What is mutex and semaphore in Java What is the main difference

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What is mutex and semaphore in Java   What is the main difference

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This question has relevant answers and link to official Java guidance  Is there a Mutex in Java

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Semaphore      A counting semaphore  Conceptually  a semaphore maintains a set of permits  Each acquire   blocks if necessary until a permit is available  and then takes it  Each release   adds a permit  potentially releasing a blocking acquirer  However  no actual permit objects are used  the Semaphore just keeps a count of the number available and acts accordingly    Semaphores are often used to restrict the number of threads than can access some  physical or logical  resource  Java does not have built-in Mutex API  But it can be implemented as binary semaphore      A semaphore initialized to one  and which is used such that it only has at most one permit available  can serve as a mutual exclusion lock  This is more commonly known as a binary semaphore  because it only has two states  one permit available  or zero permits available     When used in this way  the binary semaphore has the property  unlike many Lock implementations   that the  lock  can be released by a thread other than the owner  as semaphores have no notion of ownership   This can be useful in some specialized contexts  such as deadlock recovery   So key differences between Semaphore and Mutex    Semaphore restrict number of threads to access a resource throuhg permits  Mutex allows only one thread to access resource  No threads owns Semaphore  Threads can update number of permits by calling acquire   and release   methods  Mutexes should be unlocked only by the thread holding the lock  When a mutex is used with condition variables  there is an implied bracketing   it is clear which part of the program is being protected  This is not necessarily the case for a semaphore  which might be called the go to of concurrent programming   it is powerful but too easy to use in an unstructured  indeterminate way

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Mutex is basically mutual exclusion  Only one thread can acquire the resource at once  When one thread acquires the resource  no other thread is allowed to acquire the resource until the thread owning the resource releases  All threads waiting for acquiring resource would be blocked   Semaphore is used to control the number of threads executing  There will be fixed set of resources  The resource count will gets decremented every time when a thread owns the same  When the semaphore count reaches 0 then no other threads are allowed to acquire the resource  The threads get blocked till other threads owning resource releases   In short  the main difference is how many threads are allowed to acquire the resource at once      Mutex --its ONE  Semaphore -- its DEFINED COUNT    as many as semaphore count

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Semaphore can be counted  while mutex can only count to 1   Suppose you have a thread running which accepts client connections  This thread can handle 10 clients simultaneously  Then each new client sets the semaphore until it reaches 10  When the Semaphore has 10 flags  then your thread won t accept new connections  Mutex are usually used for guarding stuff  Suppose your 10 clients can access multiple parts of the system  Then you can protect a part of the system with a mutex so when 1 client is connected to that sub-system  no one else should have access  You can use a Semaphore for this purpose too  A mutex is a  Mutual Exclusion Semaphore

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You compare the incomparable  technically there is no difference between a Semaphore and mutex  it doesn t make sense  Mutex is just a significant name like any name in your application logic  it means that you initialize a semaphore at  1   it s used generally to protect a resource or a protected variable to ensure the mutual exclusion

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The object of synchronization Semaphore implements a classical traffic light  A traffic light controls access to a resource shared by a counter  If the counter is greater than zero  access is granted  If it is zero  access is denied  The counter counts the permissions that allow access to the shared resource  Then  to access the resource  a thread must receive permission from the traffic light  In general  to use a traffic light  the thread that wants to access the shared resource tries to acquire a permit  If the traffic light count is greater than zero  the thread acquires a permit  and the traffic light count is decremented  Otherwise the thread is locked until it can get a permission  When the thread no longer needs to access the shared resource  it releases the permission  so the traffic light count is increased  If there is another thread waiting for a permit  it acquires a permit at that time  The Semaphore class of Java implements this mechanism    Semaphore has two builders   Semaphore int num  Semaphore int num  boolean come    num specifies the initial count of the permit  Then num specifies the number of threads that can access a shared resource at a given time  If num is one  it can access the resource one thread at a time  By setting come as true  you can guarantee that the threads you are waiting for are granted permission in the order they requested

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A mutex is often known as a binary semaphore  Whilst a semaphore can be created with any non-zero count a mutex is conceptually a semeaphore with an upper count of 1

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Unfortunately everyone has missed the most important difference between the semaphore and the mutex  the concept of  ownership     Semaphores have no notion of ownership  this means that any thread can release a semaphore  this can lead to many problems in itself but can help with  death detection    Whereas a mutex does have the concept of ownership  i e  you can only release a mutex you have acquired   Ownership is incredibly important for safe programming of concurrent systems  I would always recommend using  mutex in preference to a semaphore  but there are performance implications     Mutexes also may support priority inheritance  which can help with the priority inversion problem  and recursion  eliminating one type of deadlock     It should also be pointed out that there are  binary  semaphores and  counting general  semaphores  Java s semaphore is a counting semaphore and thus allows it to be initialized with a value greater than one  whereas  as pointed out  a mutex can only a conceptual count of one   The usefulness of this has been pointed out in other posts    So to summarize  unless you have multiple resources to manage  I would always recommend the mutex over the semaphore

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Mutex is binary semaphore  It must be initialized with 1  so that the First Come First Serve principle is met  This brings us to the other special property of each mutex  the one who did down  must be the one who does up  Ergo we have obtained mutual exclusion over some resource    Now you could see that a mutex is a special case of general semaphore

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A mutex is used for serial access to a resource while a semaphore limits access to a resource up to a set number  You can think of a mutex as a semaphore with an access count of 1  Whatever you set your semaphore count to  that may threads can access the resource before the resource is blocked

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A semaphore is a counting synchronization mechanism  a mutex isn t

[java] What is mutex and semaphore in Java ? What is the main difference?

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