used to hold the process for few seconds or the time you wanted. Because you don't need to provoke any notify() or notifyAll() method to get that thread back. Or you don't need any other thread to call back that thread. Like if you want something should happen after few seconds like in a game after user's turn you want the user to wait until the computer plays then you can mention the sleep() method.
And one more important difference which is asked often in interviews: sleep() belongs to Thread class and wait() belongs to Object class.
These are all the differences between sleep() and wait().
And there is a similarity between both methods: they both are checked statement so you need try catch or throws to access these methods.
I hope this will help you.

source : http://www.jguru.com/faq/view.jsp?EID=47127
Thread.sleep() sends the current thread into the "Not Runnable" state
for some amount of time. The thread keeps the monitors it has aquired
-- i.e. if the thread is currently in a synchronized block or method no other thread can enter this block or method. If another thread calls t.interrupt() it will wake up the sleeping thread.
Note that sleep is a static method, which means that it always affects
the current thread (the one that is executing the sleep method). A
common mistake is to call t.sleep() where t is a different thread;
even then, it is the current thread that will sleep, not the t thread.
t.suspend() is deprecated. Using it is possible to halt a thread other
than the current thread. A suspended thread keeps all its monitors and
since this state is not interruptable it is deadlock prone.
object.wait() sends the current thread into the "Not Runnable" state,
like sleep(), but with a twist. Wait is called on an object, not a
thread; we call this object the "lock object." Before lock.wait() is
called, the current thread must synchronize on the lock object; wait()
then releases this lock, and adds the thread to the "wait list"
associated with the lock. Later, another thread can synchronize on the
same lock object and call lock.notify(). This wakes up the original,
waiting thread. Basically, wait()/notify() is like
sleep()/interrupt(), only the active thread does not need a direct
pointer to the sleeping thread, but only to the shared lock object.

Wait and sleep are two different things:
In sleep() the thread stops working for the specified duration.
In wait() the thread stops working until the object being waited-on is notified, generally by other threads.

sleep is a method of Thread, wait is a method of Object, so wait/notify is a technique of synchronizing shared data in Java (using monitor), but sleep is a simple method of thread to pause itself.

sleep() is a method which is used to hold the process for few seconds or the time you wanted but in case of wait() method thread goes in waiting state and it won’t come back automatically until we call the notify() or notifyAll().
The major difference is that wait() releases the lock or monitor while sleep() doesn’t releases any lock or monitor while waiting. Wait is used for inter-thread communication while sleep is used to introduce pause on execution, generally.
Thread.sleep() sends the current thread into the “Not Runnable” state for some amount of time. The thread keeps the monitors it has acquired — i.e. if the thread is currently in a synchronized block or method no other thread can enter this block or method. If another thread calls t.interrupt() it will wake up the sleeping thread. Note that sleep is a static method, which means that it always affects the current thread (the one that is executing the sleep method). A common mistake is to call t.sleep() where t is a different thread; even then, it is the current thread that will sleep, not the t thread.
object.wait() sends the current thread into the “Not Runnable” state, like sleep(), but with a twist. Wait is called on an object, not a thread; we call this object the “lock object.” Before lock.wait() is called, the current thread must synchronize on the lock object; wait() then releases this lock, and adds the thread to the “wait list” associated with the lock. Later, another thread can synchronize on the same lock object and call lock.notify(). This wakes up the original, waiting thread. Basically, wait()/notify() is like sleep()/interrupt(), only the active thread does not need a direct pointer to the sleeping thread, but only to the shared lock object.
synchronized(LOCK) {
Thread.sleep(1000); // LOCK is held
}
synchronized(LOCK) {
LOCK.wait(); // LOCK is not held
}
Let categorize all above points :
Call on:
wait(): Call on an object; current thread must synchronize on the lock object.
sleep(): Call on a Thread; always currently executing thread.
Synchronized:
wait(): when synchronized multiple threads access same Object one by one.
sleep(): when synchronized multiple threads wait for sleep over of sleeping thread.
Hold lock:
wait(): release the lock for other objects to have chance to execute.
sleep(): keep lock for at least t times if timeout specified or somebody interrupt.
Wake-up condition:
wait(): until call notify(), notifyAll() from object
sleep(): until at least time expire or call interrupt().
Usage:
sleep(): for time-synchronization and;
wait(): for multi-thread-synchronization.
Ref:diff sleep and wait

In simple words, wait is wait Until some other thread invokes you whereas sleep is "dont execute next statement" for some specified period of time.
Moreover sleep is static method in Thread class and it operates on thread, whereas wait() is in Object class and called on an object.
Another point, when you call wait on some object, the thread involved synchronize the object and then waits. :)

wait and sleep methods are very different:
sleep has no way of "waking-up",
whereas wait has a way of "waking-up" during the wait period, by another thread calling notify or notifyAll.
Come to think about it, the names are confusing in that respect; however sleep is a standard name and wait is like the WaitForSingleObject or WaitForMultipleObjects in the Win API.

