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What does it mean by waiting on object monitor [duplicate]

Someone at work just asked for the reasoning behind having to wrap a wait inside a synchronized.
Honestly I can't see the reasoning. I understand what the javadocs say--that the thread needs to be the owner of the object's monitor, but why? What problems does it prevent? (And if it's actually necessary, why can't the wait method get the monitor itself?)
I'm looking for a fairly in-depth why or maybe a reference to an article. I couldn't find one in a quick google.
Oh, also, how does thread.sleep compare?
edit: Great set of answers--I really wish I could select more than one because they all helped me understand what was going on.

Lots of good answers here already. But just want to mention here that the other MUST DO when using wait() is to do it in a loop dependent on the condition you are waiting for in case you are seeing spurious wakeups, which in my experience do happen.
To wait for some other thread to change a condition to true and notify:
synchronized(o) {
while(! checkCondition()) {
o.wait();
}
}
Of course, these days, I'd recommend just using the new Condition object as it is clearer and has more features (like allowing multiple conditions per lock, being able to check wait queue length, more flexible schedule/interrupt, etc).
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
lock.lock();
try {
while (! checkCondition()) {
condition.await();
}
} finally {
lock.unlock();
}
}

If the object does not own the object monitor when it calls Object.wait(), it will not be able to access the object to setup a notify listener until the the monitor is released. Instead, it will be treated as a thread attempting to access a method on a synchronized object.
Or to put it another way, there is no difference between:
public void doStuffOnThisObject()
and the following method:
public void wait()
Both methods will be blocked until the object monitor is released. This is a feature in Java to prevent the state of an object from being updated by more than one thread. It simply has unintended consequences on the wait() method.
Presumably, the wait() method is not synchronized because that could create situations where the Thread has multiple locks on the object. (See Java Language Specifications/Locking for more info on this.) Multiple locks are a problem because the wait() method will only undo one lock. If the method were synchronized, it would guarantee that only the method's lock would be undone while still leaving a potential outer lock undone. This would create a deadlock condition in the code.
To answer your question on Thread.sleep(), Thread.sleep() does not guarantee that whatever condition you are waiting on has been met. Using Object.wait() and Object.notify() allows a programmer to manually implement blocking. The threads will unblock once a notify is sent that a condition has been met. e.g. A read from disk has finished and data can be processed by the thread. Thread.sleep() would require the programmer to poll if the condition has been met, then fall back to sleep if it has not.

It needs to own the monitor, since the purpose of the wait() is to release the monitor and let other threads obtain the monitor to do processing of their own. The purpose of these methods (wait/notify) is to coordinate access to synchronized code blocks between two threads that require each other to perform some functionality. It is not simply a matter of making sure access to a data structure is threadsafe, but to coordinate events between multiple threads.
A classic example would be a producer/consumer case where one thread pushes data to a queue, and another thread consumes the data. The consuming thread would always require the monitor to access the queue, but would release the monitor once the queue is empty. The producer thread would then only get access to write to the thread when the consumer is no longer processing. It would notify the consumer thread once it has pushed more data into the queue, so it can regain the monitor and access the queue again.

Wait gives up the monitor, so you must have it to give it up. Notify must have the monitor as well.
The main reason why you want to do this is to ensure that you have the monitor when you come back from wait() -- typically, you are using the wait/notify protocol to protect some shared resource and you want it to be safe to touch it when wait returns. The same with notify -- usually you are changing something and then calling notify() -- you want to have the monitor, make changes, and call notify().
If you made a function like this:
public void synchWait() {
syncronized { wait(); }
}
You would not have the monitor when wait returned -- you could get it, but you might not get it next.

Here's my understanding on why the restriction is actually a requirement. I'm basing this on a C++ monitor implementation I made a while back by combining a mutex and a condition variable.
In a mutex+condition_variable=monitor system, the wait call sets the condition variable into a wait state and releases the mutex. The condition variable is shared state, so it needs to be locked to avoid race conditions between threads that want to wait and threads that want to notify. Instead of introducing yet another mutex to lock its state, the existing mutex is used. In Java, the mutex is correctly locked when the about-to-wait thread owns the monitor.

