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is the following statement correct:
" There shouldn't be any thread interference between two synchronized methods in 2 different classes . So they can run concurrently without any problems."
Thanks for your time
That is way too vague. A few pointers:
"how does synchronization work in Java": There are a couple of mechanisms, the question seems to be about the synchronized keyword. This works by marking "critical sections" that must not be executed by more than one thread at the same time, and having the threads "lock" a monitor object while they are in that section (so that all other threads wait).
synchronized methods synchronize on the object instance (or class object in case of a static method). So methods in different classes do not synchronize with each-other that way. They will run concurrently.
you can use the synchronized keyword to synchronize blocks on any other monitor object. This way, methods in different classes can still be synchronized with each-other.
"can run concurrently without problems" is not guaranteed just by having some synchronization (or lack thereof). You need to see what mutable state these methods (directly or indirectly) try to access (and who else does the same) to see what kind of concurrency control is necessary.
You misunderstood the concept a little bit. Collisions happen when two (or more) threads simultaneously try to make a change on the same data or when one of them tries the read the data while the other thread is trying to change it.
When two thread tries to change the shared resource simultaneously, a race condition occurs. Check out this link to learn more about Race Condition.
In order to prevent this kind of problems, you need to guard the shared resource for simultaneous changes. Mutexes and semaphores are invented for this purpose: To lock the shared resource for the other threads, when one thread is currently making a change on it. For this purpose, Java uses the synchronized keyword. You can read more about Synchronized in Java using the link.
Note that, using the synchronized keyword will not eliminate all of the synchronization related issues, but it is a good starting point.
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I am trying to understand multi-threading in Java, using the
features which were added as part of java.util.concurrent.* . To begin with, there is concept of lock which a thread can try to acquire; if a thread can't acquire, it can do some other tasks.
This I have read in online materials as well in some books, but never ever seen anything they have implemented in real. How is this possible that if a thread can't acquire a lock it can execute other tasks; isn't a thread supposed to do a single "piece of work"? How can it have multiple logic execution based on if it can/can't acquire a lock?
Is there any real implementation which I can refer to see to understand, to reinforce the concepts; else it seems too abstract, how to implement in real life.
Any explanations?
It's difficult to find real life examples because normally you wouldn't design your software to use tryLock(). The example given in the javadoc is as follows:
Lock lock = ...;
if (lock.tryLock()) {
try {
// manipulate protected state
} finally {
lock.unlock();
}
} else {
// perform alternative actions
}
But you wouldn't design your software like that, would you? What if the lock is never (or almost never) available, how will that affect your program? What if it's always available? You have a method that does one of two things depending on pure chance. That's not good design, it increases randomness and complexity.
Okay, so it's not something you decide to use because it's elegant. What is it good for?
Imagine you've inherited a legacy project designed by an insane programmer and it has severe issues with deadlocks. It has synchronized methods peppered all around and needs to be booted at least once every week because it locks up. First you convert all the synchronized methods to use Lock instead. Now you no longer block forever on synchronized, but can use tryLock(long, TimeUnit) to timeout and prevent deadlocks.
Now you've solved the reboot causing deadlocks, but it's still not optimal since you're spending time waiting. With additional refactoring you manage to reduce the locks, but unfortunately you can't do proper lock ordering just yet. Your end code looks like this, where inner locks are acquired with tryLock() or outerlock is released to prevent deadlock:
Lock outerLock = ...;
outerLock.lock(); // Here we block freely
try {
Lock innerLock = ...;
if (innerLock.tryLock()) { // Here we risk deadlock, we'd rather "fail-fast"
try {
doSomethingProtectedByLocks();
} finally {
innerLock.unlock();
}
} else {
throw new OperationFailedException(); // Signal the calling code to retry
}
} finally {
outerLock.unlock();
}
I think the problem is mainly with wording. The Javadoc talks about "actions" (like unlocking an outer lock) being performed based on whether the lock was acquired or not, but it's easy to read it as if the thread would have 2 separate responsibilities determined by the lock state.
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As mentioned in answer, synchronized is implemented using compareAndSwap, which is non-blocking algorithm.
On synchronized without using wait(), Does a thread state set to BLOCKED?
Does a thread in BLOCKED & WAITING state consume CPU cycles?
As mentioned in answer, synchronized is implemented using compareAndSwap, which is non-blocking algorithm.
I think you are misreading that answer. synchronized is certainly not implemented with the Java-level compareAndSwap call. It will be implemented in native code by the interpreter and JIT in your JVM. Under the covers it might use the compare-and-swap instruction, or it may use something else (atomic test-and-set and atomic exchange are also common - and some platforms don't even have a CAS primitive).
This is definitely not a "non-blocking algorithm" - by definition synchronized implements a type of critical section which implies blocking if a second thread tries to enter the critical section while another thread is inside it.