From this post : http://javaconceptoftheday.com/difference-between-wait-and-sleep-methods-in-java/
wait() Method.
1) The thread which calls wait() method releases the lock it holds.
2) The thread regains the lock after other threads call either notify() or notifyAll() methods on the same lock.
3) wait() method must be called within the synchronized block.
4) wait() method is always called on objects.
5) Waiting threads can be woken up by other threads by calling notify() or notifyAll() methods.
6) To call wait() method, thread must have object lock.
sleep() Method
1) The thread which calls sleep() method doesn’t release the lock it holds.
2) sleep() method can be called within or outside the synchronized block.
3) sleep() method is always called on threads.
4) Sleeping threads can not be woken up by other threads. If done so, thread will throw InterruptedException.
5) To call sleep() method, thread need not to have object lock.

Here wait() will be in the waiting state till it notify by another Thread but where as sleep() will be having some time..after that it will automatically transfer to the Ready state...

wait() is a method of Object class.
sleep() is a method of Thread class.
sleep() allows the thread to go to sleep state for x milliseconds.
When a thread goes into sleep state it doesn’t release the lock.
wait() allows thread to release the lock and goes to suspended state.
This thread will be active when a notify() or notifAll() method is
called for the same object.

One potential big difference between sleep/interrupt and wait/notify is that
calling interrupt() during sleep() always throws an exception (e.g. InterruptedException), whereas
calling notify() during wait() does not.
Generating an exception when not needed is inefficient. If you have threads communicating with each other at a high rate, then it would be generating a lot of exceptions if you were calling interrupt all the time, which is a total waste of CPU.

You are correct - Sleep() causes that thread to "sleep" and the CPU will go off and process other threads (otherwise known as context switching) wheras I believe Wait keeps the CPU processing the current thread.
We have both because although it may seem sensible to let other people use the CPU while you're not using it, actualy there is an overhead to context switching - depending on how long the sleep is for, it can be more expensive in CPU cycles to switch threads than it is to simply have your thread doing nothing for a few ms.
Also note that sleep forces a context switch.
Also - in general it's not possible to control context switching - during the Wait the OS may (and will for longer waits) choose to process other threads.

The methods are used for different things.
Thread.sleep(5000); // Wait until the time has passed.
Object.wait(); // Wait until some other thread tells me to wake up.
Thread.sleep(n) can be interrupted, but Object.wait() must be notified.
It's possible to specify the maximum time to wait: Object.wait(5000) so it would be possible to use wait to, er, sleep but then you have to bother with locks.
Neither of the methods uses the cpu while sleeping/waiting.
The methods are implemented using native code, using similar constructs but not in the same way.
Look for yourself: Is the source code of native methods available? The file /src/share/vm/prims/jvm.cpp is the starting point...

Wait() and sleep() Differences?
Thread.sleep()
Once its work completed then only its release the lock to everyone. until its never release the lock to anyone.
Sleep() take the key, its never release the key to anyone, when its work completed then only its release then only take the key waiting stage threads.
Object.wait()
When its going to waiting stage, its will be release the key and its waiting for some of the seconds based on the parameter.
For Example:
you are take the coffee in yours right hand, you can take another anyone of the same hand, when will your put down then only take another object same type here. also. this is sleep()
you sleep time you didn't any work, you are doing only sleeping.. same here also.
wait(). when you are put down and take another one mean while you are waiting , that's wait
you are play movie or anything in yours system same as player you can't play more than one at a time right, thats its here, when you close and choose another anyone movie or song mean while is called wait

wait releases the lock and sleep doesn't. A thread in waiting state is eligible for waking up as soon as notify or notifyAll is called. But in case of sleep the thread keeps the lock and it'll only be eligible once the sleep time is over.

sleep() method causes the current thread to move from running state to block state for a specified time. If the current thread has the lock of any object then it keeps holding it, which means that other threads cannot execute any synchronized method in that class object.
wait() method causes the current thread to go into block state either for a specified time or until notify, but in this case the thread releases the lock of the object (which means that other threads can execute any synchronized methods of the calling object.