Mostly wait is done if there is a condition say a queue is empty.
If(queue is empty)
queue.wait();
Let us assume the queue is empty.
In case if the current thread pre-empts after checking the queue, then if another
thread adds few elements to queue, the current thread will not know and will go for wait
state. Thats wrong.
So we should have something like
Synchornized(queue)
{
if(queue is empty)
queue.wait();
}
Now let us consider what if they made wait itself as synchronized. As already mentioned in one of the comments, it releases only one lock. That means if wait() was synchronized in the above code only one lock would have been released. Implies that current thread will go for wait with the lock for the queue.

What is the latency of a BlockingQueue's take() method?

I'd like to understand how take() works and if it's a suitable method to consume "fastly" elements that are pushed on a queue.
Note that, for the sake of understanding how it works, I'm not considering here the observer pattern: I know that I could use that pattern to "react quicly" to events but that's not what my question is about.
For example if I have a BlockingQueue (mostly empty) and a thread "stuck" waiting for an element to be pushed on that queue so that it can be consumed, what would be a good way to minimize the time spent (reduce the latency) between the moment an element is pushed on the queue and the moment it is consumed?
For example what's the difference between a thread doing this:
while( true ) {
elem = queue.peek();
if ( elem == null ) {
Thread.sleep( 25 ); // prevents busy-looping
} else {
... // do something here
}
}
and another one doing this:
while ( true ) {
elem = queue.take();
... // do something with elem here
}
(I take it that to simplify things we can ignore discussing about exceptions here!?)
What goes on under the hood when you call take() and the queue is empty? The JVM somehow has to "sleep" the thread under the hood because it can't be busy-looping constantly checking if there's something on the queue? Is take() using some CAS operation under the hood? And if so what determines how often take() does call that CAS operation?
What when something suddenly makes it to the queue? How's that thread blocked on take() somehow "notified" that it should act promptly?
Lastly, is it "common" to have one thread "stuck" on take() on a BlockingQueue for the lifetime of the application?
It's all one big question related to how the blocking take() works and I take it that answering my various questions (at least the one that makes sense) would help me understand all this better.

Internally, take waits on the notEmpty condition, which is signaled in the insert method; in other words, the waiting thread goes to sleep, and wakes up on insert. This should be fast.
Some blocking queues, e.g. ArrayBlockingQueue and SynchronousQueue, have a constructor that accepts the queue's fairness property; passing in true should prevent threads from getting stuck on take, otherwise this is a possibility. (This parameter specifies whether the underlying ReentrantLock is fair.)

Well, here's the implementation of LinkedBlockingQueue<E>.take() :
public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
while (count.get() == 0) {
notEmpty.await();
}
x = dequeue();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
When the queue is empty, notEmpty.await() is called, which :
Causes the current thread to wait until it is signalled or
interrupted.
The lock associated with this Condition is atomically released and the
current thread becomes disabled for thread scheduling purposes and
lies dormant until one of four things happens:
Some other thread invokes the signal method for this Condition and the
current thread happens to be chosen as the thread to be awakened; or
Some other thread invokes the signalAll method for this Condition; or
Some other thread interrupts the current thread, and interruption of
thread suspension is supported; or
A "spurious wakeup" occurs.
When another threads puts something in the queue, it calls signal, which awakes one of the threads waiting to consume items from this queue. This should work faster then your peek/sleep loop.

You can assume that take() will be notified that it can wake as soon your OS can pass such a signal between threads. Note: your OS will be involved worst case. Typically this is 1 - 10 micro-seconds, and in rare case 100 or even 1000 micro-seconds in very rare cases. Note: Thread.sleep will wait for a minimum of 1000 microseconds and 25 milli-seconds is 25,000 micro-seconds so I would hope the difference is obvious to you.
The only really way of avoiding rare but long context switches is to busy wait on a affinity lock CPU. (This allocates a CPU to your thread) If your application is that latency sensible, simpler solution is to not pass the work between threads at all. ;)

Since two threads are involved, the peek/sleep with a hypothetical micro/nano-sleep implementation would not differ too much from take() since they both involve passing information from one thread to the next via main memory (using volatile write/read and a healthy amount of CAS), unless the JVMs find other ways to do inter-thread synchronization. You can try to implement a benchmark using two BlockingQueues and two threads who each act as producer for one queue and consumer for the other and move a token back and forth, taking it from one queue and offering to the next. Then you could see how fast they can produce/consume and compare that to peek/sleep. I guess performance depends a lot on the amount work spent on each token (in this case zero, so we measure pure overhead) and the distance of CPU to memory. In my experience, single CPUs come out way ahead of multi-socket machines.

