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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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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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Aside from the properties to know the state of the task and the call backs (completed, failed, etc), is there any other differences of using JavaFX Task over a plain old Java Thread with a lambda?
I'm not asking what's good or bad, I'm not asking what you think I should do, I'm asking for the objective factual differences that would happen when using a Task instead of a plain lambda to run a background thread in a JavaFX application. Aside from the the differences that I already mentioned.
On the Oracel's tutorial for concurrency, in the section titled "Why Use the javafx.concurrent Package?" it says:
If you have special requirements or need extra power over the code, implementing a background worker by creating a Runnable object and a new thread is an appropriate way to go.
other than that, all the reasons for using a Task are equally applicable to a Runnable and I don't see what special requirements or extra power one gains, I also don't see what you lose, if anything, aside from state and callbacks, when choosing Runnable over Task. What I do see is that using Task is much more verbose:
new Thread(this::doSomething).start();
vs
new Thread(new Task<Void>() {
#Override
protected Void call() throws Exception {
doSomething();
return null;
}
}).start();
My concern would be the some unexpected side effect from choosing a shorter more concise version of the code that according to Oracle, should only be used in special cases.
doSomething is a variety of different activity on this example and I'm interested in facts that are agnostic to what they do. In my case, it's mostly network requests that then update the UI by using Platoform.runLater.
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I have a server with multithreading.
Each connection has it's own thread. The thread sometimes needs to access some methods from an Object and the method could be only called once at a time. So what would be better for the performance: Just to create a new object for every thread when it need to use it. Or sharing one global object which has synchronized methods?
synchronizing would not lead you to better performance. It could potentially make the performance worse if done incorrectly.
You are not showing any code so there is no way for us to give you any advice on what to focus on optimizing.
As a general advice:
1) Avoid any state if possible. This way synchronized is not needed
2) Make state immutable if the object needs to have a state. This way you don't care about synchronizing and avoid tedious bugs
3) If object creation is cheap then just do that using (2) if possible
4) If object creation is heavy look into singleton pattern and try to use locks on the methods.
And about
Each connection have its own Thread
Make sure you use a thread pool
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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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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.