In my opinion, the main difference between both mechanisms is that sleep/interrupt is the most basic way of handling threads, whereas wait/notify is an abstraction aimed to do thread inter-communication easier. This means that sleep/interrupt can do anything, but that this specific task is harder to do.
Why is wait/notify more suitable? Here are some personal considerations:
It enforces centralization. It allows to coordinate the communication between a group of threads with a single shared object. This simplifies the work a lot.
It enforces synchronization. Because it makes the programmer wrap the call to wait/notify in a synchronized block.
It's independent of the thread origin and number. With this approach you can add more threads arbitrarily without editing the other threads or keeping a track of the existing ones. If you used sleep/interrupt, first you would need to keep the references to the sleeping threads, and then interrupt them one by one, by hand.
An example from the real life that is good to explain this is a classic restaurant and the method that the personnel use to communicate among them: The waiters leave the customer requests in a central place (a cork board, a table, etc.), ring a bell, and the workers from the kitchen come to take such requests. Once that there is any course ready, the kitchen personnel ring the bell again so that the waiters are aware and take them to the customers.

Example about sleep doesn’t release lock and wait does
Here there are two classes :
Main : Contains main method and two threads.
Singleton : This is singleton class with two static methods getInstance() and getInstance(boolean isWait).
public class Main {
private static Singleton singletonA = null;
private static Singleton singletonB = null;
public static void main(String[] args) throws InterruptedException {
Thread threadA = new Thread() {
#Override
public void run() {
singletonA = Singleton.getInstance(true);
}
};
Thread threadB = new Thread() {
#Override
public void run() {
singletonB = Singleton.getInstance();
while (singletonA == null) {
System.out.println("SingletonA still null");
}
if (singletonA == singletonB) {
System.out.println("Both singleton are same");
} else {
System.out.println("Both singleton are not same");
}
}
};
threadA.start();
threadB.start();
}
}
and
public class Singleton {
private static Singleton _instance;
public static Singleton getInstance() {
if (_instance == null) {
synchronized (Singleton.class) {
if (_instance == null)
_instance = new Singleton();
}
}
return _instance;
}
public static Singleton getInstance(boolean isWait) {
if (_instance == null) {
synchronized (Singleton.class) {
if (_instance == null) {
if (isWait) {
try {
// Singleton.class.wait(500);//Using wait
Thread.sleep(500);// Using Sleep
System.out.println("_instance :"
+ String.valueOf(_instance));
} catch (InterruptedException e) {
e.printStackTrace();
}
}
_instance = new Singleton();
}
}
}
return _instance;
}
}
Now run this example you will get below output :
_instance :null
Both singleton are same
Here Singleton instances created by threadA and threadB are same. It means threadB is waiting outside until threadA release it’s lock.
Now change the Singleton.java by commenting Thread.sleep(500); method and uncommenting Singleton.class.wait(500); . Here because of Singleton.class.wait(500); method threadA will release all acquire locks and moves into the “Non Runnable” state, threadB will get change to enter in synchronized block.
Now run again :
SingletonA still null
SingletonA still null
SingletonA still null
_instance :com.omt.sleepwait.Singleton#10c042ab
SingletonA still null
SingletonA still null
SingletonA still null
Both singleton are not same
Here Singleton instances created by threadA and threadB are NOT same because of threadB got change to enter in synchronised block and after 500 milliseconds threadA started from it’s last position and created one more Singleton object.

Should be called from synchronized block : wait() method is always called from synchronized block i.e. wait() method needs to lock object monitor before object on which it is called. But sleep() method can be called from outside synchronized block i.e. sleep() method doesn’t need any object monitor.
IllegalMonitorStateException : if wait() method is called without acquiring object lock than IllegalMonitorStateException is thrown at runtime, but sleep() method never throws such exception.
Belongs to which class : wait() method belongs to java.lang.Object class but sleep() method belongs to java.lang.Thread class.
Called on object or thread : wait() method is called on objects but sleep() method is called on Threads not objects.
Thread state : when wait() method is called on object, thread that holded object’s monitor goes from running to waiting state and can return to runnable state only when notify() or notifyAll() method is called on that object. And later thread scheduler schedules that thread to go from from runnable to running state.
when sleep() is called on thread it goes from running to waiting state and can return to runnable state when sleep time is up.
When called from synchronized block : when wait() method is called thread leaves the object lock. But sleep() method when called from synchronized block or method thread doesn’t leaves object lock.
For More Reference