The difference is that the first thread sleeps for up to 25ms too long, whereas the second thread doesn't waste any time at all.

why does my notify method not work properly?

I am improving my concurrent program by pausing threads that doing the same thing to wait for one of them finished. However, it cannot not wake up threads properly. Here is the code.
//to store graphs, if a thread finds the graph it is going to compute is in the entry, it waits, otherwise it compute then notify all other threads waiting on it.
Map<Graph, Object> entry = new ConcurrentHashMap<Graph, Object>();
public Result recursiveMethod(Graph g) {
if (entry.get(g) != null) {//if the graph is in the entry, waits
synchronized(entry.get(g)) {
entry.get(g).wait();
}
//wakes up, and directly return the result
return result;
}
synchronized(entry) {
if (entry.get(g) == null)//if the graph is not in the entry, continue to compute
entry.put(g,new Object());
}
//compute the graph recursively calls this method itself...
calculate here...
//wake up threads waiting on it, and remove the graph from entry
synchronized(entry.get(g)){
entry.get(g).notifyAll();
}
entry.remove(g);
return result;
}
This method is called by many many threads. Before a thread starts calculation, it looks up the entry to see if there is another thread calculating an identical graph. If so, it waits.
If not, it continues to calculate. After it figures out the result, it notifies all the threads that is waiting on it.
I use a map to pair up graph and an object. The object is the lock.
Please notice that the this map can recognize two identical graphs, that is, the following code returns true.
Graph g = new Graph();
entry.put(g, new Object());
Graph copy = new Graph(g);
entry.get(g) == entry.get(copy) //this is true
Thus, the entry.get(g) should be ok to be the lock/monitor.
However, most of the threads have not been awaken, only 3-4 threads has.
When the number of threads that is waiting equals to the number of threads that my computer can create, which means all the threads are waiting, this program would never terminate.
Why exactly does not the entry.get(g).notifyAll() work?

Due to the fact that you have un-synchronized gaps between times that you check the map and times that you operate on the map, you have many holes in your logic where threads can proceed incorrectly. you either need to synchronize outside of your map checks or use some of the special atomic methods for ConcurrentMaps.
when writing concurrent code, i like to pretend there is a malicious gnome running around in the background changing things wherever possible (e.g. outside of synchronized blocks). here's the first example to get you started:
if (entry.get(g) != null) {//if the graph is in the entry, waits
synchronized(entry.get(g)) {
you call entry.get() twice outside of a synchronization block. therefore, the value you get could be different between those 2 calls (the evil gnome changes the map as often as possible). in fact, it could be null when you try to synchronize on it, which will throw an exception.
additionally, wait() calls should always be made in a loop while waiting for a loop condition to change (due to the possibility of spurious wakeups, or, also in your case, multiple wakeups). lastly, you should change the loop condition before you notify. #AdrianShum gave a pretty good overview of how to use wait/notify correctly. your while loop should not be around everything, but within the synchronized block, around the wait call alone. this is not to deal with InterruptedException (a separate issue), but to deal with spurious wakeups and notifyAll calls. when you call notifyAll all waiting threads wake up, but only one can proceed, so the rest need to go back to waiting (hence the while loop).
in short, writing concurrent code is hard, and what you are attempting to implement is not simple. i would recommend reading a good book first (like Josh Bloch's "Java Concurrency In Practice") before attempting to finish this code.