1) On synchronized without using wait(), Does a thread state set to BLOCKED?
Yes, if a thread is waiting to enter a synchronized section, its state is set to BLOCKED.
2) Does a thread in BLOCKED & WAITING state consume CPU cycles?
Generally no, at least not in an ongoing manner. There is a CPU cost associated with entering and exiting the state1, but once the thread is blocked it is generally held in a non-runnable state until it is awoken when the monitor becomes free, and doesn't use CPU resources during that time.
1 Which is exactly why good implementations will generally "spin" a bit before going to sleep, in case the critical section is short and the holding thread may soon exit - spinning in this case avoids the 1000+ cycle overhead of making the transition to sleep (which involves the OS, etc).
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I've been watching a lot of videos on data structures, and these terms are always being mentioned: synchronized/not synchronized and thread-safe/not thread-safe.
Can someone explain to me in simple words what synchronized and thread-safe mean in Java? What is sync and what is thread?
A thread is an execution path of a program. A single threaded program will only have one thread and so this problem doesn't arise. Virtually all GUI programs have multiple execution path and hence threads - one for processing the display of the GUI and handing user input, others for actually performing the operations of the program. This is so that the UI is still responsive while the program is working.
In the simplest of terms threadsafe means that it is safe to be accessed from multiple threads. When you are using multiple threads in a program and they are each attempting to access a common data structure or location in memory several bad things can happen. So, you add some extra code to prevent those bad things. For example, if two people were writing the same document at the same time, the second person to save will overwrite the work of the first person. To make it thread safe then, you have to force person 1 to wait for person 2 to complete their task before allowing person 1 to edit the document.
Synchronized means that in a multiple threaded environment, a Synchronizedobject does not let two threads access a method/block of code at the same time. This means that one thread can't be reading while another updates it.
The second thread will instead wait until the first is done. The overhead is speed, but the advantage is guaranteed consistency of data.
If your application is single threaded though, Synchronized has no benefit.
As per CIP:
A class is thread-safe if it behaves correctly when accessed from
multiple threads, regardless of the scheduling or interleaving of the
execution of those threads by the runtime environment, and with no
additional synchronization or other coordination on the part of the
calling code.
So thread safety is a desired behavior of the program in case it is accessed by multiple threads. Using the synchronized block is one way of achieving that behavior. You can also check the following:
What does 'synchronized' mean?
What does threadsafe mean?
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I find the method wait() in the class Object.
It is final which means that this method cannot be overriden.
Any ideas why it's final?
#Flavio - it's actually a very good question.
The reason you can't override it, of course, is that the designers made it "final".
A couple of potential reasons for this decision:
You don't want people to mess with the semantics of a fundamental operation on a fundamental class (class "Object").
Since it's "final", compilers can optimize performance (save a few cycles) by in-lining "wait()"
"final" increases the security of the Java object model by preventing malicious code from exploiting "wait()".
It is not designed to be overriden, that's why. The method wait() calls wait(long timeout) which is final and native. So the latter is not supposed to be overriden and the no-paremeter version is just final so it is not supposed to be overriden too.
If you override a very basic functionality available for all Objects then it is highly likely that you will blow up your software or ruin your colleague's day.
If you check the documentation of wait():
Causes the current thread to wait until another thread invokes the
notify() method or the notifyAll() method for this object.
it turns out that it works in tandem with notify() and notifyAll() so you can't change it without altering their functionality. Not to mention the dozens of concurrent libraries using those methods.
It has a fairly concrete implementation that does not need to be modified. The logic behind wait() should be synonymous across all objects
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What is this:
synchronized (this) {
// ...some code...
}
good for?
(Could you write an example?)
It prevents concurrent access to a resource. Sun's got a pretty good description with examples.
It prevents multiple threads from running the code contained within the braces. Whilst one thread is running that code, the remainder are blocked. When the first thread completes, one of the blocked threads will then run the synchronised code, and so on.
Why do you want to do this ? The code within the block may modify objects such that they're in an inconsistent state until the blocks exits. So a second thread coming in would find inconsistent objects. From that point on chaos ensues.
An example would be removing an object from one pool and inserting it in another. A second thread might run whilst the first thread is moving the object, and subsequently find the object referenced in both collections, or neither.
You can also use this mechanism to restrict multiple threads from accessing a resource designed to be used by one resource (e.g. a trivial database, for example).
Note that the following two are equivalent:
synchronized void someMethod() {
// ...
}
and
void someMethod() {
synchronized (this) {
// ...
}
}
From the now-defunct Java Quick Reference formerly at http://www.janeg.ca/scjp/threads/synchronized.html:
Synchronizing threads has the effect
of serializing access to blocks of
code running on the thread.
Serializing in this context means
giving one thread at a time the right
to execute specific block of code.