From oracle documentation page on wait() method of Object:
public final void wait()
Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object. In other words, this method behaves exactly as if it simply performs the call wait(0).
The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up
interrupts and spurious wakeups are possible
This method should only be called by a thread that is the owner of this object's monitor
This method throws
IllegalMonitorStateException - if the current thread is not the owner of the object's monitor.
InterruptedException - if any thread interrupted the current thread before or while the current thread was waiting for a notification. The interrupted status of the current thread is cleared when this exception is thrown.
From oracle documentation page on sleep() method of Thread class:
public static void sleep(long millis)
Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers.
The thread does not lose ownership of any monitors.
This method throws:
IllegalArgumentException - if the value of millis is negative
InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.
Other key difference:
wait() is a non-static method (instance method) unlike static method sleep() (class method).

wait() is given inside a synchronized method
whereas sleep() is given inside a non-synchronized method because wait() method release the lock on the object but sleep() or yield() does release the lock().

The method wait(1000) causes the current thread to sleep up to one second.
A thread could sleep less than 1 second if it receives the notify() or notifyAll() method call.
The call to sleep(1000) causes the current thread to sleep for exactly 1 second.
Also sleeping thread doesn't hold lock any resource. But waiting thread does.

Actually, all this is clearly described in Java docs (but I realized this only after reading the answers).
http://docs.oracle.com/javase/8/docs/api/index.html :
wait() - The current thread must own this object's monitor. The thread releases
ownership of this monitor and waits until another thread notifies
threads waiting on this object's monitor to wake up either through a
call to the notify method or the notifyAll method. The thread then
waits until it can re-obtain ownership of the monitor and resumes execution.
sleep() - Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers. The thread does not lose ownership of any monitors.

Java synchronize block on same object in different methods

I am trying to understand the concept of synchronized blocks in java.
As of the documents that I have read, I understood that if we acquire
a lock ( synchronized block using an instance variable ) then we
cannot acquire a synchronized lock on same object in that class. But
when I tried practically using the following snippet I found that my
understanding is going wrong.
I.e I am able to acquire lock
(synchronized block on same instance variable) in two different
methods at the same time. When the thread is started it will go to run
method and wait indefinitely and will not come out of the synchronized
block. At the same time if I call the stop method using the same
thread it goes into the synchronized block and executes notify
statement. I searched in the Java doc but I couldn't find any.
This is the code snippet:
public class MyClass extends Thread
{
private Object lock = new Object();
public void run()
{
synchronized(lock)
{
lock.wait()
}
//other code
}
public void stop()
{
synchronized(lock)
{
lock.notify()
}
//other code
}
}
Here is the code snippet of how i am managing the MyClass thread:
public class MyClassAdmin
{
MyClass _myclass;
public MyClassAdmin()
{
_myclass=new MyClass();
_myclass.start();
}
public void stop()
{
_myclass.stop();
}
public static void main(String args[])
{
MyClassAdmin _myclassAdmin=new MyClassAdmin();
_myclassAdmin.stop();
}
}
According to my understanding when the thread is started it will acquire lock on 'lock' object (synchronized block in run method of MyClass). When i call the stop method it should wait indefinitely until the run method comes out of the synchronized block (which will never happen in this case). But when i executed, call to stop method acquired lock on the 'lock' object and notified the object which resulted in shutdown of the thread.

Both of your methods use the same lock. If your MyClass thread happens to start waiting before the main thread can call the stop method, your stop method can still proceed, because the waiting thread lets go of the lock. Once a thread enters the wait method it releases the lock before it goes dormant, and doesn't re-acquire the lock until it exits the wait method.
This is the relevant API doc for Object#wait, the second paragraph covers what I described above about how wait releases the lock. Pay attention to the part where it says you have to call this method in a loop, otherwise you have an order-dependency bug that can cause the waiting thread to hang when the notification arrives in the main thread before the other thread can start waiting.
public final void wait()
throws InterruptedException
Causes the current thread to wait until another thread invokes the
notify() method or the notifyAll() method for this object. In other
words, this method behaves exactly as if it simply performs the call
wait(0).
The current thread must own this object's monitor. The thread releases
ownership of this monitor and waits until another thread notifies
threads waiting on this object's monitor to wake up either through a
call to the notify method or the notifyAll method. The thread then
waits until it can re-obtain ownership of the monitor and resumes
execution.
As in the one argument version, interrupts and spurious wakeups are
possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait();
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner of
this object's monitor. See the notify method for a description of the
ways in which a thread can become the owner of a monitor.
Understood this is a toy example, but subclassing Thread and overriding Thread methods is confusing. One of the reasons for using Runnable instead of Thread is that there's no opportunity to cause problems by mistakenly overriding Thread methods.