In fact #jtahlborn has already raised the key of problem. I am trying to supplement by telling what's the most obvious issue here.
Try to get yourself understand the basics of Condition and why it can solve those race condition as normal signalling (in windows for example)
Your logic is something like this now (assume obj is referring to same object):
Thread 1:
if (!hasResult) {
synchronized(obj) {
obj.wait();
}
}
Thread 2:
hasResult = false;
// do your work
synchronized(obj) {
obj.notify();
}
hasResult= true;
You gotta know thread 1 and thread 2 is run in parallel, therefore you may have something like
Thread 1 Thread 2
hasResult = false
if (!hasResult)
do your work
synchronized(obj)
obj.notify()
end synchronized(obj)
synchronized(obj)
obj.wait()
end synchronized(obj)
Thread 1 is going to wait forever.
What you should do is
Thread 1:
synchronized(obj) {
while (hasResult) {
obj.wait();
}
}
Thread 2:
hasResult = false;
synchronized(obj) {
// do your work
obj.notify();
hasResult=true;
}
That's one of the biggest hole that #jtahlborn is talking I believe (and there are other). Note that setting condition and checking condition are all protected in synchronized block. That's the main basic idea of how Condition variable is solving the race condition illustrated before. Get yourself understand the idea first, and redesign your piece of code with something more reasonable.

Condition vs wait notify mechanism

What is the advantage of using Condition interface/implementations over the conventional wait notify mechanism? Here I quote the comments written by Doug Lea:
Condition factors out the Object monitor methods (wait, notify and notifyAll) into distinct objects to give the effect of having multiple wait-sets per object, by combining them with the use of arbitrary Lock implementations. Where a Lock replaces the use of synchronized methods and statements, a Condition replaces the use of the Object monitor methods.
I see this is a more Object Oriented way of implementing wait/notify mechanism. But is there a sound advantage over the former?

The biggest problem is that wait/notify is error prone for new developers. The main problem is not knowing how to handle them correctly can result is obscure bug.
if you call notify() before wait() it is lost.
it can be sometimes unclear if notify() and wait() are called on the same object.
There is nothing in wait/notify which requires a state change, yet this is required in most cases.
wait() can return spuriously
Condition wraps up this functionality into a dedicated component, however it behaves much the same.
There is a question regarding wait/nofity posted minutes before this one and many, many more Search [java]+wait+notify

When you use Condition: await()/signal() you can distinguish which object or group of objects/threads get a specific signal. Here is a short example where some threads, the producers, will get the isEmpty signal while the consumers will get the isFull signal:
private volatile boolean usedData = true;//mutex for data
private final Lock lock = new ReentrantLock();
private final Condition isEmpty = lock.newCondition();
private final Condition isFull = lock.newCondition();
public void setData(int data) throws InterruptedException {
lock.lock();
try {
while(!usedData) {//wait for data to be used
isEmpty.await();
}
this.data = data;
isFull.signal();//broadcast that the data is now full.
usedData = false;//tell others I created new data.
}finally {
lock.unlock();//interrupt or not, release lock
}
}
public void getData() throws InterruptedException{
lock.lock();
try {
while(usedData) {//usedData is lingo for empty
isFull.await();
}
isEmpty.signal();//tell the producers to produce some more.
usedData = true;//tell others I have used the data.
}finally {//interrupted or not, always release lock
lock.unlock();
}
}

There are many advantages like mentioned above about Condition Interface some important are as follows:
Condition interface comes with Two extra methods that are:
1)boolean awaitUntil(Date deadline)throws InterruptedException :
Causes the current thread to wait until it is signalled or interrupted, or the specified deadline elapses.
2)awaitUninterruptibly() :
Causes the current thread to wait until it is signalled.
If the current thread's interrupted status is set when it enters this method, or it is interrupted while waiting, it will continue to wait until signalled. When it finally returns from this method its interrupted status will still be set.
The above two methods are not present in default monitor that is in object class,in some situations we want to set the deadline for thread to wait then we are able to do that by Condition interface.
In some situations we don't want thread to be interrupted and want current thread to wait until it is signaled then we can go for awaitUninterruptibly method present in Condition Interface.
For more information Condition Interface Java Documentation:
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/locks/Condition.html#awaitUntil%28java.util.Date%29

To specifically address why having multiple waitsets is an advantage:
With wait/notify if there are different things that threads are waiting for (the common example is a fixed size blocking queue, with some threads putting things in the queue and blocking when the queue is full, and other threads taking from the queue and blocking when the queue is empty) then if you use notify, causing the scheduler to pick one thread from the wait set to notify, you can have corner cases where the chosen thread isn't interested in being notified for a particular situation. For instance the queue will notify for adding something to the queue, but if the chosen thread is a producer and the queue is full then it can't act on that notification, which you would rather have gone to a consumer. With intrinsic locking you have to use notifyAll in order to make sure that notifications don't get lost.
But notifyAll incurs churn with every call, where every thread wakes up and contends for the lock, but only one can make progress. The other threads all bump around contending for the lock until, one at a time, they can acquire the lock and most likely go back to waiting. It generates a lot of contention for not much benefit, it would be preferable to be able to use notify and know only one thread is notified, where the notification is relevant to that thread.
This is where having separate Conditions to wait on is a big improvement. The queue can invoke signal on a condition and know it will wake up only one thread, where that thread is specifically waiting for the condition.
The API doc for Condition has a code example that shows using multiple conditions for a bounded buffer, it says:
We would like to keep waiting put threads and take threads in separate wait-sets so that we can use the optimization of only notifying a single thread at a time when items or spaces become available in the buffer.

In addition to other well accepted answers - since Condition is associated with Lock object you can have arbitrary sets of Lock objects (reawrite, read, write) in your class and have specific condition associated with that. Then you can use those set of condition to synchronize different parts of your class according to your implementation semantics. This gives more flexibility and explicit behavior then wait-notify imo

Why should wait() always be called inside a loop

I have read that we should always call a wait() from within a loop:
while (!condition) { obj.wait(); }
It works fine without a loop so why is that?

You need not only to loop it but check your condition in the loop. Java does not guarantee that your thread will be woken up only by a notify()/notifyAll() call or the right notify()/notifyAll() call at all. Because of this property the loop-less version might work on your development environment and fail on the production environment unexpectedly.
For example, you are waiting for something:
synchronized (theObjectYouAreWaitingOn) {
while (!carryOn) {
theObjectYouAreWaitingOn.wait();
}
}
An evil thread comes along and:
theObjectYouAreWaitingOn.notifyAll();
If the evil thread does not/can not mess with the carryOn you just continue to wait for the proper client.
Edit: Added some more samples.
The wait can be interrupted. It throws InterruptedException and you might need to wrap the wait in a try-catch. Depending on your business needs, you can exit or suppress the exception and continue waiting.

It's answered in documentation for Object.wait(long milis)
A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout);
... // Perform action appropriate to condition
}
(For more information on this topic,
see Section 3.2.3 in Doug Lea's
"Concurrent Programming in Java
(Second Edition)" (Addison-Wesley,
2000), or Item 50 in Joshua Bloch's
"Effective Java Programming Language
Guide" (Addison-Wesley, 2001).

Why should wait() always be called inside a loop
The primary reason why while loops are so important is race conditions between threads. Certainly spurious wakeups are real and for certain architectures they are common, but race conditions are a much more likely reason for the while loop.
For example:
synchronized (queue) {
// this needs to be while
while (queue.isEmpty()) {
queue.wait();
}
queue.remove();
}
With the above code, there may be 2 consumer threads. When the producer locks the queue to add to it, consumer #1 may be blocked at the synchronized lock while consumer #2 is waiting on the queue. When the item is added to the queue and notify called by the producer, #2 is moved from the wait queue to be blocked on the queue lock, but it will be behind the #1 consumer which was already blocked on the lock. This means that the #1 consumer gets to go forward first to call remove() from the queue. If the while loop is just an if, then when consumer #2 gets the lock after #1 and calls remove(), an exception would occur because the queue is now empty -- the other consumer thread already removed the item. Even though it was notified, it needs to be make sure the queue is for sure not empty because of this race condition.
This well documented. Here's a web page I created a while back which explains the race condition in detail and has some sample code.