This is multithreaded and it may wait forever may not.
In your case you were lucky that _myclassAdmin.stop(); were executed after MyClass has started executing and executed wait();
I ran the program after changing the method stop() name to stop1() it were waiting forever.
To get consistent behavior do one thing, put a 1 sec sleep between two method call in main like:
MyClassAdmin _myclassAdmin=new MyClassAdmin();
Thread.sleep(1)
_myclassAdmin.stop();
Now, the execution will always halt.
Also, when a thread calls wait() it releases the monitor associated with it, so any other thread can aquire that lock and issue a notify() / notifyAll() to wake up waiting threads. This is expect

Do code inside a synchronized block but after wait() runs in synchronized way?

I wish to know the code written inside the synchronized block but after wait() statement actually runs in synchronized way or not as during wait() lock is relinquished and once the thread is notified it again needs to take lock.
public void process(){
synchronized(sharedObj){
try{
sharedObj.wait();
}catch(InterruptedException ex){
ex.printStackTrace();
}
// when wait is over do the line below runs in synchronized way. It is inside synchronized block but during call of wait(), lock was relinquished.
sharedObj.writeValue();
}
}
Please help me clarifying my doubt.
Regards,
Krishna

http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#wait()
The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
Yes, it gets the lock back.

It is true that for a thread to successfully invoke an object's wait() method, it must hold that object's monitor, and that it relinquishes the monitor during the execution of wait(). It is also true, however, that the thread re-acquires that monitor before returning from wait(). You don't need to do anything special to make that happen.
Perhaps you are confused about the fact that the thread that executes wait() may need to contend with others for the monitor after it stops waiting, but you do not need to worry; it happens before wait() returns. The integrity of the synchronized block remains intact: code executing within can always be assured that no other thread holds the synchronization object's monitor, including after a wait() is preformed.

According to the Javadoc:
The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
So, yes, when the wait call returns, that thread will own the monitor, which means that it is running "in synchronized way" (no other thread will be able to execute that block of code or any other block of code that is synchronized using that same sharedObj before the current thread leaves the synchronized block).
By the way, also according to the Javadoc, wait should always be called in a loop, because the wait can be interrupted for some other reason that is not the notify you are waiting for.
For example:
public void process(){
synchronized(sharedObj){
while(!sharedObj.valueCanBeWritten()) {
try{
sharedObj.wait();
}catch(InterruptedException ex){
ex.printStackTrace();
}
}
sharedObj.writeValue();
}
}

How to understand this simple example of thread?

it is an example about wait() and notify() in Java Concurrency. My theory knowledge about it doesn't explain me about this code and I cannot explain why this gives me a misunderstood result.
So, This is code to get an idea:
public class ExampleOne {
public static void main(String[] args) {
Test b = new Test();
b.start();
synchronized(b){
try{
b.wait();
} catch(InterruptedException ex){
ex.printStackTrace();
}
System.out.println(b.total);
}
}
}
class Test extends Thread {
int total;
#Override
public void run(){
synchronized(this){
for(int i =0;i<50;i++){
total+=i;
System.out.println("I am here");
}
notify();
}
}
}
The result is : 4950
So, how to understand this process (how total can be 4950)?
I understand that If I call wait() it stops the object's thread which called this method and woken up it then another thread calls notify(). Also, synchronized() block restricts threads and accept only one thread in one time.
So when thread calls notify(), it becomes non-active until other
thread calls wait()?
How wait() and notify() play it's role in this code? Also synchronized() block?
So, how many threads are created in this code?
I am confused about this. Help me to figure it out.