There might be more then just one worker waiting for a condition to become true.
If two or more worker get awake (notifyAll) they have to check the condition again.
otherwise all workers would continue even though there might only be data for one of them.

I think I got #Gray 's answer.
Let me try to rephrase that for newbies like me and request the experts to correct me if I am wrong.
Consumer synchronized block::
synchronized (queue) {
// this needs to be while
while (queue.isEmpty()) {
queue.wait();
}
queue.remove();
}
Producer synchronized block::
synchronized(queue) {
// producer produces inside the queue
queue.notify();
}
Assume the following happens in the given order:
1) consumer#2 gets inside the consumer synchronized block and is waiting since queue is empty.
2) Now, producer obtains the lock on queueand inserts inside the queue and calls notify().
Now,either consumer#1 can be chosen to run which is waiting for queue lock to enter the synchronized block for the first time
or
consumer#2 can be chosen to run.
3) say, consumer#1 is chosen to continue with the execution. When it checks the condition,it will be true and it will remove() from the queue.
4) say,consumer#2 is proceeding from where it halted its execution (the line after the wait() method). If 'while' condition is not there (instead an if condition), it will just proceed to call remove() which might result in an exception/unexpected behaviour.

Because wait and notify are used to implement [condition variables](http://en.wikipedia.org/wiki/Monitor_(synchronization)#Blocking_condition_variables) and so you need to check whether the specific predicate you're waiting on is true before continuing.

Both safety and liveness are concerns when using the wait/notify mechanism. The safety property requires that all objects maintain consistent states in a multithreaded environment. The liveness property requires that every operation or method invocation execute to completion without interruption.
To guarantee liveness, programs must test the while loop condition before invoking the wait() method. This early test checks whether another thread has already satisfied the condition predicate and sent a notification. Invoking the wait() method after the notification has been sent results in indefinite blocking.
To guarantee safety, programs must test the while loop condition after returning from the wait() method. Although wait() is intended to block indefinitely until a notification is received, it still must be encased within a loop to prevent the following vulnerabilities:
Thread in the middle: A third thread can acquire the lock on the shared object during the interval between a notification being sent and the receiving thread resuming execution. This third thread can change the state of the object, leaving it inconsistent. This is a time-of-check, time-of-use (TOCTOU) race condition.
Malicious notification: A random or malicious notification can be received when the condition predicate is false. Such a notification would cancel the wait() method.
Misdelivered notification: The order in which threads execute after receipt of a notifyAll() signal is unspecified. Consequently, an unrelated thread could start executing and discover that its condition predicate is satisfied. Consequently, it could resume execution despite being required to remain dormant.
Spurious wakeups: Certain Java Virtual Machine (JVM) implementations are vulnerable to spurious wakeups that result in waiting threads waking up even without a notification.
For these reasons, programs must check the condition predicate after the wait() method returns. A while loop is the best choice for checking the condition predicate both before and after invoking wait().
Similarly, the await() method of the Condition interface also must be invoked inside a loop. According to the Java API, Interface Condition
When waiting upon a Condition, a "spurious wakeup" is permitted to
occur, in general, as a concession to the underlying platform
semantics. This has little practical impact on most application
programs as a Condition should always be waited upon in a loop,
testing the state predicate that is being waited for. An
implementation is free to remove the possibility of spurious wakeups
but it is recommended that applications programmers always assume that
they can occur and so always wait in a loop.
New code should use the java.util.concurrent.locks concurrency utilities in place of the wait/notify mechanism. However, legacy code that complies with the other requirements of this rule is permitted to depend on the wait/notify mechanism.
Noncompliant Code Example
This noncompliant code example invokes the wait() method inside a traditional if block and fails to check the postcondition after the notification is received. If the notification were accidental or malicious, the thread could wake up prematurely.
synchronized (object) {
if (<condition does not hold>) {
object.wait();
}
// Proceed when condition holds
}
Compliant Solution
This compliant solution calls the wait() method from within a while loop to check the condition both before and after the call to wait():
synchronized (object) {
while (<condition does not hold>) {
object.wait();
}
// Proceed when condition holds
}
Invocations of the java.util.concurrent.locks.Condition.await() method also must be enclosed in a similar loop.