If I call wait() it stops the object's thread which called this method and woken up it then another thread calls notify()
Right. notify() or notifyAll() on the same object instance will awaken the thread.
So when thread calls notify(), it becomes non-active until other thread calls wait()?
No. Notify just signals a thread that is waiting on the same object. The thread that calls notify() continues running and does not release the synchronized lock that it is holding.
How wait() and notify() play it's role in this code?
Without wait/notify. the main thread would not wait for the calculation to finish so would probably print 0.
Also synchronized() block?
When you use wait() or notify() you need to be in a synchronized block. It is the monitor associated with the object instance that you are synchronized on that is being waited on and being notified. Also, the synchronized block ensures memory synchronization so the main threads sees the changes to the notify field. Each thread has a memory cache and there needs to be mechanisms in place for threads to publish their changes and see changes to fields made by other threads.
So, how many threads are created in this code?
1 thread is created by your code and the main thread is created automagically by the JVM.
It is important to realize that there is a race condition in this code. It is possible for the new thread to start up, spin through the loop, call notify and exit before the main thread actually calls wait. This is unlikely because starting a thread takes some time but it could happen. In that case, the main thread would be stuck in wait forever because there is no one there to notify it.
Couple other comments:
I recommended that you use this.notify(). It's always good to reiterate what is being notified.
As #JBNizet points out, it is better to use a private final lock object as opposed to a Thread object.
It is always recommended to define a class that implements Runnable as opposed to extending thread. Then you would do new Thread(new TestRunnable()); ....

To complement Gray's excellent answer, here's what happens:
main thread:
---------------------------------------------------->
| | |
start Test thread waits until notified wakes up and prints the result
Test thread:
------------------------>
| |
starts running notifies main thread
computes the sum
Also note that it's generally considered bad practice to call wait() and notify() on a Thread instance. A shared object, acting as a lock, should be used instead. notifyAll() should be preferred generally, or even better, a higher-level abstraction like a CountDownLatch should used.

Java: How can I notify a thread only if it is waiting?

In a first Java thread I have:
while (!isDone) {
try {
synchronized (this) {
wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
doSomeVeryLongRunningTask();
}
In another thread, I want to send a notify signal:
synchronized (thr1) {
thr1.notify();
}
However, if the doSomeVeryLongRunningTask() method is running, I don't want the second thread to get blocked. I only want to notify the first thread if it is waiting so that the second thread can continue it tasks without getting locked.
How might I fix the code above to accomplish this?

They problem you want to fix does not exist. synchronized blocks will block the thread only if another thread is already inside a synchronized block synchronizing on the same object. Since your doSomeVeryLongRunningTask() will be called outside the synchronized block the notifying thread will never get blocked if the other thread is inside the doSomeVeryLongRunningTask() method.
But this raises another problem. You seem to be thinking, that wait and notify invocations are always paired. This is not the case, you may call notify as often as you wish without anyone listening to it. It might be also the case that a wait invocation returns “spuriously”, i.e. for no apparent reason. You therefore need to define another “hard condition” which is defined by a state that is modified and checked inside the synchronized block.
E.g. inside the class whose instance you have in your thr1 variable, you can define a boolean flag:
boolean condition;
Then you modify you waiting method like this:
while(!isDone) {
try {
synchronized(this) {
while(!condition) wait();
if(isDone) break;// skip doSomeVeryLongRunningTask()
condition=false;
}
} catch(InterruptedException e) {
e.printStackTrace();
}
doSomeVeryLongRunningTask();
}
And the notifying code to:
synchronized(thr1) {
thr1.condition=true;
thr1.notify();
}
This way your notifying code still won’t get blocked (at least never for a significant time) but the waiting thread will wait for at least one notification to happen within one loop cycle.

It seems what is blocking your program is not the notify() (it doesn't block ever) but the two synchronized blocks that are synchronizing on the same object.
I don't think there is a workaround to what you ask. Check this link to know why: http://javarevisited.blogspot.com/2011/05/wait-notify-and-notifyall-in-java.html

The notify() call doesn't block. Only wait() blocks. You can call notify even if there isn't another thread waiting, but then make sure your algorithm is correct. If you expect to notify only once, then another thread arriving after the notify will wait() forever.

The advised pattern is to use notifyAll() AND to have ALL waiting threads check their wake-up condition each time they are notified AND before starting the first Wait.

The synchronized in modern Java is about as fast as --i, because this is about what is internally happening thanks to hardware compareAndSet mechanisms. The only moment this slows down noticeably, is when more than one thread is arriving at the synchronized block and therefore at least one has to wait.