Before getting to the answer, lets see how wait is probably implemented.
wait(mutex) {
// automatically release mutex
// and go on wait queue
// ... wait ... wait ... wait ...
// remove from queue
// re-acquire mutex
// exit the wait operation
}
In your example mutex is the obj with the assumption that your code is running inside synchronized(obj) { } block.
A mutex is called as monitor in Java [some subtle differences though]
A concurrency example using condition variable with if
synchronized(obj) {
if (!condition) {
obj.wait();
}
// Do some stuff related to condition
condition = false;
}
Lets say we have 2 threads. Thread 1 and Thread 2.
Lets see some states along the timeline.
at t = x
Thread 1 state:
waiting on ... wait ... wait ... wait ..
Thread 2 state:
Just entered the synchronised section, since as per the thread 1's state, the mutex/monitor is released.
You can read more about wait() here java.sun.com/javase/6/docs/api/java/lang/Object.html#wait(long).
This is the only thing that is tricky to understand. When 1 thread is inside the synchronized block. Another thread can still enter the synchronized block because wait() causes the monitor/mutex to be released.
Thread 2 is about to read if (!condition) statement.
at t = x + 1
notify() is triggered by some thread on this mutex/monitor.
condition becomes true
Thread 1 state:
Waiting at re-acquire mutex, [Since thread-2 has the lock now]
Thread 2 state:
Doesn't go inside if condition and marks the condition = false.
at t = x + 2
Thread 1 state:
Exits the wait operation and about to mark condition = false.
This state is inconsistent as condition is supposed to be true but is false already, because thread 2 marked it false previously.
And thats the reason, while is required instead of if. As while would trigger the condition to be checked again for thread 1 and thread 1 will begin waiting again.
Result
In order to avoid this inconsistency the correct code seems to be like this:
synchronized(obj) {
while (!condition) {
obj.wait();
}
// Do some stuff related to condition
condition = false;
}

From your Question:
I have read that we should always called a wait() from within a loop:
Although wait( ) normally waits until notify( ) or notifyAll( ) is called, there is a possibility that in very rare cases the waiting thread could be awakened due to a spurious wakeup. In this case, a waiting thread resumes without notify( ) or notifyAll( ) having been called.
In essence, the thread resumes for no apparent reason.
Because of this remote possibility, Oracle recommends that calls to wait( ) should take place within a loop that checks the condition on which the thread is waiting.

Three things you will see people do:
Using wait without checking anything (BROKEN)
Using wait with a condition, using an if check first (BROKEN).
Using wait in a loop, where the loop test checks the condition (NOT BROKEN).
Not appreciating these details about how wait and notify work leads people to choose the wrong approach:
One is that a thread doesn't remember notifications that happened before it got around to waiting. The notify and notifyAll methods only effect threads that are already waiting, if a thread isn't waiting at the time it is out of luck.
Another is that a thread releases the lock once it starts waiting. Once it gets a notification it re-acquires the lock and continues on where it left off. Releasing the lock means that thread does not know anything about the current state once it wakes back up, any number of other threads could have made changes since then. The check made before the thread started waiting doesn't tell you anything about what the state is currently.
So the first case, with no checking, makes your code vulnerable to race conditions. It might happen to work by accident if one thread has enough of a head start over another. Or you may have threads waiting forever. If you sprinkle in timeouts then you end up with slow code that sometimes doesn't do what you want.
Adding a condition to check apart from the notification itself protects your code from these race conditions and gives your code a way to know what the state is even if the thread wasn't waiting at the right time.
The second case, with if-checks, is likely to work if you have only 2 threads. That puts a limit on the number of states things can get into and when you made faulty assumptions you don't get burned so badly. This is the situation for lots of toy example code exercises. The result is people come away thinking they understand, when they really don't.
Protip: Real world code has more than two threads.
Using the loop lets you re-check the condition once you re-acquire the lock so that you're moving forward based on current state, not on stale state.

In simple words,
'if' is a conditional statement , once condition is satisfied remaining block of code will get executed.
'while' is a loop which going check the condition unless condition is not satisfied.